AustralianPhotography

photography glossary

Edited by Prashphutita Greco, with contributions by Prashphutita Greco, Andrew Fildes and James Ostinga.



3D Matrix Metering: Nikon system for exposure calculation, and automatic exposure compensation, based on scene brightness and contrast, and distance information (obtained from the lens). (See also: Exposure Compensation.)

ABC (Auto Backlighting Control): A camera metering feature that automatically recognises a subject in a backlighting condition, and increases the exposure to compensate.

Aberration: An inherent flaw or failing in the ability of a lens (optical system) to produce a true image. Examples: chromatic (colour) distortion; astigmatism (differing focus points); vignetting (darkening of the image in the corners).

Accessory Shoe: A mounting point located on top of a camera. The camera accessory shoe is commonly used to attach a flash unit to the camera. In most modern cameras the attachment allows the camera and flash unit to relay information. In some cases, the accessory shoe may be used to connect other accessories, eg. an electronic viewfinder. or LED lighting. (See also: Hotshoe.)

Active AF (autofocus): In Active AF the camera sends out a beam of something (usually infrared light) and times how long it takes to return. This is "bat" focussing and Polaroid once put out a range of cameras that sent out a sonic signal, like a bat listening for sound reflections - they had a microphone over the lens. Most SLRs and some compacts now use an infrared beam to get focus in very low light or they pre-flash the subject. They measure the time the reflection takes to return and set the focus accordingly. It's rather slow and may reflect off glass if shooting through a window - that's why it's only used in an “emergency” such as flash shooting in dark places.

Acutance: The quality of an image's sharpness, based on how quickly the contrast changes across an edge.

Adams, Ansel: Ansel Adams (February 20, 1902 – April 22, 1984) was a legendary American photographer most famous for his black-and-white prints of Yosemite National Park in California.

AE (Automatic Exposure): Camera determines the exposure required to obtain a nominally correct-looking image (based on a “typical” scene, i.e. averaging out to around 13% reflectance).

Aerial Photography: Photography taken from a high vantage point, mostly aircraft.

Airbrushing: Although no longer a tool identified as such in newer versions of Photoshop, still refers to the technique of smoothing imperfections, e.g. in a model’s skin.

Air Lens: A lens using an “air gap” (which has a refractive index of 1.0) between a pair of lens elements.

Algorithm: A series of pre-defined steps followed by a computer program (e.g. built into a camera) to produce a certain result, e.g. sharpening, noise reduction, colour saturation boost, etc.

Ambient Light: The available light (including natural, and any other artificial light “sources”) that is currently illuminating the scene.

Analogue: Until the last few decades, only ("conventional") analogue photography existed. Contrary to what the marketing hype might have led you to believe, the universe still is analogue. For instance, light -- the very basis for all forms of photography -- can assume any of an infinite variety of levels, subtleties and shades of colour. It's only the digital (see definition, later) representation that imposes fixed, finite, discrete, individually identifiable steps.

Anti-aliasing (Low-Pass) Filter: Situated in front of a digital camera's imaging sensor, its role is to limit bandwidth, i.e. reduce the higher spatial frequencies (think of progressively more closely-spaced black and white line pairs on lens resolution test charts) from the lens. The Nyquist Sampling Criterion states that, in order to fully and correctly reconstruct a signal, it must be sampled at a rate at least twice as high as the highest frequency component in the signal. If not, unwanted, spurious components will appear in the output which were not present in the original; this is called "aliasing".

Aperture: Aperture refers to an opening inside the lens which can be made larger and smaller to control how much light reaches the sensor (or film plane). Along with shutter speed and ISO, aperture is used to control the exposure of the image -- in other words, how dark or light it is.

Aperture also has an important secondary role. Along with focal length, it influences depth-of-field, or how much of a scene is in focus either side of the point you actually focused on. A narrower aperture setting increases depth-of-field, while a wider aperture setting reduces it.

With a wide aperture it’s possible to put a person’s face in sharp focus, while everything in the background and foreground falls away into a soft blur. This is really useful for isolating an element so it appears to “pop” out of the photo.

Conversely, with a narrow aperture it’s possible to create an image where every part of the scene -- from foreground to background -- is rendered in sharp detail.

Aperture is measured in terms of f-stops. Common f-stop settings are: f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f32. A little confusingly, it’s a case of the smaller the f-stop number, the wider the aperture. So, an aperture of f/2 is wider and lets in more light than an aperture of f/22. Got it? You get used to this inverse relationship quite quickly, particularly if you start thinking of f-stops as fractions, for example 1/2 is bigger than 1/22.

There’s another thing you need to know about aperture. Most lenses produce their sharpest results in the middle aperture settings, usually around f/8 and f/11. It’s not a depth-of-field issue. Rather, the physics of optics makes it difficult to build affordable lenses that are pin-sharp over the entire aperture range. Stopped down to f/22 or f/32 or wide open at f/2.8 or f/4 many lenses are slightly soft. (See also: f-stop).


Aperture (software): An Apple program for Digital Asset Management, RAW conversion, and image editing/adjustments and printing.The software is Mac only. (See also: Lightroom).

Aperture Priority (A, Av): Aperture Priority is a semi-manual (or semi-automatic) camera setting that allows the user to select the aperture while the camera selects the required shutter speed to balance the exposure. Aperture Priority is abbreviated to Av on Canon and Pentax bodies, and A on Nikon, Sony, Olympus and Panasonic models. If your main concern is depth-of-field, Aperture Priority is a great choice.
When you’re using Aperture Priority, keep an eye on the info display to make sure the camera is choosing a suitable shutter speed. If the shutter speed gets too slow you’ll need a tripod, a bigger ISO, and/or a wider aperture. If the shutter speed display is flashing, it's likely an indication that the shutter speed setting you’ve chosen is too wide or too narrow. (See also: Shutter Priority).

APS-C: APS-C is digital sensor size, similar to the dimensions of Advanced Photo System (APS) film. APS film measured 25.1 × 16.7mm with an aspect ratio of 3:2. In Nikon, Sony and Pentax cameras, APS-C is around 23.6 x 15.8mm and results in a Crop Factor of 1.5x. In Canon’s implementation, APS-C is 22.2 x 14.8 mm, and results in a Crop Factor of 1.6x. (See also: Crop Factor).

Artefact: In digital processing, artefacts are undesirable, unfortunate side-effects of processing information, e.g. excessive (or repeated use of) JPEG compression. (See also: Compression, JPEG).

Artificial light: Light made from a man-made source e.g. studio photo lamp or electronic flash unit.

Aspect Ratio: Refers to the ratio between the linear dimensions of a particular imaging format. For example, with the familiar 35mm (24 X 36 mm) format, 36/24 = 1.5 (= 3:2, hence suitable papers for printing this image on (without any cropping) would be 4” X 6”, 8” X 12”, 20” X 30”, etc.)

ASA (American Standards Association): System for measurement of film speed; interchangeable with ISO system. (See also: ISO).

Autofocus: System by which the lens, or camera body (or combination of both) automatically focuses the image (formed by the lens) of a selected part of the picture subject onto the imaging sensor, or film. Currently all major camera makers such as Canon, Leica (a contraction for LEItz CAmera), Mamiya, Nikon, Pentax and Sony all sell complete camera systems with Autofocus capability.

Axis Lighting: Lighting used in photography, referring to the light source being positioned as close as possible to the lens itself (resulting in a shadowless light). Also known as “frontal lighting”.

Background: The scene that appears behind the main subject of a photograph.

Backlighting: Any light that comes from behind the subject, towards the camera lens, making the subject stand out against the background.

Balance: Composition of colours or objects in a picture to create equilibrium.

Bare bulb: A large flash used within a studio that doesn't involve a flash head or reflector dish.

Barrel: The cylindrical body of a camera lens (usually made from plastic or metal).

Barrel Distortion: Photographs of straight lines taken when using a wide-angle lens show a “bowing-out” of those lines (similar to the shape of a barrel, hence the name).

Bayer Interpolation: The method by which information from the typical Bayer pattern (Red, Green, Green, Blue) imaging sensors (see later) in cameras is reconstructed to produce a representation of the scene. Also known as de-mosaicing.

Binary: In our familiar decimal number system, we have, in order of increasing "weight": units, tens, hundreds, thousands, tens of thousands, etc. Each position further to the left has a value of 10 times more than the position adjacent to it. The binary number system uses only ones and zeroes; there is a factor of two between adjacent positions.

Bit: Contraction for Binary Digit, the fundamental quantity of the binary number system.

Bit Depth: In the analogue (real) world, physical quantities can take on any of an unlimited range of values, e.g. a child growing up progresses *continuously* through all possible heights. However, in the digital system, there are only a finite number of possible values which a particular variable can assume, according to the number of bits assigned to that variable. 8 bits per channel, or 24 bits per pixel (since there are three channels in the RGB [Red, Green, Blue] colour space, though often just called "24 bit") allows for representing 256 levels, from zero to 255, for each channel, giving a total of over 16 million possible colours (256 X 256 X 256). This isn't nearly enough when major adjustments need to be made to the tonal range of the image. Hence, the need for 16 bits per channel ("48 bit"), which yields billions of colours (two to the power of 16 = 65,536; hence 65,536 X 65,536 X 65,536 equals 2.8 to the power of 14).

Bitmap/Bit-mapped Image: An image (a.k.a. raster) file, containing information based on specific locations, organised as a rectangular array, on a fixed grid; e.g. so many across, and so many down. The maximum size at which this image can be reproduced is then limited by the dimensions of the file. Photoshop deals only with raster images. (See also: Vector).

Blocked-up: Term referring to under-exposed (e.g. shadow) areas in a photograph being devoid of detail.

Bokeh: Lens characteristic, denoting the nature or quality of the out-of-focus areas of the image (particularly pronounced where bright point sources of light exist).

Bracketing: Shooting several shots of the same subject using different exposure settings. This can be done manually (the photographer changes the exposure settings between shots), or automatically (the camera takes a series of images in quick succession – usually three – each with a different exposure value). Bracketing is widely used to ensure a correct (or, at least, usable) exposure, or as the basis for creating a High Dynamic Range Image.

Brightness: The intensity of photographic light energy.

Byte: In computer terminology a byte is a unit of storage that comprises 8 bits. Using the Greek prefixes kilo ("thousand"), mega ("million") and giga ("thousand-million") allows for quoting big numbers. But, being based on the binary number system, in this case one kilo means X 1024 (so one kilobyte (KB) equals 1024bytes), mega denotes 1024 X 1024 (one megabyte equals 1,048,576bytes), while Giga means 1024 x 1024 x 1024 (one gigabyte (GB) equals 1,073,741,824bytes). Be wary, though, that your file manager may be reporting file sizes in "decimalised" form, i.e. a smaller number.

C-41: Process for the development of Colour Negative (dye-based) films.

Calotype: A print created using either the first negative or positive printing process .

Candela: SI (International System of Units) unit for the measurement of light intensity. Monitor brightness is measured in Candela per square metre, eg. 300  Cd/m2. (See also: Brightness).

Candid Photography: Unposed pictures often taken without the subject’s knowledge. Leads to natural and relaxed photos.

Carbon Fibre: Composite plastic material, with high strength-to-weight ratio and vibration-absorbent characteristics.

Cartier-Bresson, Henri: Henri Cartier-Bresson (August 22, 1908 – August 3, 2004) was a French photographer considered by many to be the forerunner of modern photojournalism. Cartier-Bresson co-founded Magnum Photos in 1947 with Robert Capa,  George Rodger and David "Chim" Seymour. Cartier-Bresson coined the term “Decisive Moment” and had the uncanny ability of being in the right place at the right time for many of the major events of the 20th century. (See also: Decisive Moment).

CCD (Charge-Coupled Device) Sensor: CCD is a type of digital imaging sensor. The CCD is the original form of light sensor chip and it is still widely used in compact digital cameras. CCDs are also used in DSLRs, though CMOS sensors (see CMOS) are more common. CCDs are quick and efficient though tend to produce more image noise at high ISOs than CMOS sensors (see CMOS).

Centre-weighted Metering: Popular in older cameras, centre-weighted metering biases most of the priority for basing the exposure on the central portion of the viewfinder. It takes in a much larger region than spot metering. It can be handy for backlit subjects, but matrix metering (aka evaluative, multi-zone, honeycomb) generally produces better results in normal lighting conditions.

Chemical Photography: Based around traditional photography processes that used film and chemicals.

Chroma Noise: That component of noise in a digital imaging system which is due to random variations in colour. Generally, chroma noise is more noticeable/disturbing/distracting than Luminance noise. (See also: Luminance Noise; Noise).

Clipping: In a digital system, "Clipping" means that there are no longer tonal values remaining to represent those particular parts of the image. Thus, clipped highlights appear white and clipped shadows look black. Once clipping occurs detail is lost.

Close-up: A photo taken when the subject is less than a metre away from the camera.

The Cloud: Metaphoric term for the Internet. Generally used to describe a way of working whereby users’ digital files (images, movies, documents, applications, etc.) are stored remotely on the internet rather than on a local drive.

CMOS Sensor: CMOS (Complementary Metal Oxide Semiconductor) is a type of digital imaging sensor. CMOS sensors are widely used in digital SLR cameras. CMOS is generally preferred over CCD (see CCD), for its ability to  generate less image noise at high sensitivity (ISO) settings. A new type of “back illuminated” CMOS is showing up in the better compact cameras now, promising better speed and sensitivity.

Colour Cast: Usually unwanted condition where some or all colours in an image are skewed towards one part of the colour spectrum.
 
Colour Space: Various mathematical models exist to represent the range of colours visible to human eyes. Digicams typically use sRGB (standard RGB, see also: sRGB), or Adobe RGB (1998), which has a larger gamut (see also: Gamut). When editing your images, you might choose other colour spaces, e.g. L*a*b, which is a 3-dimensional model based on Luminance ("Lightness") and two colour axes. Colour magazines are often produced using the CMYK (Cyan, Magenta, Yellow and blacK) colour model, while images on computer screens are produced using RGB (Red, Green, Blue).

CompactFlash (CF): An early, and still popular, form factor of memory card used by professional and/or prosumer cameras. SD has become the de-facto standard. (See also: SDHC).

Composition: The arrangement of the elements within a scene creating a visually appealing image.

Compression: Mathematical techniques by which a digital file can be made smaller, usually for the sake of saving storage space on the digicam's memory card, or on the computer's hard disk, or to minimise the time required to transfer the file across a network or the Internet, etc. Familiar examples: lossless compression -- .Zip; lossy compression -- .MP3 (audio), or .JPEG (still image).  (See also: Lossless; Lossy).

Contrast: The differences in brightness between the light and dark areas of a photo. Often referred to as "tonal difference".

Contrast Detect AF (autofocus): Contrast Detect AF is the primary autofocus system used in compact digital cameras. It may also used be used by digital SLR cameras when used in Live View mode. Contrast Detect AF uses the camera's main imaging sensor to assess and adjust focus. Contrast (intensity) between individual pixels increases with improved focus so it simply adjusts the lens for maximum contrast at the sensor. Contrast Detect AF is slower than Phase Detect AF as it is unable to detect which way to focus the lens at first and may “hunt” back and forth. Focussing is slow but potentially very accurate.

Crop Factor ( or Lens Crop Factor): Digital cameras typically have sensors of size less than “full-frame” (24 X 36 mm). As a result, the image circle formed by the lens falls on a receiving device with a surface area of less than 864 square millimetres. In effect, the imaged scene is being “cropped”, which results in image magnification, ie. as if you were using a longer lens (more towards the Telephoto end of the range). Quite often – though erroneously – this is referred to as a Lens Multiplier Factor. In truth, the focal length of a lens doesn’t vary: it’s a fixed quantity, as determined by the original design.

Cropping: The process of resizing and redesigning an image by removing unwanted areas.

Curves: Derived from “Transfer Curve”, Curves refers to a graph describing the relationship of the Output (on the vertical, or “Y” axis) and the Input (on the horizontal, or “X” axis). A 1:1 correspondence (ie. Output = Input, for all values of the Input variable) would mean a straight line at a 45 degree angle. Image Editing programs (the most famous of which is Adobe Photoshop) allow users to manipulate Curves for the separate Red, Green and Blue Channels. This could be done, for example, to introduce a classic “S-shaped” Curve, which yields more contrast in the image. Or, to extract more detail from the shadows. Or, to selectively brighten just the midtones. Or, to darken too-bright highlights. Curves afford a very high degree of flexibility and creativity. However, used carelessly, Curves can produce unwanted colour casts. (See also: Posterisation, Colour Cast).

Darkroom: Light-tight room used to process and print film. A red “safety” lamp is able to be used when printing Black & White.

Decisive Moment: A term coined by French photographer Henri Cartier-Bresson, meaning: “… the simultaneous recognition, in a fraction of a second, of the significance of an event as well as of a precise organisation of forms which give that event its proper expression." (See also: Henri Cartier-Bresson).

Diffusing: The act of softening light or detail in a print. Light can be diffused by bouncing it off a surface, such as a wall or ceiling, or by using a light modifier such as a softbox or umbrella.

Depth of Field (DOF): Depth of field refers to how much of a scene is in focus either side of the point of focus. An image with a small depth of field will have a narrow area of sharp focus, while the areas behind and in front of that point will fall away to a soft blur. An image with a large depth of field will appear to be in focus from the foreground through to the background.

Digital: A representation of physical reality (eg. audio, still images, video, etc.) as a series of discrete steps, by use of the binary number system. In the case of an 8-bit greyscale image, for example, all zeroes represents black, all ones represents brightest white, while 10000000 represents medium grey.

Double Exposure: An image involving two exposures on the same frame. Some cameras provide this capability. Alternatively, this can be readily achieved by combining two separate exposures in post-processing.

DPI (Dots Per Inch): Often used interchangeably – though erroneously – with PPI (Pixels per Inch). More correctly, DPI apply when a digital image is printed out e.g. on an inkjet printer. Let's say we want to make a 4" X 6" print, and insist on having a minimum of 300PPI (as, to our eyes, that's the least we require for acceptable "photo quality"). This means our file must be sized at least 1800 pixels in the long dimension.

DSLR (Digital Single Lens Reflex): A DSLR is an SLR that uses a digital sensor rather than film. All DSLRs use a mirror to deflect light to the viewfinder. In simple terms, a mirror, angled upwards at 45°, is positioned inside the camera between the lens and the shutter. The mirror deflects light from the lens up to a prism and into the viewfinder. This allows the photographer to view the scene through the lens. When the shutter button is pressed to take a photo, the mirror swings up out of the way, the shutter opens and light from the lens exposes the sensor.  

Dynamic Range: The range of tones captured by a camera, scanner or other imaging device. Also referred to as “contrast range”.

Electronic Flash: A flash unit powered electronically, designed to provide light approximating the colour temperature of noontime daylight.

Electronic Viewfinder: In digital cameras, the image produced by the lens is displayed electronically on a miniature LCD.

Equivalent Focal Length: 35mm-Equivalent Focal Length is a measure which allows photographers to compare the angle of view of a particular lens and sensor combination with the 35mm camera format. The term is useful in digital photography because field of view is a product of focal length and sensor size, and there is no uniform sensor size in digital photography.
Focal length has long been used to describe field-of-view. That is, whether a lens is wide-angle, normal or telephoto. That system worked well when most people were using 35mm film cameras, but got a little confusing when photography went digital.
Field-of-view is determined by two things: focal length and the size of the film plane or digital sensor. Since digital sensors come in all shapes and sizes, focal length, on its own, is not a useful indicator of angle of view. A 28mm lens might produce a wide-angle view on one camera and a telephoto view on another. 35mm Equivalent Focal Length provides a common standard by which we can compare angle of view. Equivalent focal length is produced by multiplying the camera's Crop Factor (which is determined by its sensor size) by the lens focal length.
Examples: The Canon EOS 600D has a FOV crop factor of 1.6. With a 20mm lens, the 35mm-equivalent focal length is 1.6 x 20, or 32mm. The Nikon D40x has a FOV crop factor of 1.5. With a 20mm lens, the 35mm-equivalent focal length is 1.5 x 20, or 30mm. TheIf you attach a 20mm lens to a Nikon D700, which has a FOV crop factor of 1x, the equivalent focal length is 20mm! (See also: Crop Factor).

ETTR: Expose To the Right; because of the finite number of bits representing a digital image, and the way that these are distributed in the binary number system, we're admonished to always: "ETTR!". This means setting the digicam's exposure so that the distribution of pixels (see Histogram, later) is as far to the "(b)right" side as possible, without the highlights being clipped (burned-out).

Evaluative Metering: Evaluative Metering (also known as matrix, honeycomb, multi-zone metering) is the default metering system for most modern cameras. Set to evaluative metering, the camera effectively splits the scene into sections and takes exposure readings at multiple points across the image (over 1000 in some cases). An algorithm then weighs the importance of the different sections according to variables such as subject distance and position to produce an overall recommended exposure. This is the most sophisticated of the three main metering modes (evaluative, centre-weighted and spot) and it usually produces the best results. That said, it can be tricked in some high-contrast situations.

EVIL (Electronic Viewfinder Interchangeable Lens): EVIL is a term widely used to describe a type of digital, interchangeable-lens camera that, unlike a conventional single lens reflex camera, does not use a mirror system and pentaprism to direct light from the lens to an optical viewfinder. Rather than using an optical viewfinder, users compose images with an electronic viewfinder or the primary LCD monitor.

EXIF (Exchangeable Image File): A file specification for the recording of informative data within a JPEG or TIFF file itself. Amongst a very large possible number of tags (meta data) are:  camera make and model; date and time of picture; lens aperture and focal length; shutter speed; ISO setting; exposure compensation; etc.

Exposure: The process of light hitting a photographic material. For better and/or more creative photography, it is essential to understand and control exposure, which is determined by: ISO setting, Aperture and shutter speed.

Exposure Compensation: Adjustment of the exposure by “adding” (+EC) or “subtracting” light (–EC).

Exposure Triangle: ISO, aperture and shutter speed all combine together to determine the final exposure. Doubling the ISO setting, for instance, makes the camera twice as sensitive to light, meaning that the shutter speed required to maintain the same exposure would be only half of what it was previously. This is called “reciprocity”. (See also: Aperture; ISO; Shutter Speed).

Eye Cup: The rubber ring positioned around the camera eyepiece to reduce stray light rays from entering the viewfinder.

f-Stop:  Originally, referred to boards (with various sized holes drilled in them) which allowed set quantities of light to pass through to the lens in an old-fashioned bellows camera. Mathematically, derived by dividing a lens’ effective (optical) diameter into its focal length, e.g. for long focal length (telephoto) lenses to allow in the same amount of light as shorter focal length lenses, the telephoto lens needs to be of a bigger diameter.

Fast Lens: Lens with a wide maximum aperture: allows in more light, hence the shutter speed is correspondingly reduced (ie. “Faster”).

Filesize: Computer terminology for the size which a file (eg. image from a digital camera, or derived by scanning film, or a text document, or a music or video file, etc.) occupies on storage media (eg. Hard Disk Drive, memory card, etc.). When transmitting over a Network (eg. Local Area Network (LAN), or over the Internet, etc.) compression techniques may be applied to reduce the filesize, in order to save both bandwidth and time.

Filter: A transparent material (usually glass or plastic resin), used to cover the lens to affect the transmission of radiation and light, thereby changing the image in the required, controlled fashion.

Firmware: Software embedded in a particular, microprocessor-based system, eg. digital camera, Smartphone, GPS unit, etc. Usually, it is possible to upgrade the firmware, as it is stored in (non-volatile) EEPROM (Electrically Erasable Programmable Read-Only Memory). A camera manufacturer, for example, is likely to release upgraded firmware to overcome operational problems with a newly-released model, or to provide different/enhanced functionality.

Fisheye Lens: An ultra wide-angle lens that gives a 180 degree angle of view and causes severe distortion of straight lines.

Flash: Illumination of an area by a burst of artificial light, usually from a glass tube filled with Xenon gas (triggered by a High Voltage discharge).

Flash Memory: A type of memory technology which allows the data written electrically to the storage cells to be maintained even in the absence of power. It can be erased and written to again almost indefinitely (e.g. newer devices are typically rated at one million read/write cycles). The most common form factors of memory cards are CompactFlash (CF) and Secure Digital (SD). Sometimes, people refer to "Memory Stick" (which is proprietary to Sony!), but the generic term is flash memory.

Focal Length: Focal lengths are usually measured in millimetres (mm), and determine the magnification of the image projected onto the sensor or film plane. (See also: Telephoto; Wide-Angle).

Focus: The adjustment of a lens at which the image so formed of an object is defined sharply and details appear clearly.

Four Thirds System: The Four Thirds (FT) System describes a common standard for the design of cameras and lenses. Current manufacturers of Four Thirds products include Olympus (cameras and lenses), Panasonic (cameras and lenses) and Leica (lenses only). All Four Thirds lenses and cameras are compatible, irrespective of brand. Four Thirds sensors measure 17.3 x 13mm, which is around half the dimensions of a full-frame sensor (36 x 24mm) and a quarter of the area. Four Thirds cameras have a Crop Factor of 2x. The Four Thirds system uses an image aspect ratio of 4:3, rather than the 3:2 usually used by DSLRs. (See also: Crop Factor, Micro Four Thirds).

Frame: A single image on film, e.g. 35 mm (24 X 36 mm), or Roll film (6 X 6 cm, or 6 X 7 cm, or 6 X 9 cm, or 6 X 17 cm).

Fresnel Lens: A flat type of lens that distributes light evenly over the surface.

Full-Frame: Refers to a digital imaging sensor having physical dimensions the same as traditional 35 mm format, ie. 36 x 24mm.

Full HD Video: High-definition video is of a format having resolution higher than that of SD (Standard-Definition). Full HD, for example, would be 1920 X 1080 pixels (progressive, or, interlaced).

Galen Rowell: Galen Rowell (August 23, 1940 – August 11, 2002) was an American mountaineer and internationally renowned wilderness photographer. His images were published in Life, National Geographic, Outdoor Photographer and many other publications.

Gamut: Range of colours which can be represented; e.g. within the sRGB (see later) colour space, or reproduced e.g. by an inkjet printer.

Geotagging: “Geo-tagging” your images means applying geographical-related metadata (e.g. latitude and longitude coordinates) to help facilitate your searches based on location information. Your Smartphone (e.g. iPhone) may already provide GPS data. There are also GPS data logger add-ons for various model cameras (e.g. certain Nikon and Fujifilm DSLRs with 10-pin Remote Sockets). More cameras now have this capability built-in. Also, more software now allows you to search and display your images based on location information.

Ghost images: Bright or blurred spots appearing on an image when the lens is pointed towards a bright light, e.g. the sun, or streetlamps at nighttime. In optical terms, caused by lens flare and/or veiling glare (which also results in reduced overall contrast in the image).

GIF: Graphics Interchange Format. A digital image format only supporting depth of 8 bits (and therefore not very useful for photographs, as it can only represent a limited range of tones). Useful for graphics, though, due to the resultant small filesizes.

Gigabyte: 1024 MB  (See also:  Byte; Megabyte).

Graduated Filter: A type of filter tinted progressively along its length to create specific types of effects – often used to emphasize sunsets.

HD (High-Definition) Video: High-definition video is of a format having resolution higher than that of SD (Standard-Definition). (See also: Full HD Video).

HDR (High Dynamic Range) Image: A high dynamic range (HDR) image contains a greater range of tonal detail than a given camera could capture in a single photo. HDR images are commonly created by shooting a series of images of the same subject, and altering the exposure with every shot to produce overexposed, correctly exposed and underexposed images. This process targets specific tonal regions of the scene: shadows, midtones and highlights. The images are then blended into a single image using special HDR software.

HDMI (High Definition Multimedia Interface): A Standard for the transfer of digital data, audio, and video over an interface.

High Key: In photography, refers to an image which is exposed more than what would constitute a normal exposure, ie. it’s overly bright (though not usually overexposed (with burnt-out highlights). “Key” refers to the Key (main) light. Overall, the lighting on the subject is very flat. Also notable is the low contrast throughout, achieved by carefully controlling the lighting to ensure that shadows, and any dark tones, are minimised. Emotion conveyed: happy, light & breezy. (See also: Low Key).

High-speed Photography: Photography with extremely short exposure times with the effect of freezing rapidly-moving subjects/scenes.

Histogram: A histogram is a graph, which allows users to quickly assess the tonal makeup of an image. The vertical axis represents number of pixels, while the horizontal axis represents the different tones, starting with black on the far left, moving through progressively lighter shades of grey to white on the far right.  Underexposed images appear "bunched-up" on the left, while over-exposed images appear "bunched-up" on the right. (See also: ETTR).

Hot Shoe: A mounting point located on top of a camera. The camera accessory shoe is commonly used to attach a flash unit to the camera. In most modern cameras the attachment allows the camera and flash unit to relay information. In some cases, the accessory shoe may be used to connect other accessories, eg. an electronic viewfinder. or LED lighting. (See also: Hotshoe.)

Hue: An attribute of colour, which describes how it compares with the unique (primary) hues of Red, Green, Blue and Yellow. Other attributes of colour are saturation, and brightness.

Image: A visual representation of a scene or object; a picture.

Image Sensor: Silicon-based sensor chip in a DSLR detecting light rays and recording information as digital data. (See also: CCD or CMOS Sensor).

Image Stabilisation (IS): System used within Canon lenses to counteract the effects of camera motion, and allow handholding at much lower shutter speeds. Other camera manufacturers have devised their own systems. (See also: Vibration Reduction).

Intensifier Filter: A filter that enhances the image by subtly brightening through increasing the saturation of certain colours.

Interchangeable-lens camera: Refers to a camera system whereby a range of different lenses lenses can be attached to a particular camera body. (See also: Wide-angle; Telephoto).

Interpolation: Required for resampling. Using various mathematical formulae (which produce different results, according to the nature of the image being dealt with), it's possible to estimate (based on the existing pixels) what pixels ought to be present if that image were to be resized to different dimensions.

ISO: International Organization for Standardisation. The ISO system describes exposure film speeds using a numeric system where the higher the number, the greater the sensitivity ("speed"). (See also: ASA).

ISO (digital): In digital photography, ISO is the measure of the sensor’s sensitivity to light. As you increase ISO the sensor becomes more sensitive (also referred to as “faster”) and requires less light to make an exposure. Conversely, as ISO decreases the sensor becomes less sensitive, and requires more light to make an exposure.
It’s important to understand that as ISO increases, so too does image noise. Noise, in this context, has nothing to do with the beeping sounds your camera makes when it focuses. Rather, it describes those ugly spots and coloured artefacts you sometimes see in some images.
Noise is present in all images, though at low ISO settings it’s not visible unless you zoom right in to the smallest details of the image. At high ISO settings it can be so bad it’s a struggle to make out important details – like watching an analogue TV with bad reception.
Some cameras produce more noise than others. Try shooting a series of images at different ISO settings with your camera. Open the images in a computer, zoom in to 100 or 200% and compare the results. While some compacts can get very noisy at 400 and 800 ISO, most entry-level DSLRs produce relatively clean images up to 800 and 1600 ISO. Some high-end pro DSLRs produce great images at 12,800 ISO and beyond.
If you’re shooting outdoors in good light, it’s generally best to use your camera’s lowest ISO setting (eg. 100). (See also: Noise).

ISO (film): In film photography ISO is the measure of a particular film's sensitivity to light. Common film ISO ratings include 50, 100, 200, 400, 800 and 1600. As the ISO rating increases the film becomes more sensitive to light and produces more grain.

JCII: Japan Camera Industry Institute.

JFIF: JPEG File Interchange Format (JFIF) is the Standard defining the JPEG file format.

JPEG (Joint Photographic Experts Group): JPEG defines file compression, while the file format used is JFIF (JPEG File Interchange Format). People normally just refer to "JPEG files". Very popular: designed specifically for photographs, where it can achieve reductions of up to 95% of the original file size, albeit with corresponding loss of image quality. Excessive compression results in the familiar look of: even tones being built of "blocks", fuzzy edges, distortion of colours, and loss of detail, etc. Be aware that making any change to a file, then saving it, causes the JPEG compression algorithm to be applied again, resulting in further image degradation. JPEG can only represent 24-bit images.
Used on the Web and with many digital cameras. (See also: Compression; Lossless; Lossy).

Kelvin: Measurement scale used to measure colour temperature.

Kit: An SLR sold as a package with one or more lenses (usually zoom).

Kit lens: A lens sold in a package with an SLR.

Landscape Photography: Photography of outdoor landscapes, generally attempting to convey mood and emotion, as well as being a pictorial record of a scene. To this end, natural light makes a major contribution to the impact of a landscape photo. Of the many, great practitioners of  landscape photography, Ansel Adams and Galen Rowell figure prominently.

Layer: Image editing software such as Photoshop allows for creating complex compositions by means of a stack of separate images. This provides for very powerful capabilities.

LCD (Liquid Crystal Display): An electronically generated display found in many DSLRs and other cameras.

LED (Light Emitting Diode): Comprising solid-state (semiconductor) materials which, when current flows as a result of an applied voltage, emit light. Commonly available colours include: Red, Amber, Green, Blue, White.

Lens: A piece of plastic, glass or crystal optical element which is designed in a shape that will refract light to form a sharp image on an imaging sensor or film.

Lens Crop Factor (or Crop Factor): Digital cameras typically have sensors of size less than “full-frame” (24 X 36 mm). As a result, the image circle formed by the lens falls on a receiving device with a surface area of less than 864 square millimetres. In effect, the imaged scene is being “cropped”, which results in image magnification, ie. as if you were using a longer lens (more towards the Telephoto end of the range). Quite often – though erroneously – this is referred to as a Lens Multiplier Factor. In truth, the focal length of a lens doesn’t vary: it’s a fixed quantity, as determined by the original design.

Levels: Another means available for adjusting images within Editing Programs such as Adobe Photoshop. When using the Levels Tool, you can directly observe the effect on the Histogram. Depending on your intent, you can manipulate the brightness distribution within the image, or adjust the Contrast, or work on the shadows, or the highlights, or any combination thereof. To achieve this, there are Sliders for: Black point, Grey-point, and White point.

Lightroom: Adobe (Windows, and Mac) program for Digital Asset Management, RAW conversion, and image editing/adjustments and printing.

Live View: Live View is the feature that allows DSLR users to frame pictures using the camera's LCD monitor. With Live View switched on, the scene in front of the lens is displayed “live” on the camera’s LCD monitor, allowing photographers to frame the shot using the big screen on the back of the camera – just like a compact camera. That said, with just a few exceptions, the implementation of Live View in DSLRs is generally not as good as it is with compact cameras. That’s because the mirror that sits in front of the sensor (to reflect light into the viewfinder), has to flap up and down to allow Live View focusing and metering. That makes for slow shooting, which is why many photographers only use Live View for stationary subjects; landscapes, macros, product shots, etc.

Local Contrast Enhancement: A technique to increase the apparent detail in an image (or to cut through atmospheric haze), by means of increasing the contrast in a localised area relative to the average contrast of the surrounding area.

Lossless: Refers to a file format which maintains all of the information originally available. This ensures that none of the originally available data (information) is discarded, even though the filesize can be (somewhat) reduced. Applicable to still images, or video, or music, etc. Lossless formats allow offer an unlimited number of “sessions” of editing a file, and saving it again, with no loss of quality. Some formats allow for compression of the file size, whilst still remaining lossless. (See also: Compression; JPEG).

Lossy: In order to achieve very significant quantities of file size reduction, information contained within the original file is discarded, resulting in a degradation of the image quality. This cannot be fully recovered (reconstructed) later.

Low Key: In photography, refers to an image which is exposed less than what would constitute a normal exposure, ie. it’s overly dark (though not necessarily underexposed – however, this can be an advantage for creating a very moody scene, and/or to hide details in the scene which might otherwise be a distraction). Also notable is the maximised contrast, achieved by carefully controlling the lighting. Emotion conveyed: moody, dark, brooding, menacing. (See also: High Key).

Luminance: The quantity of light being emitted in a single direction from a surface.

Luminance Noise: That component of noise in a digital imaging system which is due to random variations in luminance (loosely, brightness levels). Generally, luminance noise is less noticeable/disturbing/distracting than chroma noise. (See also: Chroma Noise; Noise).

Macro: A lens that provides continuous focusing of tiny objects that traditionally reflect the object’s size (1:1 reproduction ratio, or at least 0.5:1).

Manual Focus: Any system that involves the photographer manually adjusting the lens to focus a scene.

Matrix Metering: Matrix Metering (also known as evaluative, honeycomb, multi-zone metering) is the default metering system for most modern cameras. Set to matrix metering, the camera effectively splits the scene into sections and takes exposure readings at multiple points across the image (over 1000 in some cases). An algorithm then weighs the importance of the different sections according to variables such as subject distance and position to produce an overall recommended exposure. This is the most sophisticated of the three main metering modes (matrix, centre-weighted and spot) and it usually produces the best results. That said, it can be tricked in some high-contrast situations.

Maximum Aperture: Maximum aperture refers to a lens' largest aperture setting. The maximum aperture may be given as a single aperture (for example f/4), or as a range of apertures (for example, f/4-5.6). If the maximum aperture is given as a range, it indicates that the maximum aperture becomes smaller as the lens zooms. For example, a zoom lens with a maximum aperture of f/4-5.6 has a maximum aperture of f/4 at its widest focal length and a maximum aperture of f/5.6 at its longest zoom setting.
A typical kit zoom has a maximum aperture of f/4.5 and gets darker as you zoom – closing down to around f/5.6 at the full telephoto setting.
Pro lenses have large maximum apertures – like f/2 and f/2.8 – so they can let in more light. And pro zooms don’t get slower as you zoom either. They also give you a brighter view in the viewfinder (all cameras hold the lens open at the maximum aperture until you press the shutter button – then they stop down).

Megabyte:  1024 kB  (See also: Byte; Gigabyte; Terabyte).

Megapixel: Common unit of measurement for digital camera resolution. Based on binary multiples: 2 to the power of 10 is 1024. Hence, 1024 X 1024 = 1,048,576 pixels. (See also: Pixel; Resolution).

Memory: Any storage system used to keep digital data.

Memory Card: Solid-state device, based on non-volatile/memory technology, on which a digital camera stores its  captured images. (See also: CompactFlash; Flash Memory; SD; SDHC; SXHC).

Metadata: Literally, “data about data”, contained within the image file itself (e.g. MP3, JPEG, or TIFF). Contained herein are the “Tags”.

Metering Mode: Metering mode refers to the method the camera uses to determine the exposure of the scene. Different metering modes include Evaluative Metering (also known as matrix metering), Centre-Weighted Metering and Spot Metering.

Micro Lens: Terminology used by Nikon for their close-up, macro lenses, e.g. Micro Nikkor: a lens used for close-up photography.

Micro Four Thirds (MFT): The Micro Four Thirds system describes a common standard for the design of cameras and lenses. Based on the Four Thirds system, Micro Four Thirds allows for the design of smaller, mirrorless, interchangeable-lens cameras. Current manufacturers of Micro Four Thirds products include Olympus (cameras and lenses), Panasonic (cameras and lenses) and Leica (lenses only). While Micro Four Thirds cameras use the same size sensor as Four Thirds cameras (17.3 x 13mm) the lens mount and the distance between the lens and sensor are smaller. MFT cameras are designed to be used with MFT lenses, however they can also be used with Four Thirds lenses with an adapter.

Midtone: Situated between the darkest tone (Black), and the brightest tone (White). For a 24 bit colour image, this occurs when Red = Green = Blue = 128.

Monopod: A camera support with a single leg. Less stable than a tripod, but lighter and more flexible. Widely used by sport photographers. Also referred to as a unipod. See also: Tripod.

Multi-Zone Metering: Multi-Zone Metering (also known as evaluative, matrix, honeycomb) is the default metering system for most modern cameras. Set to Multi-Zone metering, the camera effectively splits the scene into sections and takes exposure readings at multiple points across the image (over 1000 in some cases). An algorithm then weighs the importance of the different sections according to variables such as subject distance and position to produce an overall recommended exposure. This is the most sophisticated of the three main metering modes (multi-zone, centre-weighted and spot) and it usually produces the best results. That said, it can be tricked in some high-contrast situations.

Negative: A photo, which is created upon photosensitive material in which light areas are seen as shadow and vice-versa. To assist the printing process, there is usually an orange mask on the film base. Used to produce larger prints.

Night Mode: Pre-programmed exposure mode built into cameras to help photographers with taking photos at night. Works by allowing camera to use flash with a longer shutter time, thereby allowing for the generally poorly illumined background to show. Also known as "Dragging the shutter".

Noise: An unwanted (though inevitable) fact of life in electronic systems, noise is a random variation in the signal. In digital imaging, noise is more obvious when using the highest ISO settings and in areas which receive the lowest exposure (e.g. in the shadow regions).  Such underexposed regions will have the lowest Signal To Noise ratio, which is the all-important factor. In post-processing, there exist various techniques, and/or built-in Filters, and/or third-party Plug-In Filters, for tackling and dealing with the ever-present problem of noise. Better (ie. more expensive cameras), with large-sized sensels (and hence greater light-gathering capabilities – a bigger “bucket”, if you will) produce less noise to begin with, and manage noise better, and yield acceptable results, even at high ISOs.

OLED (Organic Light Emitting Diode): Organic compounds, sandwiched between electrodes, emit light when a voltage is applied. This technology has advantages over LEDs or LCDs, however, it is yet to achieve maturity. (See also: LED).

Opaque: Does not allow the passage of light.

Overexposure: When a sensor or film frame gets too much light it is said to be overexposed. An overexposed image will normally lack detail in the highlights. That is, the brightest parts of the image will appear all white.

Panning: Moving the camera to track a moving object. Often resulta in an effect where the subject is clearly-defined and the background is blurred background to create a strong sense of movement.

Panorama: An image with a wide view, scenic.

Perspective: The impression of 3-D space in a 2-D photo or image. Determined by the inclusion of lines converging towards a vanishing point, different object sizes and different focus. Also refers to the relationship of foreground to background, as determined by the position of the picture-taking lens relative to the subject. Note: Changing the focal length of a lens does not change the perspective (if the camera remains in the same position).

Phase Detect AF (autofocus): Phase Detect AF is the primary autofocus system used in SLR cameras. In this system, the light coming through the lens (TTL) is split by a small prism and sent to sensors. The camera's brain compares the signals from different sensors and adjusts the lens until the signals match up - come into phase. It is an automatic, digital version of the old mechanical rangefinder process. The advantage of Phase Detect AF is that it is fast; because it can tell whether the current focus point is behind or in front of the desired focus point, so it starts to turn the lens in the right direction immediately.

Photograph: An image traditionally recorded on light sensitive material. From the Greek words "Phos" and "Graphikos" meaning – literally – "To write with light".

Photokina: Biannual photography show held in Cologne, Germany.

Photoshop: A professional image-editing program produced by Adobe Systems, serving the needs of a wide variety of users, e.g. Photographers, Pre-press, Web designers. The default industry-standard.

Pincushion Distortion: A form of distortion (made very obvious when taking architectural images), exhibited by lenses in the telephoto range, particularly seen in pictures taken with cheap, wide-range zoom lenses.

Pixel: A contraction of Picture Element, the most fundamental entity which a digital image is comprised of. Possibly the most widely used term in digital imaging, yet also the most misunderstood! A pixel is a mathematical construct, and does not exist "in reality".

Polarised Light: The optical phenomenon of light waves only vibrating in one plane, rather than the multi-directional vibrations of normal rays.

Polarising Filter: Removes polarised light (in the same way that sunglasses function). This is the single most versatile and useful filter, with a wide variety of functions. These include: darkening of blue sky; increase in colour saturation; removal of reflections; revealing stress patterns in transparent materials; neutral density filter (eg. useful to increase exposure times); haze reduction; glare reduction.

Polaroid: Instant film.

Portrait: Telephoto lenses are used to take portraits of people, due to the more flattering perspective resulting from the flattening of features throughout the image plane. A dedicated portrait lens ensures high quality, e.g. pleasant-looking out-of-focus areas in the background.

Positive: Photographic image in which light areas correspond to highlights and dark areas with shadow. AKA Transparency, or Reversal; when mounted, a “Slide”.

Posterisation: Andy Warhol's 1960s Pop Art style might still be popular, but generally we want to see smooth, continuous tones in our photos, without big, sudden jumps. This "posterisation", or "banding" often occurs when large editing changes (e.g. Levels, Curves, Brightness or Contrast) have been made to an image with insufficient bit depth.

Pre-visualisation: Term coined by Ansel Adams. Using the “mind’s eye” to imagine a completed print before taking the picture.

Professional: Person deriving the majority of their income from the pursuit of photography.

PSD (Photoshop Document): The native, lossless document format, used by Adobe Photoshop, which supports the use of Layers.

Purple Fringing: A phenomenon, attributable to a wide variety of causes, whereby a faint, purple “glow” is visible, particularly around edges in high contrast lighting situations, eg. a subject photographed against a bright sky.

RAM: Random Access Memory. Memory in the computer which needs a constant flow of electricity to store contents. All data in RAM is lost when power is switched off.

Raster: A representation of an image based on information (pixels) placed at the intersection points of a rectangular grid. Attempting to display such an image at greater than its resolution limits will result in the dreaded “Jaggies” (jagged edges, or pixelation). (See also: Pixel; Bitmap/Bit-Mapped Image).

RAW: A file format for the complete and unprocessed (hence, "raw") data from the camera’s imaging sensor and image processing system. Requires interpretation and conversion. Will look very dark, as a Gamma correction curve is yet to be applied.  RAW files are usually 10 or 12 or 14 bits, and therefore contain more visual information than the eight bits of a "ready to use" JPEG file. Unfortunately, there exist many different, proprietary, RAW formats, even within the same manufacturer (e.g. Canon's CRW or CR2, Nikon's NEF, Olympus’ .ORF; Pentax’s .PEF). Compression (usually lossless) is applied to reduce file sizes.

Rear Curtain Sync: Flash is fired when the second (rear) curtain commences its travel (thereby concluding the exposure).

Reciprocal Rule: In the days of film photography, a useful Rule Of Thumb was that a photographer could safely hand-hold the camera using a shutter speed which was at least as fast as the reciprocal of the lens’ focal length, e.g. 1/250th of a second, for a 200 mm lens (or, zoom setting). With digital cameras, high-resolution sensors of a physical size much smaller than the image frame of a 35mm format camera mean that the same amount of movement (due to the inability to hold the camera perfectly steady) will affect a large number of sensor elements. Hence, it is necessary to use an even faster shutter speed again, e.g. twice (or more) as fast as the old Reciprocal “Rule” would suggest.

Reflector: Any material (e.g. white foam core, silver foil, gold foil) used to reflect light onto an object to improve exposure balance and reduce extremes of contrast (eg. Shadows & highlights), such as when taking a portrait.

Reflex Lens: A camera lens achieving a long focal length in a compact physical size by using mirrors to reflect the light multiple times within the lens. Also known as catadioptric.

Render: To reproduce a computer image by calculating and defining size and tone.

Resampling: Various mathematically-based options are available in Image Editing programs to allow you to either “up-sample” (interpolate, and “guess” what pixels would have been there if the resolution of the starting image had been higher) or, conversely, to “down-sample” (discard the appropriate number of pixels to create an image of a specified (smaller) number of pixels).

Resizing: For an image of a given resolution (number of pixels), to resize means to allocate those pixels at a different resolution, e.g. a 1200 X 1800 pixel image would be able to be printed at 4” X 6” at 300 Dots Per Inch, or 6” X 9” at 200 Dots Per Inch. (See also: DPI; Pixel; Resolution).

Resolution: The ability of photographic equipment to discern fine details from one another, e.g. closely-spaced pairs of lines (as used in Test Charts).  When applied to digital imaging, as a general term, resolution might best be defined as "the number of pixels per linear unit of measure". For instance, Australian Photography -- and most other magazines -- stipulates a minimum resolution of 300ppi (pixels per inch), at the final reproduced size, for any images supplied. A digicam's resolution is ultimately set not only by the number of pixels available on the imaging sensor, but also by the resolving power of the lens, the design of the antialiasing/infrared filter pack, de-mosaicing algorithms, signal processing (e.g. sharpening, contrast, etc.).
Also, refers to the total number of pixels in an image.

Retouching: In traditional photography, altering a print after development to create an effect, or remove flaws and blemishes. With digital imaging, all of this, and a lot more, can be achieved when post-processing using an image editing program.

Saturation: The percentage or intensity of colour. Low saturation is dull and pastel-like with lots of white light; saturated colour is intense and contains no white.

Scattering: Light and radiation is reflected off small particles in the air creating an effect of light diffusion.

SD: A memory card, of dimensions 32 X 24 mm. Most popular of the memory card Standards, which define the physical, electrical and interconnection requirements. (See also: CompactFlash; Flash Memory; Memory Card; SDHC and SDXC).

SDHC (Secure Digital High Capacity): A very popular form factor for memory cards using flash memory technology, the SDHC Standard also specifies all of the numerous interface and electronic requirements for compatibility. SDHC has a maximum capacity of 32 gigabytes (GB).

SDXC (eXtended-Capacity): A later development of the SD Standard, allowing for capacities beyond the 32 gigabyte (GB) limit of SDHC and up to 2 terrabytes (TB).

Self-Timer: Used to delay the shutter after release, usually between 5 and 10 seconds. Allows photographer to be in the frame or to limit chance of shaking when pressing shutter. Also useful for minimising vibrations when taking long-exposure photos, eg. with a shutter speed of slower than approximately 1/30s.

Sensel (Sensor Element): A sensel is a photo-receptor (light-sensitive site) on a silicon-based digital imaging sensor. Sensels are spaced on a regular, fixed-pitch grid.

Sensitivity: In digital photography, sensitivity usually refers to the sensor's responsiveness to light. Sensitivity can be adjusted using the ISO setting. As ISO increases, the sensor becomes more sensitive (also referred to as “faster”) and requires less light to make an exposure. Conversely, as ISO decreases the sensor becomes less sensitive, and requires more light to make an exposure.
As sensitivity increases, so too does image Noise (see Noise). If you’re shooting outdoors in good light, it’s generally best to use your camera’s lowest ISO setting – 50, 100 or 200 ISO. As the light dims, a higher ISO will help you keep the shutter speed up so you don't need a tripod or artificial light. Similarly, if you’re shooting sport a higher ISO can help you obtain a faster shutter speed to freeze the action. (See also: ISO; Noise).

Sensor: In a digital camera the sensor converts light into an electronic signal. A silicon-based imaging sensor has light-sensitive sites, called "sensels" (sensor elements), arranged on a regular grid. There are two main types of sensor, CCD (Charge-Coupled Device) and CMOS (Complementary Metal Oxide Semiconductor). (See also: CCD, CMOS, Anti-aliasing (Low-Pass) Filter; Pixel; Sensel). 

Sensor Size: Digital camera sensors come in various sizes. Bigger sensors allow designers to not only add more pixels, but also give each pixel more space. That means less interference from neighbouring pixels and less image noise. That’s most noticeable in low light conditions when high ISO settings are often required. Professional DSLRs with their big sensors can capture high-quality images at high ISOs, such as 3200 and above, with very little visible image noise. At the other end of the scale, cheap compact cameras – with their fingernail-sized sensors –can struggle with noise at ISO settings as low as 200 or 400. Sensor size also affects depth-of-field, with bigger sensors making it easier to create short depth-of-field effects. Further, the size of the sensor influences field of view, with small sensors having a larger crop factor and a greater magnifying effect on lenses. In the DSLR space, there are three commonly used sensor sizes: Full Frame (36 x 24mm), APS-C (around 23.6 x 15.8mm for Nikon, Sony and Pentax models, or around 22.3mm x 14.9mm for Canon models) and Four Thirds (17.3 x 13mm). There are several large-sensor compact cameras but most have tiny sensors, between around 4 x 3mm and 6.2 x 4.6mm. (See also: APS-C; Four Thirds System; Noise).

Sepia: Reddish-brown pigment common to many aged, monochrome photographs.

Sharpening: Both the term and the practice are used frequently, defined rarely, and misunderstood commonly. Sharpening is an essential step for all digital images, whether derived from digital capture, or scanned from film. No new detail is added to an image by sharpening. Instead, using various possible algorithms, edges in an image are identified and the contrast between light and dark pixels is bumped up, resulting in the visual impression that sharpness has been increased. Unsharp Masking (USM) is perhaps the most widely known, and used, technique; its name derives from the traditional darkroom technique of using a sandwiched mask of an inverse, blurred, low density and low contrast image of the original. A perceived increase in sharpness results, since the edges (at which transitions in tone occur) become more obvious. In image editing programs, the USM filter allows for user control of three parameters: Amount, Radius and Threshold, which act on the pixels in the image.

Shutter: A barrier inside a camera that controls the time light is allowed to pass and reach the imaging sensor, or film.

Shutter Priority (S, Tv): Shutter Priority is a semi-manual (or semi-automatic) camera setting that allows the user to select the shutter while the camera selects the required aperture to balance the exposure. Use shutter priority if you want to control the way your camera records movement. If you want to show a moving object as a blur, use a tripod and choose a slow shutter speed. If you need to freeze the action, choose a fast shutter speed.
Get used to checking the display as you adjust the shutter speed. If you see the aperture setting flashing, it may be an indication that the shutter speed setting you’ve chosen is too fast or slow – meaning the aperture cannot produce a balanced exposure. (See also: Aperture Priority; Exposure Triangle; ISO; Shutter Speed;).

Shutter Speed: The amount of time, measured in seconds, that a camera’s shutter remains open. The longer the shutter remains open, the more light hits the sensor or film plane. Apart from its role in controlling the amount of light in the exposure, shutter speed also affects the way the camera renders movement. A fast-moving sports car photographed with a slow shutter speed (for example 1/2 or 1 second) will appear blurred as it speeds past the camera. Use a fast shutter speed, say 1/1000s, and the same car will appear as still as if it were parked on the side of the road.
Camera shake can cause problems at slow shutter speeds, making images appear soft. And the problem becomes more pronounced with telephoto lenses and big zooms.
There are a few solutions. One option is to put the camera on a stable platform like a tripod or monopod. Another is to increase the shutter speed. A rule of thumb for hand-held shooting says the shutter speed should be at least “1/focal length”. For example, if you are zoomed to an equivalent focal length of 250mm, the minimum shutter speed should be 1/250s to ensure sharp images.
Fortunately, most DSLRs these days also offer some form of image stabilisation (IS, OIS, VR) which allows users to shoot sharp hand-held shots at slower shutter speeds -- around two to four steps slower than normal. This gives you a bit of extra help, though there are still plenty of situations where you’ll find a good tripod handy.
Sports photographers mostly use fast shutter speeds, which allow them to freeze fast motion. Because fast shutter speeds don’t let in much light, they need to compensate by using lenses with wide apertures. Big apertures translate to big lenses, which accounts for the size of the lenses you see at big sporting events.

SLR: Single Lens Reflex, a system whereby the mirror (which is required to deflect light from the lens to the viewfinder), is rapidly moved out of the way at the moment of exposure so that the light can instead be sent directly to the imaging sensor or film.

Soft Focus: Image that uses reduced contrast in detail areas.

Spectrum: Range of electromagnetic waves of various frequencies. Visible light spans from long wavelength red  (750 nm) to green (550 nm) to short wavelength violet (400 nm). Commonly represented on a diagram showing colours of varying wavelengths visible to human eyes, from deep reds, to oranges, yellows, greens, blues, indigos, and finally violets.

Spot Metering: Spot metering is the most basic of the three metering modes (matrix, centre-weighted and spot), taking an exposure reading from a very small circle at the centre of the image. That can be very useful when you’re photographing a high contrast scene and you need to make sure a particular part of the scene is properly exposed. For example, if you’re shooting a backlit portrait it can be useful to take a spot reading off the skin.

sRGB: Standard Red, Green, Blue; a standard developed jointly by Microsoft and Hewlett Packard. When these three primary colours of the additive colour model are mixed in equal proportion, white light results. This model suits electronic systems well (e.g. computer monitors, or digital imaging sensors). In 24 bit RGB, 2 to the power of 8 = 256 possible levels are available for each of the three channels. At the midway point, where R = 128, G = 128 and B = 128, the resultant colour is medium grey. At the extreme ends, for R, G, B = 0, 0, 0: we have black; for R, G, B = 256, 256, 256: we get brightest white. (See also: Colour Space).

Telephoto Lens: A lens with a focal length longer than what it is "normal" or "standard" for a given format of system. For the 35mm system (24 mm X 36 mm frame size), the diagonal measurement is 43.2 mm. Thus, 50 mm is the normal lens. Hence, a telephoto lens is one in the range above about 70 mm.

Terabyte (TB): 1024 GB (= 1024 X 1024 MB). (See also: Bit; Byte; Gigabyte; Megabyte).

Thermal Imaging: False-colour images (obtained by using an image sensor sensitive to long wavelength infrared) that registers heat emitted by the subject.

TIFF (Tagged Image File Format): Developed from the early days of desktop publishing. Well-suited for photographs, TIFF allows for both lossless compressed, and uncompressed, forms. Uncompressed, a TIFF file's size is three times the resolution of the image (since there are three channels, viz: Red, Green And Blue). (See also: Bitmap; JPEG; Raster).

Tripod: A supporting stand with three “legs” used to hold a camera steady.

TTL: Through The Lens (eg. TTL metering, for Exposure and/or Flash Exposure determinations).

Underexposure: When a sensor or film frame doesn’t get enough light it is said to be underexposed. An underexposed image will often lack detail in the shadows. That is, the darkest parts of the image will appear all black.

USB (Universal Serial Bus): Due to its cheapness and ease of implementation, this serial communications Standard and Protocol has become by far the most popular. A commonly-used means of attaching various peripherals to a computer, e.g. external Hard Disk Drives, Flash (memory) drives, keyboard, mouse, printer, etc.

Vector: Programs such as Adobe Illustrator deal with vector-based images, which allow for scaling an image as big as you wish, since the components are being described as a mathematical function e.g. a section of a circle, with a radius of whatever you specify.

Vibration Reduction: VR is the lens-based image stabilisation system implemented by Nikon. (See also: Image Stabilisation).

Viewfinder: A camera viewfinder provides an optical or digital representation of the scene in front of the camera, allowing the photographer to compose and focus. The optical viewfinders found on compact cameras are separate from the lens and are only able to provide an approximation of what the lens “sees”. Parallax errors makes these systems less accurate as the camera and subject become closer. The viewfinders used in SLR cameras allow the photographer to see through the lens and, as a result, are much more useful. (See also: Electronic Viewfinder).

Vignette: In a photographic and artistic sense, a “vignette” is a darkening of the image at the corners of the frame. Some poor quality lenses produce strong vignetting, with the effect becoming more noticeable at the maximum aperture. The effect is often introduced in post-processing, as an artistic device to focus viewers’ attention on the central area of an image.

White Balance: The colour of light varies enormously throughout the cycle of the day, from very cool (e.g. the blue cast of shade), to "daylight" (e.g. at noon), to very warm (e.g. the fiery reds of a dramatic sunset). Our eyes and brain do a remarkable job in, as it were, "filtering out" these huge variations. Film, and digital imaging sensors, will -- in general -- accurately record these colour casts. Most digicams are reasonably good at automatically setting the correct white balance (AWB), so that white looks white, under the prevailing illumination.

Wide-Angle Lens: A lens that allows for a large area to be covered in the shot. It has a shorter focal length (relative to the “normal” or “standard” lens for that format), and a hence a  correspondingly greater field of view. (See also: Telephoto Lens). Using such a lens to “fill the frame” when taking a portrait, for example, will result in the familiar and very distorted faces (e.g. overly large nose in proportion to the rest of the face.

Workflow: The sequence of steps used to manage digital images from the time of capture through to post processing (RAW processing and editing),  and backing up.
 a digital image to achieve an end result, e.g. getting the image(s) ready for display on the Web, or to produce a Fine Art print.

Zoom Lens: A type of lens where the focal length can be varied by the user over a range specific to that lens, e.g. 17-55mm (wide-angle), 70-200mm (telephoto), 200-400mm (super-telephoto).