Photo Tip of the Week: How To Correct Colour Casts
Overly saturated reds (lobster skin) are a fairly common problem in digital photography. Fortunately it's easily fixed if you know what you're doing.
If you’ve ever taken photos with your digital camera’s colour balance set to Auto, only to
find a strange colour tint in your photos, don’t be disheartened. This and a
range of other colour issues can be dealt with easily in post-production – if
you know how. Peter Wilson-Jones explains.
In an earlier era, choosing the correct film to match your intended light source was part and parcel of the photographic process. Not that there were many choices to make – daylight or tungsten-balanced film were the main contenders – ‘daylight’ for outdoors and ‘tungsten-balanced’ for incandescent/indoor-type lighting.
In those days, modifying the ‘temperature’ of an image to enhance the vibrance, warmth or colour (taken on a cloudy day, perhaps) would invariably mean adding warming, cooling or other types of filters directly to the lens, or alternatively, manipulating the final print in the darkroom by dialling the colour wheels on the enlarger.
Modern DSLRs – and in fact many consumer digital cameras – have colour balance controls built in. The feature typically offers a default auto setting, and a selection of others which encompass the more common lighting situations you might encounter – ‘indoor’, ‘cloudy’ and so on. For most of us, this feature is often overlooked and then is rarely changed from ‘auto’. One reason for this could be that when using your camera on auto colour balance, in most instances the camera gets it right – or at least pretty close. But there will be situations where it gets confused.
A scenario where there are multiple light sources (daylight together with indoor) will certainly test your camera’s ability. Different cameras and/or brands will also have their own particular colour bias.
For example, both my Canon cameras can apply a slight red cast to some of the images which they capture. For those photographers who shoot in RAW mode this isn’t necessarily a problem. Those familiar with RAW editing will know it’s very easy to correct a colour balance, or colour cast problem, before saving from the RAW editor to a working file. Dealing with exposure mis-demeanours on a JPEG image is a slightly different kettle of fish, but a degree of success can be achieved if you know how. Here I will demonstrate some powerful yet simple-to-use Photoshop tools and techniques, which I use on a daily basis for getting the colour right.
the colour balance on ‘auto’ in unusual lighting conditions can result in
unpredictable and inconsistent results. A good example of this might be when
photographing in an exhibition space or place where the lighting is a mixture
of daylight (windows, skylights, etc) and one of the three most common lighting
sources. These include incandescent, fluorescent and High Intensity Discharge
(HID) systems like Mercury/Sodium Vapour and Metal Halide, often used in
situations such as night lighting at sports grounds. In one of these situations
you might find your camera sometimes gets the colour balance wrong, calibrating
the colour temperature to the unintended source and resulting in an image with
a colour cast.
Fortunately, Photoshop software has an expansive range of photo filters [Image > Adjustments > Photo Filter] which can reproduce (and more) the effects created by traditional lens-based filters. This veritable arsenal includes all the classic orange/blue tungsten/daylight correction filters plus numerous preset colours and a palette option which spans the entire Photoshop colour spectrum. In addition to mimicking the effect of traditional glass filters, Photoshop photo filters are user-adjustable from 0-100 per cent in intensity. Using these filters is as simple as selecting the appropriate colour, scaling the intensity and then using the ‘Preview’ check box to compare the effect before clicking ‘OK’.
The example below illustrates how a photo filter can be employed to correct the cool bluish cast encountered when shooting on an overcast day (this also applies to many other daylight situations). In this case it was possible to correct the balance simply by using just this tool.
PHOTO FILTERS: A Photoshop photo filter is a simple way to add some warmth to a grey, overcast photo. Try experimenting with different filter colours – they are very useful for correcting white balance problems in your images.
Colour tool [Image > Adjustments > Selective Colour] is very handy for
reducing and enhancing specific colours in your image. It also has an
exceptional ability to remove colour casts from images, however it’s a little
trickier to control as it requires manual inputs. I find it works well with the
Hue/Saturation filter (described in detail later). Within the ‘Selective
Colour’ dialogue window, a pull-down menu allows for the choice of specific
colour ranges in an image. The range of particular interest for removing casts
is ‘Neutrals’. With neutrals selected, cyan magenta, yellow and black can be
added or removed simply by adjusting the appropriate sliders. You might find a
bit of trial and error works best with this tool. Using the ‘Preview’ check box
and having an accurate, calibrated monitor also helps.
The example below shows how to remove a cast from an image using the Selective Colour tool.
SELECTIVE COLOUR: If you’re shooting in mixed light and your camera’s auto colour balance gets it wrong, then by choosing “neutrals’ in the Selective Colour dialogue you can modify the individual channels to achieve a more desirable result.
tool, like Curves, has been a part of Photoshop for many years, and its
function hasn’t really changed in that time. I use it as an alternative to
curves and find it a lot more user-friendly. For the novice it’s a good way to
easily achieve a similar result to the Curves tool. It’s important for your
image to be in RGB colour mode when using the Levels tool.
In its default state, by using any of the three slider controls underneath the histogram, the Levels adjustment [Image > Adjustments > Levels] only adjusts the exposure. The black triangle/slider on the left will affect the shadow areas, the middle slider affects the mid tones and so on.
However, the Levels tool has another more useful function. By using the pull-down menu in the Levels dialogue and choosing the individual RGB colour channels, one by one, colour casts and colour balance can be adjusted and corrected. In each of the three channels usually only the shadow and highlight sliders need to be moved towards the centre - towards the base of ‘the mountain’ - the two endpoints where the histogram initially rises from the baseline. The example below mainly required shadow slider adjustment in the channels to boost the vibrance, colour and contrast.
LEVELS: By adjusting the individual RGB channels in the Levels dialogue, you can correct casts while adding a little extra contrast to your shots. Adjustments should be made in small increments because the result of Levels changes can be quite pronounced.
Hue/Saturation tool has a couple of important uses for a digital photographer.
It can be applied as a way to modify specific areas of over-saturation (ruddy,
lobster-like skin tones are a good example) or used in moderate amounts to
restore the intensity of colour, especially if mixing some of these post-production
techniques has left your image looking a little flat. To restore overly red
skin to a more natural state use the pull-down menu in the Hue/ Saturation
dialogue and select ‘Reds’.
Then adjust the saturation slider to reduce the colour. To boost the saturation you can either select individual colours as described above or more simply set the pull-down menu to ‘Master’ and use the Saturation slider accordingly. If you’ve been working a lot on an especially difficult photo which is looking a little washed out after you’ve used a combination of the above described filter options, a little boost to the saturation can go a long way towards producing an acceptable final print. For those using Photoshop CS5, the Vibrance Tool [Image > Adjust > Vibrance] is a more refined version of the Hue/Saturation tool. Example 4 shows the beneficial effect of the Hue/Sat tool on overly red, saturated skin. By choosing ‘Reds” from the pull-down menu it is possible to desaturate just those areas.
HUE/SATURATION: Overly saturated reds (lobster skin) are a fairly common problem. Selecting ‘Reds’ in the Hue/ Saturation dialogue window you can quickly and easily de-saturate ‘problem’ skin. It’s worth exploring this tool’s benefit for boosting or de-saturating other specific colours too.
Each of the
filters, techniques and settings described here can be used in combination to
achieve a desirable result. Keep in mind that this article isn’t exhaustive,
and there are other methods for fixing colour issues using Photoshop. Every
imperfect photo you work on will present a different set of challenges that
require a different creative approach to achieve a satisfactory outcome.
Experimentation with various options is always beneficial in learning new
Article first published in Australian Photography, March 2012.