Hi Arlie,
Photographing rodeos is not easy, and it is made even more difficult when you are working at night time. What makes the task a lot easier though, is the ability of new cameras to literally see in the dark. Back when I photographed my first rodeo back in the early 1990s, 1600 ISO was considered fast film; nowadays cameras are easily coping at 6400 ISO, and what is even more incredible is that the newest cameras on the market now let you set the ISO to 100,000 and even 400,000. What this means is that nowadays, photographers can make photos in any conditions.
However, your question "What lens should I be using to photograph rodeos at night?" is still a good one.
Here is a fun fact to remember – while cameras now have these magical ISO settings, the actual autofocus sensors on cameras still require a reasonable amount of light to focus, particularly if the subject you are photographing is moving.
Now this is where the speed, or what is also called the maximum aperture of the lens comes into play. Most amateur photographers when they buy their first camera, get a kit lens, and these lenses usually only have a maximum aperture of f4.5 or f5.6. Good zoom lenses however, usually have a maximum aperture of f2.8 through their zoom range. What this means is that the lens will actually let in four times more light than an f5.6 lens.
This increase in aperture does a few things. Because it is letting more light into the camera, it makes it easier for the focusing system to do its job. More light also means that you have the choice of using faster shutter speeds, or lower ISO settings. And then there is the advantage that every photographer loves – the wider the maximum aperture, the more out of focus the background can become at maximum apertures.
My advice to you, if you are looking at doing more rodeo photography at night, is to invest in a 70-200mm f2.8 lens. They are expensive, but they are a standard investment for anyone who is seriously into sport, portraiture, wedding photography, reportage, wildlife – anything really. What is also worth considering is whether you are using an APS-C type sensor or a full-frame (35mm) sensor. With the APS-C sensor you will get more reach, although you will also get less out of focus effect. Whatever the case though a 70-200mm f2.8 lens is a good investment.
I hope this is a help – keep up the good work!
Cheers,
Anthony