The ability to take wildlife images in low-light is something that is very important for me.  Despite the fact that I do much of my shooting outdoors on a continent with lots of sunshine, I have no control over my subjects, and they are often at their most interesting when the sun isn’t shining.
Rather than put my camera away, I try to keep shooting at these times, and I am often suprized by the results.
These days it is getting easier and easier to shoot successfully in low-light.  Using lenses that have a large maximum aperture like f/2.8 or f/4.0 is a big help to begin with.
However, even this is getting less important as each new generation of digital cameras becomes more and more capable of shooting at higher sensitivity settings (Iso) without compromising image quality too much.
What this means is that one is able to shoot at very high iso settings to make sure that the shutter speed is still fast enough to capture a sharp image, regardless of what lens is being used.
Canon 1Dmk4, Canon 300f28 IS, 1/30sec at f/4.0, Iso 800, handheld.
The lion image above was taken at Malamala, west of the Greater Kruger.  I took the image very, very early on an overcast morning.  There was hardly any light at all to work with.  As soon as we stopped our vehicle and began shooting, I noted that a wide-open aperture setting of f/2.8, combined with an Iso setting of 1600, gave me shutter speeds in the 1/160s to 1/250sec.  I know from experience that those speeds are barely fast enough to freeze any movement, whether it be my own camera shake, or movement of the lions.  I began shooting, and when I was happy that I had a few shots that I might keep, I changed strategy.
The lion in the frame was not moving around much, and he had a full belly, so wasn’t too likely to leap up anytime soon.
I already had a portrait picture of him that I was happy with, but I wanted to give myself a chance at getting the best quality portrait of him that I could.
With this in mind, I lowered the iso setting on my camera to iso 800, at which setting I am completely happy with the image quality of the Canon 1Dmk4.  I also closed down the aperture a little on my lens, to increase the depth of field.  This would mean that I might get his nose, his eyes and his ears all in focus, rather than just one of those three which was the situation at a bigger aperture setting of f/2.8.
These two changes improved the quality of the picture I could take, but also slowed the shutter speed down to 1/30s, which is much too slow for handholding a long lens.
However, my lens has image stabilization, our vehicle was completely still, I rested the lens on the side of the vehicle, and most importantly, the lion never moved during at least one of the shots that I took.
As soon as I had taken a few images and I thought I had a sharp one, I set the camera back to f2.8 and iso 1600, to give me a better chance at getting the lion sharp should he or his buddy who was nearby have begun moving about.
I have trained myself to shoot in this way.  In low light, I set the camera to a fast enough shutter speed by opening the aperture wide, and upping the iso setting as far as I need to, whether that be iso 3200 or iso 6400.  However, if I see that the subject is keeping still for any length of time, I will see if I can lower the iso, and take a chance on getting a sharp shot at the consequently lower shutter speed, just to maximize the image quality that comes with lower iso settings.  When it works out, I have a ‘cleaner’ image that needs less processing work and I can do more with cleaner images.
What is very important is that I have trained myself to be able to carry out those changes without lifting my eye away from the viewfinder, and I am able to do it very quickly.  The last thing you want to do is miss out on action because your shutter speed is too slow!
This picture is only a resting lion, but I am a big admirer of lions, and this was a particularly impressive specimen.  He had the most unusual eye colour I have seen in a lion.
And, just like my camera, he was responding to the low light conditions with his own wide-eyed, dilated pupil look!