Focusing in Low Light Conditions
Black bear at dusk in heavy vegetation obscuring much of the available light
Getting images in low light is challenging. Even more so if your subject is dark against a dark background. Your camera type and your lens selection are as important as the settings in having success in these conditions.
There was a prevailing belief that cameras with high megapixels like my CanonR5 do not perform as well in low light as cameras with fewer megapixels like the new R3. It is believed that larger resolution sensors, with therefore smaller pixels, are worse than lower resolution sensors, with therefore larger pixels, in low light. Some argue it is an issue and others state with the new mirrorless cameras you cannot discern a noticeable difference. What is my advice on this debate? Personally, I would not be running out buying a new camera to replace my R5 unless I planned on making a living photographing black wolves or black bears deep in the forest. There are software programs to reduce noise such as Topaz DeNoise, but even with these you need to be careful, Images with high noise inevitably lose sharpness when excessive denoising is applied. Sharpness is important in producing strong images. Lenses in my opinion are way more important for handling low light than any other feature.
Coastal brown bear photographed before sunrise with a 400mm f2.8 lens which was able to gather enough light
The lower the f value the more light the lens can gather. Lower means better sharpness. Prime lenses are expensive, but there is a reason they are expensive. It's because it requires premium large glass to get low f values. I believe the 400mm f2.8 is the best wildlife lens for low light. It will give you reach, gathers a ton of light, and still be fairly fast at autofocusing. One of the things that happen as you lose light is the camera struggles to autofocus, the more light the lens can gather for the camera's sensor the quicker it will autofocus, Sharpness requires pinpoint accurate focusing. A camera struggling to focus will not get consistently give sharp images. Do not use extenders as they raise the f value considerably and if you are using a zoom lens try to get close to your subject and zoom out. The more you zoom out the more the f value will lower.
Fox kit photographed in low light, shutter speed 1/160 ISO6400
Shutter speed also helps determine the amount of light that will reach the camera's sensor. The slower the shutter speed the more light it will gather. Low light photography is exceedingly difficult if you have a moving object since you cannot use high shutter speeds without having a very massively underexposed image. Hence, low-light photography is best suited to portraits of your wildlife subjects. Using a tripod helps as well in low light conditions as it minimizes shake and the impact it will have on an image's sharpness Loss of sharpness is considerably more noticeable every time you lower the shutter speed.
Porcupine in low light after sunset, ISO 64000, risky to shoot in such high ISO but every now and then it works
If you are using a DSLR camera it is best to set your camera for autoISO. Let the camera keep increasing the ISO to compensate for the loss of light until at some point in time the ISO gets too high (for me more than 10,000) then I generally stop shooting but on occasion depending on the species and behaviour I may continue and push the ISO, If you shoot with a mirrorless you want to shoot on manual and adjust your ISO as needed based on your histogram. With mirrorless cameras, even the slightest change in light, like moving out of shadow will cause large swings in your ISO.
Each camera handles noise and low light differently. Some cameras produce minimal noise even with a relatively high ISO. Settings and lens selection all are important factors when shooting in low light. To be successful in low light conditions requires you to know your gear and be willing to experiment.