I am in the process of deciding which prime to get, for the time being, to use for wildlife. I have my mind set on the 300 f2.8 IS II, but I was wondering: What's the difference, practical and theoretical, between this lens and, say, the 400 f2.8, the 500 f4 , the 600 f4? The obvious answer would be the different focal lengths and, of course, price tag. But is there anything else that is significant, as far as the differences between them are concerned?!
The difference between the 300/2.8 L II and the 600/4 L II is massive. Both are phenomenal lenses, don't get me wrong, but if/when you need extra reach, the 600/4 can be paired with a 2x TC for 1200mm of bliss. Keep in mind that subject size in frame is the square of the difference in focal length. So, a 600mm lens will result in the subject being four times larger in frame. With that TC, the subject is 16 times larger in frame.
At the very least, 600mm is often the difference between needing to crop and not. With 1200mm, it is sometimes the difference between scaring off your subject, and getting the perfect shot. I have recently been trying to photograph coyotes hunting prairie dogs. At 600mm, you have to be pretty close to frame the shot nicely, and get good detail. Coyotes are always on the move, and they KNOW when they are being followed. They will even use tactics like splitting up the group in order to lead the photographer astray, so they can hunt in peace. At 1200mm, you can stay back at a relatively comfortable distance without losing the quality your looking for.
It really depends on what you want to shoot, and how much you want/can handhold. There is no question that the 300mm f/2.8 L II is a superb hand-holdable lens, and quite versatile with TCs with 420mm f/4 and 600mm f/5.6 options. If your primary subject is deer, elk, moose, etc. then the 300mm should be ideal. If you like to photograph more elusive subjects, like coyote or mountain goat and the like, then I would recommend the 500mm or 600mm lens and both the 1.4x and 2x TCs. Not only are goats and canines and similar animals smaller than your average deer, then often tend to be more wary and maintain a greater distance, so extra focal length really helps.
Even in the case of deer, 600mm at a moderate distance gets you some amazing detail. This photo of a doe in the shadows of a tree at sunset was shot hand-held with the EF 600mm f/4 L IS II. I'd never seen this kind of quality and detail in a deer's fur coat from the distance I was standing until I took this shot:
Click for full size (warning, its retina size, 2880x1800, so quite large).