There are pretty few rules in wildlife photography, but there is one which you'd want a pretty good reason to break. This is it: the only thing in the picture that really needs to be in focus is the eyes.
Case in point: with this head-on shot of a dwarf caiman in the upper Amazon, there was simply no way I was going to get everything in focus. Even stopped down to f10 (as this was), my depth-of-field was going to be measured in fractions of an inch. The math just doesn't add up: a six foot long reptile and a 300 f2.8 lens: getting everything in focus simply at this angle was NOT going to happen. Instead, the challenge here was simply to make sure the camera was focussed on the eyes.
Just as with pictures of people, we make instant, instinctive eye contact with the subject in photographs of animals. The eyes are our primary source of information, giving us a window into personality, mood, and even the potential for threat. But if those eyes are blurred, it is so visually jarring that it makes the entire image seem out-of-focus.
The fact is, even if every other part of the picture is tack-sharp, if the eyes are blurred - so goes the whole image. Conversely, if the eyes are in focus - as they are here - the rest of the picture can be blurry, yet the picture seems acceptably sharp. It is an odd quirk of our visual make-up: but it's one that photographers of wildlife should always keep in mind.
The other challenge, of course, is that with most subjects the eyes are rarely dead-center in the frame - or at least they shouldn't be. Most compositions seem to work better if the focal point is slightly off-center, but that means moving your focus point away from the all-too-convenient middle of the frame. If I had relied on the default focus point, I would have had a nice sharp image of the caiman's back - but his eyes would have been irredeemably blurred.
To get this shot, I was prone in the bottom of a small boat, trying to get as close to the level of the water's surface as I could. Would I have preferred to have been in the water with the camera angle even lower? You bet I would - except for the schools of hungry piranhas...
Nikon D3, 300mm f2.8 lens