Sat 09-Jun-12 02:54 PM | edited Sat 09-Jun-12 07:16 PM by Jim Pearce
(I apologize to readers for referring to an earlier iteration of this argument.) I guess you didn't back up fast enough this time! As it happens, I agree that the D800 can be a useful camera for small birds, given a sufficiently powerful lens, which is why I mentioned the 800 f5.6.
I also agree that it would be useful to have sufficient pixel density to frame a little looser on any camera. Let's see if we agree on this. My theory as to why we cram the bird into the viewfinder on a D300 is that we are implicitly relying on downsizing the image to raise the SNR for posting (about one stop, 3 decibels, for a 50% reduction if I recall correctly). Change the D300's sensor to the 3200's sensor we can back off to a 1.2x crop and open up about 1/3 of a stop with equivalent DOF. This is precisely analogous to the way I think most good bird photographers will adapt to the D800, although they'll have the 1.2 x crop lines in the viewfinder.
As I've argued elsewhere, everything else is pretty much a push (unless you're printing very large) - even diffraction as long as I shoot one stop more open with the DX camera for equivalent DOF. So what it comes down to is whether I want to make optical compromises to get 50% more focal length on a D800 (Put the TC-17E or TC-20EIII on the 500 instead of the TC-14E?) or buy a longer lens and upgrade my tripod, routinely use my Wimberley instead of the Arca Swiss, etc..
Of course, at a certain point in pixel density we'll be asking for the lens to resolve at too high a spatial frequency on the DX camera and suffer a loss of contrast. That is where the physics kicks in. But we're not there yet.