There is simply no way a new crop sensor is going to rival the full frame sensors. Not happening.
Other than in this case where you may find even 800mm too short. It's not easy to get close to wildlife lots of times. In the highly reach limited cases the crop sensors can more than rival FF. And comparing 5D2 to 7D, closer generation cams than the 5D3, the 5D2 just about never did better, even at high ISO if you scaled the 7D down to # of pixels 5D2 put on the distant bird you actually got a touch LESS noise (plus less de-bayer errors and greater acutance).
Lettherightlensin, I was speaking in terms of framing the subject the same size with a longer lens on the FF, obviously. I certainly was not factoring in the crop factor and the finer pixel pitch of the crop sensor. I didn't realize that what Kerry B was referring to, was the problem of achieving sharpness. I thought he was referring to noise.
I certainly do think a 24mp 1.6x crop sensor, is going to reveal limitations and flaws of the lens' quality, and will be very difficult to get sharply focused results at times, even at a distance...especially in servo AF mode.
Let's face it, at distances of greater than 100 meters (for any Europeans out there), autofocus can be a lot less accurate, especially on a crop camera...at least with all the lenses I've ever tried or owned (not tried the 800 f/5.6 yet.)
Frankly, I have tried the 1.4x iii TC, but on an older version 1 500mm f/4L, on my 15mp 50D. I was disappointed with the results. That particular lens just wasn't all that sharp (even with no TC), so the TC made it worse. It was tested, supposedly they found nothing wrong. I'm sure it looked somewhat sharp on a 21 or 22mp full frame sensor.Here's a 100% crop via 1.4x iii + 500 f/4L + 50D, see for yourself.
Attempted focus was on the taller sailboat on the left. This was on a carbon fiber tripod, shutter speed 1/400 second, and no panning or movement of the lens/camera. The boats were moving very slowly, especially the taller one. ISO 250, closed to f/8...it was even softer when set wider than f/8, and also got softer going above f/8 as well. I purposely didn't add much NR in Adobe Camera Raw, and used the "sharpness" slider at 75 (out of 150), the "radius" at 2.3 (a "sharp" picture requires a radius from .5 to 1.4), and "detail" at 10 (out of 100).
So, it's not like I'm presenting it here without trying to optimize sharpness in post, because I did.
It made no difference how I set the AFMA (it only got worse both directions), and also made no difference if I focused manually
, with live view, at 10x magnification (which I did here, the shot is done with mirror up in live view mode, so there's no mirror shake). What I saw on the LCD, even before I ever snapped a picture, was never all that sharp...the full motion video via the live view, barely looked sharper than this still image.
Sure, there is a bit of atmosphere between me and the sailboats (I was on shore)...but I feel that is only causing 15 to 20 percent of the softening you see here. A faster shutter speed of 1/2000 would have maybe helped another 5 percent. (As evidence of a faster shutter speed not making much difference...notice the American flag on the back of the larger boat. It seems to be relatively sharp with no motion blur...yet it had to be flapping at least a bit faster than the boat was moving.) So I say the rest of the problem, is the lens combo's softness.
So...the sharpness is nothing like my 135 f/2, or the 200 f/2 I later rented. I believe either of those (without any TC) would still be sharp enough for a 32 MP 1.6x crop sensor...and coupled to a 1.4xii or iii, would maybe still be sharp enough for a 26 to 28 MP 1.6x crop sensor.
What you see here, is barely sharp enough for an 18 MP full frame (1DX), in my opinion. I mean, I'll admit it is
the older 500mm + a TC, so it's 700mm (and full frame equivalent 1120mm)...but at the pixel level on my 50D, it's very soft. White egrets at much closer range, looked similarly soft. Can't fault the atmosphere with those.
The true resolution test for a truly sharp lens, would be on Sigma's SD1 crop sensor, in my opinion. But of course, you can't mount Canon glass on Sigma bodies. And of course their camera itself, is nothing like as useful as a 7D, yet costs more.