You are right, almost all lenses are sharper when stopped up a little, but I think you may have missed his point... basically he was saying on a APS-C camera you don't have as big a drop is IQ, using EF lenses, you don't lose as much sharpness as with FF, and avoid most vignetting. You are only using the centre of the lens, where it is sharpest.
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I understood his point, I think you are making a different and valid point. It's true that APS-C take advantage of the "sweet" spot of the lens -- But also true that even APS-C lenses, tend to get better results when stopped down.
But you more often shoot at 2.8 out of necessity, not preference.
On Canon APS-C, even the great 17-55L, is best between F4 and F5.6, even on APS-C
http://www.photozone.de/canon-eos/425-canon_1755_28is_50d?start=1
Anyway, my point was that having 2.8 zooms is less critical on a fullframe camera, and it's not like shooting with a 2.8 on APS-C is just as good as shooting F4 on full frame.
Let me give an illustration -- For easier math, I'll assume Nikon/Sony/Pentax 1.5 crop ratio:
Taking a portrait......
APS-C, 33mm focal length, 50mm equivalent.....
From 5 feet away, aperture of 2.8, You get a DOF of about 9 inches. And let's say there is adequate light that you need ISO of 3200.
Now, let's put the 50mm lens on full frame -- 5 feet away, aperture of 2.8 -- Depth of field is 6 inches. Depending on the angle of the head, that might not be wide enough to even bring a whole head into focus. If it's a 2-person portrait, it makes it that much more challenging to bring them both into the DOF.
I'd say the difference between DOF of 6 inches and 9 inches is hardly insignificant.
So to get the 10 inches of DOF on the full frame.. You need to stop down to F4. You will obviously need to boost the ISO up 1 stop... to 6400.
So would I rather take this portrait with an APS-C, F2.8, ISO 3200, or Fullframe, F4, and ISO6400 ---
Having experience with both, I get much much better results with the latter set up.
Not to say I can't get good results with the APS-C setup. But having experienced both, the latter is noticeably better.