It turned out the lens I used for the review at the beginning of this thread was faulty. The fall-off of sharpness at 70 mm was significantly uneven across the frame. The lower left corner always was worse then the rest of the frame; it almost looked like motion blur. I suspect bad centering and, as a consequence, severe astigmatism. So I sent it back for a replacement, and the new copy has arrived last week.
The new copy indeed is better than the first one. The sharpness across the frame, at all focal lengths, still isn't
perfect but it's much better now. I guess it's next to impossible to align 18 elements perfectly in a zoom lens (unless you want to pay Leica prices), and the residual unevenness I can easily live with. It takes very close inspection and a trick to notice. To check a lens for symmetry, take a test shot from atop a tripod, then rotate the lens by 180° in the tripod collar and take another shot of the same subject. Rotate the upside-down image back to normal and compare at 200 % view.
What surprises me most is the difference in vignetting and distortion between the two copies. The new copy basically has the same flaws as the first one, but to a lesser degree. The vignetting at 70 mm and full aperture is still there but maybe half as bad as it was in the first copy. The same is true for the distortion: virtually absent at 70 mm, pincushion-shaped at all other settings---but significantly weaker than in the first copy. The distortion at 200 - 400 mm used to be fairly strong and possibly a problem in critical shots such as architecture; now it's pretty low and hardly a problem even with critial subjects.
Despite the flaws, I rated the first copy the best telephoto zoom lens I ever used. The second copy now is even better. I think even central sharpness, being very good to begin with, has improved ... but that's hard to tell precisely because I don't have the first copy anymore.
There is a
comparison between the Minolta AF Apo HS 400 mm 1:4.5 G lens and the SAL-70400G at
dyxum.com. I agree with their final verdict: the AF Apo 400 G still is the better performer at 400 mm but the difference between the two is very small. Being a zoom, the SAL-70400G is just excellent.
-- Olaf
and i was just wondering if other people were experiencing this problem or was it just a nit picky thing they picked out in the review?
