On 3/7/2013 7:13 AM, Floyd L. Davidson wrote:

On the other hand the new 80-400mm G lens has S30 at
0.73 and M30 at 0.74. Not only is that a very large
jump above the 0.55 of the 70-300mm, the fact that the S
and M values are so close indicates virtually no
astigmatism with the 80-400mm G.

In fact the 80-400mm G lens, at 20mm from the center
(basically this is in the very diagonal corners of a
full frame image) has S30 at 0.65 and M30 at 0.67, still
showing no astigmatism and being sharper than the 70-300
is even at 10mm from the center (M30 is 0.55).



Have the same S and M values does NOT imply that there is no
astigmatism!

That's because these are white light MTFs. Therefore they include
lateral chromatic aberration. A lens could have lateral chromatic
plus astigmatism adding up the the same blur in S and M.

Of course, having both small is better. Nevertheless, white light
MTF has the problem that if part is lateral chromatic, and it is small,
one can digitally fix it. What one needs is both white light MTF and
MTF in R, G, and B.

Doug McDonald