Originally posted by Designosophy My Sigma 30 f/1.4 has good contrast wide open - better than my FA 50 f/1.4, but I have found that the center AF point is most reliable. The left-most cross-type points are pretty bad, but the right-most are better. I can't use the far right and left points at all. With the FA 50 f/1.4, I can rely on all of the AF points pretty well. Is the contrast you're talking about something other than the contrast I see in photos?
Since Digitalis appears to have misinterpreted the post
where I hypothesized about the off-center PDAF difficulties with the Sigma 30/1.4,
it may be helpful to clarify the word "resolution" first
(contrast is also involved).
There are at least three uses of the term "resolution" that are relevant.
"Scientific" resolution is basically the same as "spatial frequency,"
a quantity measured in line pairs per millimeter or picture height.
"Photographic" resolution is what gets measured in MTF graphs.
It is specified by a contrast level at a particular spatial frequency.
Photozone and Optyczne/Lenstip present the frequency for a 50% contrast level.
"Perceived" resolution is what was tabulated by Yoshihiko Takinami:
http://www.takinami.com/yoshihiko/photo/lens_test/index.html
The distinction between that and "photographic" resolution
is very apparent in Takinami's listings for the FA*24/2 and M100/2.8.
Takinami rates the former very highly, even off-center, while Photozone does not.
Takinami suggests the latter has poor "perceived" resolution (on film),
as opposed to the high "photographic" resolution
documented by the official MTF curves I posted in my review at
SMC Pentax-M 100mm F2.8 Reviews - M Prime Lenses - Pentax Lens Reviews & Lens Database
In post #46 of this thread, I was using "resolution" in the sense of "photographic" resolution,
while Digitalis seems to have thought I meant "scientific" resolution.
My hypothesis was that the off-center point AF difficulties with the 30/1.4
are due to low "photographic" resolution at the spatial frequencies needed for the AF system.
Designosophy, the contrast you are seeing in your photos
is probably more like the "perceived" resolution, at least at low spatial frequencies.
I do not have personal experience of either the Sigma 30/1.4 or FA*24/2,
but the Photozone tests suggest that their "photographic" resolutions behave similarly.