[Messier77]

Good but short of superb

http://www.opticallimits.com/sonyalphaff...y1635f28gm

Surprised to see your finding of low field curvature.  The several copies I’ve tested had immense field curvature on the edges of the frame noticeable up to f/8, which was really the only negative I could find.  Need to be very careful about composing landscapes with highly variable depths.

The best center and best borders/corners were mostly in the same frame.
So any softness that we've seen came from performance but not field curvature.
That being said - this may differ across the focus range.

Definitely found some odd behaviors when composing certain landscape scenes and a high sensitivity for nailing focus properly.  

I've found that sharpening up edges and corners (at least to what should be expected for the chosen aperture) can be highly dependent on choosing an appropriate are of focus.  For instance, setting focus more towards the edge of the frame on landscape scenes with focus expected to be at or near infinity (even for objects on the same plane, but more often with scenes of varying depths) can result in a shot with better sharpness across the entire frame, especially at more wide open apertures.  I took this to be a characteristic of the lens (as it happened consistently on several copies that I tried) and an indication of astigmatism/field curvature.  One of the reasons that I didn't think this is purely a matter of designed sharpness is that it goes away between f/5.6 and f/8 even though measured MTF's would indicate that f/5.6 should have superior sharpness across the frame to f/8.  This is in contrast to the Batis 18mm f/2.8, which essentially felt like a point and shoot at times, with center focus being satisfactory in nearly all instances.

I admit that this may all may be a result of overzealous pixel-peeping on a 5k monitor with a challenging 42 megapixel sensor.  Overall though, I'm happy with the lens and find it to be very good and extremely versatile.  

Thanks for the very thorough and thoughtful review!  Looking forward to reading more of them.