Thanks Brad, interesting test and of course informative and very much enjoyed reading and looking at your posted article.
What I personally observed when testing some moderate wide angle lenses in general, are that some exhibit mild to severe field (spherical) curvature, so that at the wider apertures, center sharpness may be fine but side/edge sharpess is soft. Yet if one examines the closer foreground of the same image, the side/edges are indeed quite sharp, in fact shaper than the sides/edges of the image thats in the same plane as the center. Therein lies a problem since when sides/edges and corners are not as sharp as some other lenses tested against it, it may simply be due to this curvature and not actually a loss of resolving power away from the center of the frame. I found the VC 35mm f1.2 II has this trait and interestingly, it appears more so than the Ver. I I tested it against. Seems that to slightly increase shapness in the VII of this lens at wider apertures (compared to VI), and therefore some field curvature had to be introduced.
It's a trademark of some optical designed whereby they have to balance out sharpness vs. flat field (or something close to flat field). An after market company that markets a 14mm SLR lens, had their initial version of their 14mm, which only had moderate sharpness, but an update version of the same lens had incredable sharpness, but it also resulted in introducing a tremendous amount of image curvature. Seems Nikon too had to make such a compromise with their recent 16-35mm VR lens.
With the floating element design of the 35mm FLE, this might have helped greatly with this issue.
The Leica 35mm f1.4 Lux asph (version prior to the FLE) also showed moderate field curvature at closer range.
Only mentioning all of this, as personal observations such as I just described, might initially influence judgement when comparing two or more lenses in terms of their center/corner sharpness.
As you demonstrated, the Biogon is a excellent buy and I often miss the one I had some time ago. Thanks again Brad.
Dave (D&A)