thank you for your comments and observation. When we speak about field curvature and lens design in general, we should consider few things in addition…
Most important is focal length. While 35mm is sometimes considered as a standard focal length, in reality it is quite wide and wide lenses have to deal with many aberrations. Like with most other optical designs, it is always question of priorities and compromises.
Biogon is indeed known for “rather” flat field curvature in comparison to Distagon e.g.,but it is even better known for low distortion. (Biogon was originally based on Biotar, which was another way to call Planar and Planar name was based on planography – prints from flat surface)
Distortion is usually one of the most problematic aberrations with wide angle lenses and Loxia 35/2 shows extremely good control over it. However, good control of the distortion needs some compromises and some of them are partially paid off in field curvature while other partially reflects in increased sagittal oblique spherical aberration (butterfly effect).
Thing is, that wide angle lenses which seems to perform better in the extreme corners then Biogon/Loxia, usually have so called mustache (irregular, complicated) distortion. This type of correction is today very popular (thanks to much more precise and cheaper production of aspherical elements), especially when combined with automatic software correction. In most situations, image looks sharp from edge to edge with such a lens. But looking more carefully for the critical distortion control (when shooting architecture e.g.), one might be rather disappointed.
Loxia/Biogon would perfectly sharpen the extreme corners from f/11 onward, but we are facing here problems with diffraction on modern sensors.
While I understand your concern, it won’t be easy to find perfectly “flat” 35mm lens yet. As you can see from my tests, only Sigma 35/1.4 Art will beat Loxia in that regard (showing indications of mustache distortion though) and I was quite impressed with new Zeiss 35/1.4 Distagon ZM on Leica M body, even if Distagon usually means quite heavy field curvature.
In my review Loxia shown better corner performance (by very slight margin) then Sony FE 35/2.8 from f/4 onward, and except Sigma and new Distagon ZM, neither of tested lenses did come even close.