Since Fred Miranda made a test showing a change in field curvature from outward bending at short focus distance to inward bending at infinity focus, I wanted to see how my sample behaves. Bastian Kratzke also showed a test at 2 meter focus versus the Thypoch Simera 28mm f/1.4 where the Nokton appears to have strong outward bending field curvature compared to the Simera, which, notably, has clearly less optical vignetting. These tests prompted my into doing my own tests with my sample of the 28mm Nokton on my Kolari Ultra-Thin sensor glass modified Nikon Z6, which has (close to) the same sensor stack thickness as digital Leica M cameras.
In the below image, I don't see the outward bending field curvature at minimum (0.5 meter) distance wide open, using "find edges" in Photoshop, like Fred Mirandas sample shows. My test image looks pretty flat. If there is any field curvature, it's so weak that it's negligible.
The composite image below: In order to minimize the effect of optical vignetting, while checking for field curvature, I stopped the Nokton down to f/2.8 and shot an infinity scene at various focus distances from 5 to 0.7 meters. Again, I don't see any field curvature in that test.
In the third image, I found a stripe of asphalt on the icy road, and took an image wide open, focused at 2 meters. Still not much, if any, sign of outward bending field curvature.
My conclusion, at least as far as my sample on my camera goes, is that there is not much field curvature going on. I did see some inward bending field curvature when testing for infinity sharpness, but nothing dramatic, only at the edge of the long side.
In short, the decreasing blur towards the sides and corners I have seen in my wide open images, I attribute to largely, if not exclusively, optical vignetting.
