I'm using a Sony A7R IV camera with a 12mm f/2.8 ED AS NCS Fisheye Lens. From the rescharts and MTF information that I've seen of this lens, the center sharpness is much better than near the edges. However, when I performed a similar sharpness analysis using the Imatest software my results showed that the MTF50 values near the image edges were higher than near the center. See attached photos; original image is found here.
This doesn't make sense to me, since I would intuitively expect that the center of the image, where the barrel distortion is the smallest, would be sharper than the edges of the image. In trying to work through this problem, I've narrowed it to some possible reasons:
My interpretation of the MTF50 measure is wrong and so I have misinterpreted the plot
The way that I focused the camera did not correctly focus on the center. For some context, I used the Focus Assistant of the Sony camera set to High to assist with manual focusing (maximizing the number of red lines that appeared)
Uneven lighting effects are causing the edges to appear sharper
Any help or suggestions would be greatly appreciated. I'm really new to photography so I'm not sure if this was the right place to be posting my question and if how I maybe set the camera settings was correct. Thanks!
As a side note, I don't have the camera settings I used on hand, but I can try to find that information if needed.
EDIT: I retook some photos with a test chart here and here, keeping in mind the suggestions about tilt. Visually, I think that the top and bottom seem to be improved, but the right and left sides seem a bit off. When I ran my Imatest module on some slanted edges on the left and right, they both still had a slight improvement over an edge analyzed in the center. Any suggestions for how I could reduce or eliminate this sort of problem?