So maybe I can compare on the same lens, on four edge corners
to see if the corner sharpness is consistent... Is this ok?
Can be done. Gotta be reeeeeally careful aligning lens
axis perpendicular to the flat test target/subject. Most
lenses focus to a slightly different distance at the outer
regions than the focus at the center. This not a defect
but a typical shortcoming of nearly all general purpose
lenses. Some lenses are very specifically "flat field"
lenses, and will focus center and outer regions to the
same distance. Most lenses are not flat field and have
a degree of curvature of field. Usually the outer regions
are focused to a shorter distance than the center. What
this means is that if you carefully align the flat target to
be perpendicular to the lens axis, you expect to find the
corner regions need refocusing. You should accept that,
but upon refocusing to sharpen up the corner regions
a lens that is properly centered will show all four corners
to be at the same focus as each other [acoarst allowing
that the center is now imperfectly focused].
I think that explains it :-)