11-21-2017, 10:27 AM
It is not just that increased DOF masks field curvature, field curvature can be gone or worse at different focus distances (see the images from the UMN 55mm f2 in link above for a clear example, for e lens which does not even have moving elements). I do not know how to test that reliably, and obviously I do not have that lens even if I did. 
Other issues also can vary depending on focus distance with lenses, like bokeh character, astigmatism.
Other issues also can vary depending on focus distance with lenses, like bokeh character, astigmatism.