Canon said in the Lens Work III book anything above .8 was excellent and anything about above .6 was satisfactory as in no problems.I wish we had MTF-like charts that showed us what corners would look like beyond simple lp/mm, because it’s not until someone sticks a tree in the extreme corner that we can see if we’re talking just a bit of resolution decrease, or a smeared, mushy mess of purple.
Unlike Canon, Nikon seem not to have moved to an MTF system that takes account of diffraction.
There is bound to be some diffraction at 400 mm F8. This implies the Nikon 400 mm MTF is likely to be a little optimistic