I would qualify that as starting at where the lens is made to be sharpest. Most real world sharpness limitations that I have encountered with quality optics are set by the technique, circumstance or equipment, not diffraction. I am not, of course denying the principle, or that it shouldn't be considered, I just find it unlikely in this case. In reviewing the OP, I have realized he is using a TLR and now suspect the calibration of the focusing lens, or focus issues as a side effect of parallax (taking lens closer than focusing lens) as my primary suspicion.
Originally Posted by Q.G.