The 60mm difference in performance in my opinion, gets back to the micro lens issue and how the image is projected and received by the sensor.
The 60mm is sort of a transitional lens from a traditional, wide angle focal length lens, to a normal configuration, which starting at 70mm move towards a normal lens in design philosophy. Normal lenses and longer focal lengths, generally project a much flatter, physically, image shape. This is easier for any sensor to accept.
A retest and perhaps, a test at other apertures such as even F16 might be in order.
There are some interesting lens phenomonon's, with various lenses. The 47mm APO-Digitar XL, for example, is not stellar with 80mp backs in most cases, yet is more than adequate up close, for still life at F16(!) and on 80mp backs. The longer focal lengths say 120mm and longer generally do better stopped down a bit further even at F16 with 80mp backs.
The 60mm XL may prove to be one of those which just has an eccentric way of performing with some digital backs.
Anyway, Doug did a fantastic job, and we all owe him a heartfelt thanks for his in depth, hard work on this test. Thanks again, Doug.
