MPE 65 sharpest at f/?

Generally, lenses are sharpest at 1-2 f-stops down from fully open (notable exception the Zeiss Otus 55 mm 1.4, but I digress). Some macro people consider the MPE 65 to be sharpest fully open. What is your experience? I wonder what the variability of this lens/its assessment is. Thanks!

It varies depending on the magnification used. Check the macro forum, there is quite a bit written about this

Larry is right. Around 1-2x I use f/10-f/8, opening to around f/5.6 at maximum magnification. At f/16 diffraction severely limits sharpness with this lens. At f/2.8 it might be sharp, but you can hardly tell because the DOF is so thin almost nothing is in focus.

One lens I've always been tempted to give a go but haven't yet. Comes up for sale used once in a while. One of these days.....