FF or Crop for testing Lens sharpness (300 2.8 + telecons)

Hello,

Have many various bodies including FF and crop, high Mpix and low Mpix. I want to compare various II and III extenders and see real difference in sharpness. Which body to pickup? Which will show greater effect on sharpness. Thanks.

In an absolute sense, the camera with the highest pixel density should show the biggest difference.

Right now that is typically a crop sensor. However, some crop sensors have pretty strong anti-aliasing filters that might undo some of that.

However, it won't give you sharpness info on the edge of the FOV on a full frame sensor (where there might actually be a bigger difference) so both might be useful.

I see more tcon degradation with my 50D than with my 5Dc.

I can't speak to the which body issue as I have only used three and not done comparisons.
Having owned both MK II and MK III extenders (both 1.4x & 2x) I can say from my experience that the 1.4Xs are close in performance, (so much that I kept one) and with the 2Xs there is a noticeable improvement on the MK III