As with any new camera, I try to calibrate the lenses I use to make sure they are doing what the camera thinks. With shorter lenses I use the Lens Align Pro which works extremely well.
This product does not work so well from say 200mm and longer. The resolution of the camera and the minimum focal distance make it inoperable. So I decided to make my own long lens calibrations system.
Using the new Mark IV, I first wanted to test the 400 and 1.4 I would be using at the cricket next week. As with the MK III I needed to back the focus back quite a bit to get the FOD sharp. At 10 meters, f4, 560mm (400 X 1.4) the DOF is 0.060229 m (9.969976 m to 10.030205 m), so 6 centimetres. The width of the test cards is 6 centimetres. The sharpest image was with +15 far on the camera.
From here I changes to the 400mm by itself. The DOF for this is even shorter with 4 centimetres to play with. I needed to back this off to the maximum +20 FAR on the camera to get it close.
I might talk to Canon to see if they can factory calibrate the lens a bit better. Sure it’s old, but the may be able to tweak it closer for me.