Letien said:
I simply use a ruler lay flat on the ground. For open back camera like canon rf, I also use a piece of groundglass to fit it on film plane. It works.
If you find that yours is one of those that wasn't adjusted correctly for focus point from new:
The focus point is determined by a brass ring inside the lens, which shims the optical section from the focusing mount. You can remove the entire optical section by (carefully!!) unscrewing the slotted ring around the rear lens element; the optical section lifts out in one piece.
If you find your lens invariably focuses closer than the RF indicates (assuming you're
sure the RF is adjusted correctly!) that means the brass ring is too thick; if you're brave, you can (CAREFULLY!!!) thin it down by rubbing it against fine-grained abrasive paper placed on a flat surface. Use a circular motion and change your grip frequently to randomize your hand movements.
Proceed slowly -- correct adjustment is a matter of hundredths of a millimeter!!
If you find your lens invariably focuses farther than the RF indicates, it means the brass ring is too thin. You can shim it by cutting a supplementary ring from brass shim stock, which is available in very thin dimensions at any good hardware store. The thinnest shim stock is considerably thinner than ordinary paper, and cuts easily with scissors or a sharp hobby knife. Use the original ring as a pattern and carefully cut the shim so it matches exactly.
Note that I DO NOT RECOMMEND this procedure except as a last resort! That goes especially for the "thinning" process, since it's non-reversible. Adding a shim isn't so bad -- if it doesn't help, you can just throw the shim away.
Still, if you've got a 50/1.2 that's not worth keeping "stock" and which seems to perform well except for depth misadjustment, this is one thing you can do about it. Might be a good use for a lens which has slight element separation, a marred barrel, or other wear/damage that makes it too ugly to bring collector prices...