Rigid Cron focus shift?

[Ko.Fe.]

I have 50mm Summicron Rigid with matching focus part and lens part (same serial number).

I have tested it on film M and on digital M, it does the same. From f2 to f4 it is front focusing for 2-3 cm at one meter distance. At f5.6 it becomes OK, focus scale mark sharp, far and close marks blurred at similar distances. At greater distances it is sharp enough at 2 and 2.8 (I guess, because of DoF).

Could it be the focus shift?

 

[Mark C]

All of the classic fast lenses I've tried do show some focus shift, but not very significant other than the sonnar types. My Summicrons (DR and 4th version) seem to focus correctly at f2 on my properly adjusted film M's. I'm sure slight differences will be more noticeable on digital and don't know exactly what mine would do there.