This situation happens when you use the wrong section of cam curve.

If you ever read my post of lens curves, then you knew that the relationship of distance between 'lens to focused object' (L-O) and 'lens to focal plane' (L-F) never be a linear ratio. When focusing at infinity, we set the 'lens to focal plane' as ZERO point, the least length required to utilize this lens, and an 'infinity stop' is engraved on the plate, for quickly and precisely set lens on this position.


When focus objects in distance less than infinity, rail moves and bellows extends to increase L-F distance to fulfill the requirement, as object get closer, extended distance increase more. that is, if object at about 10 meters away, maybe the L-F needs only 3mm increment, but when object at 2 meters away, increment increase to 18mm.

That expained why cam curve designed to be a parabolic curve, and if you set the wrong section, all adjustments are in vain.


Left on image shows the correct curve section on cam, infinity stop is aligned with the engraved line on cam, the ZERO point, and the curve section is match with the lens focus characteristic curve. But if the ZERO point is not aligned with the mark, then curve section must be shift, like what shows on right of image. Thus before you do any calibration, you should check if the infinity stop aligned with mark line or not.