Infrared focus

[ampguy]

right

right

as mentioned, there are a lot of variables. My M-Hexanon 28/2.8 lens has the IR mark between 2.8 and 4, much closer to the 4.

Yep, but it is a 28mm. The focus offset is proportional to the focal length. On long lenses, it may be around or past the f/8 DOF mark, for the same reason.