The Pentax 645D has a fine focus adjust feature (custom setting #33). It can be used to force focus forward or back of where the camera would otherwise focus.
The fine focus adjustment applies to both autofocus and the focus assist in manual focus mode.
After about 90 minutes of effort, I felt frustrated:
- With the 75/2.8, there is so much haze at f/2.8 (probably spherical aberration) that determining where focus actually is became a challenge. Testing at f/3.2 helped a great deal over f/2.8.
- Manual focus is prone to minor errors (my ability to focus the lens the same way each time) that made it difficult to separate user variation from actual focus confirmation, in terms of spot-on precision each time.
- The 645D has enough variation in its focus that I was unable to conclude that my results could be improved with the fine focus adjust.
- Focus error changes depending on whether the lens is focused from near to far, or from far to near. That includes the focus assist for manual focus mode.
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