AF fine tune and focus shift for fast lenses

Started Sep 12, 2016 | Discussions thread
Marianne Oelund Veteran Member • Posts: 7,788
Viewscreen adjustment

fPrime wrote:

When I installed a Katzeye split prism focusing screen in my D700 I could immediately tell that it was out of alignment with PDAF focus. There appeared to be a slight back focus on MF when using the split screen. Fortunately the resolution was to adjust the manual focus adjustment screw with a 1.5mm Allen wrench. A simple 10 degree counter clockwise turn brought the view screen into perfect alignment.

There isn't a "manual focus adjustment screw."  The one you probably turned, is the adjustment for the reflex mirror rest position.  That is there to adjust vertical framing for the viewfinder, not focus.  In some cameras, it may also impact the AF-point vertical registration and the AF tuning.

In all cameras, it also affects viewfinder image vertical skew, which means that although you may have improved focus accuracy at the center of the screen, it's been altered differently at the top and bottom.

Viewscreen focus corrections need to be done by changing the shims - unless you don't care whether or not the viewfinder gives you the correct image framing, or top and bottom focus.

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Source credit: Prov 2:6
- Marianne

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