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| p.5 #16 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes |
How do you incorporate BFCV, and the idea of lens calibration in general, into your thinking?
That a lens can be consistently out-of-spec in regards to its mechanical positioning relative to where its directed, and that this condition can only be fixed through manufacturer shimming/adjustment as the 50mm manual you referenced indicates, is actually further support for the notion that AF tune cannot correct for such conditions, and that it wasn't designed to.
Okay, I see where you're coming from. BFCV doesn't have anything to do with mechanical positioning. (It doesn't matter for AF accuracy if there is a mechanical positioning error, since the body will command the lens to move until the AF sensor sees an in-focus subject - accuracy depends on detection, not motion. You can induce a mechanical positioning error by using a short extension tube, and the system will still achieve accurate focus.)
AFAIK shimming only affects where "infinity" focus via the focus ring ends up.
BFCV is a number sent by the lens to the body on request, which tells the body how to align a phase match with optimum focus, just like MA does with a value stored in the body. Check out Doug Kerr's Canon EOS Autofocus - BFCV and micro-adjust (MA) for more info.