Martin,
That's interesting to read, thanks for the input. I did my test with two lenses, the AF 80mm PQS and the 180/2.8 and got that same result. I am struggling to see how the lenses could affect this setting - any thoughts? I mean its clear that differences between the film plane or sensor chip alignment could change the AF focus calibration as its downstream from the AF electronics and focusing screen, however the lenses are upstream - how could they change things? Not saying it isn't true, just would like to know how.
Hi Eric,
There are several elements in the AF cycle that can (or need to) be adjusted.
First there is the mechanical tolerance of the lens(mount). We are utilising the extremely narrow DOF of a wide aperture AF sensor. The exact depth of focus (in the projected image plane) of the AF sensor is probably in the order of the thickness of a human hair (or less).
Next there is the potential discrepancy between the optical path to the AF sensor versus the optical path to the image sensor. we are again faced with construction accuracy versus several tens of microns of depth of focus. When we're unlucky, these tolerances can add instead of canceling each other out.
Next there is the mechanical drive train of the focus mechanism in a lens, with its hysteresis tolerance when changing focus direction. In addition there is the accuracy of the stepper motor(s), how small a change of focus plane is the AF mechanism capable of.
A common way of dealing with all that is an electronic 'factory' calibration which can (partly) be stored in the lens. Most likely that is implemented as a lookup table which is built from an offset and a multiplier (or more complex if a lens also uses internal focusing). The easy part of that equation is the offset, and that's one of the reasons why it has become available as a user adjustable parameter which is probably stored in the camera body. The other parameters used to build that lookup table are too complex for an average user to calibrate by hand. A computer application with knowledge of the mechanical focus properties of the specific lens is required to find an optimized solution.
Due to the differences in lenses, the offset can be different between lenses. I'm not sure how it's implemented, but one could store the offset in the camera body, and the other parameters/lookup table in the lens. One could also store a second offset in the camera body which is added to the generic offset (which cannot know the tolerances in the camera AF path) used in the lens' lookup table.
Cheers,
Bart