Originally posted by zapp Nice explanation, but you did not get the question.
Whatever you do to your lens, will not change the AF sensor to image plane calibration. The AF sensor will focus the lens based on phase shift meaning that it compensates automaticlly for all the problems you mention abnove. What can be adjusted is the offset between AF sensor and image plane. So again why do some lenses reveal different fine adjsutments. Is there a feedback between AF and lens that results in something like an offset, Does the size of the AF area change too much for a given lens...
All the causes mentioned above will only change what you read off the foucs ring - who cares?
The camera will move the AF motor based on how far the AF module thinks the (lens) focus is out of phase (usually using the smallest defocus amount from the sensors, weighted per some complicated algorithms re:focus area, contrast & sensor type). The AF motor has encoder pulse feedback to the camera CPU to let it know how far the motor has moved. If the phase detected defocus amount being used is not within the "in focus" focus width, the CPU will calculate how many pulses it needs to see to drive the AF motor the right amount. So the motor encoder counts down, and eventually (hopefully) the next defocus amount is within the focus width, and focus lock can be indicated. The focus width is a range which the defocus amount can still be considered in focus w/regards to focus depth. This is a number that can get calculated based on various lens/focus info, and must apparently allow for some margin of error.
The AF motor has a mechanical connection to the lens focusing group...there are several gear blocks involved (in the body and the lens), and coupling points, and build tolerances, and maybe some backlash and other various built-in slop, and lots of different lenses to interface with. This means the CPU counts down how far the AF motor moves, without REALLY knowing how far the focusing group in the lens moved, or how exact it is positioned.
Besides normal factory differences, mechanical tolerances that otherwise should be tight and small enough to allow for quite accurate - & centered - calculations, may tend to get a bit sloppier when used and handled. Fine adjustment is provided to help compensate for these errors to dial in the exact target focus.
I imagine if BF/FF slop was so bad, the camera would never be able to find lock, except by luck - as any calculation determined/used to drive the AF motor would be off for the next go around.
Of course knowing EXACTLY what target the camera (with its large sensors) focused on is important too.
Last edited by jmg257; 04-04-2012 at 05:31 AM.