An autofocus system is system based on a feedback loop, where the AF sensor error reading if fed back to the lens focus motor. Being a closed loop, then theoretically there should be no focusing errors (at least when lighting is good enough).
So I was wondering, what is the source for this kind of error that requires MFA different for each lens?
The one reason I could think of is that the AF sensor is not perfectly aligned with the image sensor - a thing that will create a constant misfocus. But then, this probably needs to be constant for all lenses.
Whet exactly is being calibrated with MFA? Is is it the AF sensor position, the image sensor, control loop gain, etc.?