My 35 Art´s AF is drifting again (a third AFMA with Focal showed a further 4 step adjustment, on top of the 7 steps I got between the one I did when I got it and Christmas), so I must admit I am a bit skeptical to that part of sigma. But since so many are happy with it, I hope my AF problem is a one-off.
I have a technical query here:
As far as I understand, the purpose of AFMA is not to 'fix' defective lenses, but calibrate a specified lens to a given camera to account for manufacturing tolerances.
Once the AFMA is done, the camera knows how much to compensate for this lens, and everything is hunky-dory.
But in what condition can AFMA drift as is happening in Eldar's case? Is it because something is moving within the lens and a gap is getting bigger or a cog is becoming more loose?
I am particularly interested since I just acquired a 35A (so far it looks like it is focusing right on target as shown below- spot focused on "6" using a peripheral point and center point respectively), I haven't run it through FoCal yet.