Quote Originally Posted by canoli View Post
As I understand it the point of micro-adjusting is to determine the plane of focus for a particular body/lens combo. Once you've found it you're done...aren't you? For that specific body/lens combo I mean.

I imagine certain (extreme) weather conditions could throw off the adjustment a bit and I would certainly recheck the adjustment from time to time... but what is the benefit of changing it "on the fly?"
Subject distance affects AFMA - testing is usually done at a distance of 25-50x the focal length, when the effect of distance is minimized. But, what is an ideal adjustment for a subject at that distance might not work for for a subject near the MFD.

Also, newworld666 referred to 'eyes behind a helmet' - the AF system is going to grab onto the highest contrast feature, for a driver/rider wearing a helmet, that's going to be the edge of the helmet, not the person's eyes. So, one might want to intentionally dial in a bit of backfocus using AFMA, so the eyes are in focus. Probably sounds easier than it is...