Originally Posted by Carlos Lindado
First, lets catch everyone up on how phase detection autofocus works:http://www.lensrentals.com/news/2010.07.05/how-autofocus-often-works. Scroll down to the section about phase detection for info related to this discussion, though the rest of the article is very interesting as well.
The problem lies in the fact that the autofocus sensors and the image sensor aren't one in the same. These two components need to be aligned properly so that light focusing on the autofocus sensors when the mirror is down is also in focus on the image sensor. This might be why some lenses work wonderfully on one camera body and not so great on another.
Toward the end of the article is a section on disadvantages of phase detection autofocus. Here is a quote from the article:
The light path to the imaging sensor must be calibrated with the light path to the autofocus sensor, so what is in focus for the autofocus sensor is also exactly in focus for the imaging sensor. Each lens contains chips that provide feedback to the camera, telling it exactly what position the focusing element is in and how far it moves for a given input to the lens motor. This must agree completely, so that the lens actually moves exactly where the camera told it to move, and so the camera knows exactly what position it’s in. If any of these systems aren’t calibrated perfectly, autofocus becomes inaccurate. Even if they are calibrated perfectly at the factory, if they expand and contract slightly with heat or cold they may become inaccurate, at least temporarily.
I would image the purpose of AF microadjustment is to tell the camera to modify the distances it tells the lens to move.
Hopefully this helped. However, I may be wrong about some of this, since my knowledge comes only from this and a few other articles I've read. If someone knows more than I do about this topic, please share your knowledge as I'm always happy to learn.
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