Why do *lenses* front- and rear-focus?
I can understand that a particular body will produce focus errors, if there is some sort of misalignment of the AF sensor. But this error should be consistent across all lenses. I have experienced rear-focusing that has only affected a single lens, and the problem has gone away after that lens has been back with Nikon for “lens recalibration”.
The Nikon auto-focus system used in DSLRs uses phase detection, described here. The system is designed to compute the exact distance to move the lens position based upon the signal input. There is no feedback mentioned, but there obviously is an element of feedback involved. If my lens fails to achieve focus (e.g. in bad light with a low contrast subject), it tries again (i.e. it “hunts”) until focus is achieved.
However, when a lens is rear-focusing the in-camera focus sensor lights green so the camera clearly believes that it has achieved focus. It is only when you examine the image that you discover that it has not.
Can anyone explain this?