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?

One response:

Lens re-calibration

There are (in a Nikon fixed focal-length lens) at least three points to calibrate if you want the auto-focus system to function correctly: Infinity position, close focus distance, and a parameter that controls the ramp speeds and allowance for mechanical play in the helicoid/motor coupling.

The first two are mostly important for AF-S focusing, and the third is quite important for AF-C. There are actually more things also, but that's beside the point here.

This means that a lens can be very much off target even if the body is perfect. It can also show a behaviour that gives alternating rear/front focusing errors depending on if you approach focus from a far/short focus position, or if you use AF-C or AF-S.

Errors in the first two parameters are (at stationary targets) largely negated by the use of AF-C, unless you're close to infinity focus.

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