Shocking insight: AF is not a closed feedback loop!

Started Jul 18, 2003 | Discussions thread
OP Mishkin™ Contributing Member • Posts: 917
about lens adjustments

Now that we forget about closed loop, it becomes clear why Canon asks you to send ALL lenses in even if you have a problem with one lens only.

First, Canon calibrates the body (by applying a shift to sensors' signals in AF processor's firmware) by using some "perfect" lens (their well-calibrated copy of say 50/1.4).

After body is calibrated, all lenses you sent in are calibrated with the body. Their drive quantity detectors get adjusted (by updating code in lens' controller chip) to be a perfect match with the body.

The myth about lens being a dumb device is finally misspelled. AF is rather a handshake of the chips in camera and lens. Both must be properly calibrated for handshake to be a perfect focus.

If lens were a dumb device with a motor only driven by a feedback loop signal, this dumbness would produce ultimate sharpness all the time. Unfortunately, closed loop system cannot compete with "open" one in speed and moving subject tracking, while "open loop" produces excellent result when all components are properly calibrated.

Due to bad Canon's QC, users may need to send the camera and all lenses in almost EACH TIME they buy a new lens. Oh well, life is life.

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