Shocking insight: AF is not a closed feedback loop!

Started Jul 18, 2003 | Discussions thread
OP Mishkin™ Contributing Member • Posts: 917
Re: hmm, AF sensor vs image sensor

Frank Nichols wrote:

1. Are you confusing the AF sensor resolution with the Sensor
resolution?

No. AF sensor doesn't know anything about what kind of sensor (or film - "what's film?") is in image plane. AF sensor resolution (pixels/mm) directly relates to the size of maximum CoC projected on sensor/film plane. For 10D's AF this is up to 0.03mm at f/2.8, for 1D/1Ds it's 0.01mm at f/2.8. This is the maximum diameter of each point point in the image blurred due to permissible [that is, to maximum ability of AF sensor] misfocus.

2. If I follow what you are saying, then I would expect that error
would be a function of subject distance and would appear a a
"ripple" of error. In otherwords, what ever the stepping error is,
it will repeat so that as you walk forward toward the subject, you
will get more and more then less and less error. At some distances
you should get "none".

Possible. But even if you're at such distance, a jump of 1 pixel in AF sensor is still possible intermittently (due to noise, imperfect matching of the signals from half-pairs, etc.) and that will cause jump in AF.

3. Don't forget the "lens type specific database" in the 10D - hmm
step functions?

Who knows...

4. This would also say non-mfr lens owners are SOL, but it doesn't
explain why some 3rd party copies focus and others don't - ie some
Tamron 28-75's focus and some don't.

I don't think so. As long as lens is made to report a matching signal about distance back to camera, it doesn't matter who is manufacturer.

I think there is still a bit more to this story. Otherwise, lens
focus perfomance would be predictable.

I think 99% is explained already. Thinking outside the box of closed feedback loop solved the puzzle why some lenses focus perfectly, yet some do not.

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Mishkin

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