Why do lenses need AF calibration?

Started Oct 15, 2003 | Discussions thread
Doug Warner Senior Member • Posts: 1,128
Re: directly from Chuck Westfall

processor wrote:

Doug Warner wrote:

I don't agree with his analysis of the optical pulse counter as a
focus position device. It's more likely for speed feedback, a n
omportant part of any servo system.. (Speed sensing for the large
fast movement, AF sensor for the final null)

Chuck says:

"3. As the USM turns, the phototransistor of the photocoupler
detects changes in the amount of light reflected from the LED,
which occur as the encoder ring moves. The phototransistor converts
this data to pulse signals which are relayed to the lens CPU.

4. The CPU counts the pulses to determine when correct focus has
been reached. When the correct focus is reached, the CPU stops the

AF processor calculates the number of pulses to achieve focus
(first estimates, then refines - usually only 2 measurements) and
sends this number to lens' CPU. When CPU counts the same number of
pulses, it stops USM.

To summarize, here's how Canon AF works:

1. Determine the shift between the curves on half-pair AF sensors,
offset it additionally by body's calibration amount.

2. Calculate the number of pulses the lens should count to achieve
focus, based on the shift, focal length, and lens' calibration.

3. Send this number to lens and start to move it towards focused

4. Repeat 1-2-3 one more time shortly before reaching the first
estimate, and refine it. If data is bad (poor light or contrast),
repeat 1-2-3 as necessary.

5. Lens rotates and counts pulses until the number determined by
1-4 sequence is reached. Stop.

Open loop.

I concede.. This is also what I found in Canon's filed patents recently. I guess the repeat / refine readings are what allowed it to compensate for the insertion of the extra lens.

I can also agree with those who say it's partially-closed, since it makes at least one refinement reading.

So, if the lens fails to move the required number of pulses, or the distance moved per pulse is not what's expected by the camera, then the lens needs to be "calibrated".

And, one of the patents explained how this method helps in multipoint focusing.. It can take an average of the points and focus somewhere in the middle by sending the lens to focus on that imaginary point, instead of what one of the sensors is reading. (Same for AI predictive focusing..)

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