Why do lenses need AF calibration?

Started Oct 15, 2003 | Discussions thread
processor Contributing Member • Posts: 648
directly from Chuck Westfall

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 USM."

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 position.

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.

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