Why do lenses need AF calibration?

Started Oct 15, 2003 | Discussions thread
Derek Hawkins Senior Member • Posts: 2,049
It's a closed loop algorithm...

except that it isn't continually monitoring focus error. I suggested this in my post. I would be surprised if Nikon isn't doing the same thing. Step 4 confirms that it is closed loop and the loop is steps 1 to 4.

If data is bad (poor light or contrast),
repeat 1-2-3 as necessary.

The data is the calculated focus error. The above is the response to the focus error feedback. In fact the only difference I see here to what I said is that an optically encoded servomotor is being used instead of a stepper motor.

processor wrote:

Doug Warner wrote:

http://www.photoscene.com/sw/tour/inside.htm
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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