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denoir wrote:
PetKal wrote:
Well, you seem to have more detail on the subject than I do, however, my understanding has always been that Canon AF in a single shot mode is an open loop (feedforward) type of control. The camera moves the lens once and that's the end of one focusing action, as initiated by the photographer. No residual focus error measurement, and no ensuing control action aimed at reducing the measured error.
However, please give me your references so that I can learn more.
Oh, I know absolutely nothing about it - just speculating. Sorry that I didn't make that clear. I would guess that in order to increase the speed of the focus it would do an iterative focusing i.e first a large jump into the right direction and then smaller corrections. But as I said, pure speculation, I have nothing to back it up.
Assuming you are right, I'd still guess that it's the lens motor that is the cause of it. How else would one explain that it happens more often on one lens compared to another?
It is very educating conversation. I think Peter is right on Single-Shot AF.
Back to your 135 issue, I think that it may caused by loose/improper coupling between focus mechanism and lens housing or group. The motor rotates properly based on the camera's command, however, the actual distance moved is not what the camera AF system indicates. Did you hear any wobbling sound when you gently shake your lens? If so, then it could be the reason I just mentioned above.
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