Why do lenses need AF calibration?

Started Oct 15, 2003 | Discussions thread
processor Contributing Member • Posts: 648
Wrong - Re: Closed lopp: Proof:

AF first analyses the curves on half-pair sensors, finds the shift between them, estimates the correct focal plane position, and begins to move the lens. Close to focused position, it gathers the data again and refines the focal plane position. Sometimes - but rarely - additional points of data are gathered, if the previous data was not of good quality. Therefore, AF on Canon SLRs is OPEN loop and strongly depends on perfect calibration of all components.

It's like if it you drive from A to B and it's 100 miles, and you see on the map that A to B is half of A to C, you know A to C is approximately 200 miles. You don't need to make measurement EACH mile - that would be more accurate, but also slow. But you need your odometer and map be calibrated precisely for "open loop" to give correct result.

The curve "lens shift vs. focal plane offset" is a second-order polynomial and can be drawn (and the root - precise focus - can be found) by knowing only 2 points (the focal length is the third parameter which is reported by the lens).

What you observed in your experiment, you moved the diopter in before AF gathered the last data point (which happens very shortly before AF declares focus lock). When AF made the measurement, the new data was completely out of line, so the camera repeated the AF sequence.

Open loop.

Doug Warner wrote:

Derek Hawkins wrote:

for the system to be open loop the camera would have to initially
calculate in advance the amount of pulses to send to the lens
stepper motor based on just an OOF image.....Heck, without focus
error feedback, the camera wouldn't even know whether it was

I agree, and to prove it, I set my 10D on a tripod with my 24-70L.
and..

1. Aimed it at a page on the wall 2 feet away.
2. Manually defocus lens to infinity.
3. Press * to focus. When the focus mechanism started to rotate, I
moved a +1 diopter close-up lens in front of the camera lens.

The focus scale actually reverses and backs up to compensate for
the +1 diopter.

IF it was open loop and took a reading at the start of the focus
cycle and moved the elements a fixed distance based on that
measurement, it would be OOF with the +1 in place. Instead, it
compensates for the change, on the fly..

Now that I think of it, the same text can be done by setting the
focus at minimun distance while aiming at a distant scene. As the
focus cycle begins, quickly move a test target in front of the lens
at it's minumum focusing distance and watch what happens. It'll be
heading for infinity focus, and will instantly reverse and zero in
on the new, closer focus point.

Closed loop, closed issue.

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John

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