Why do lenses need AF calibration?

Started Oct 15, 2003 | Discussions thread
Doug Warner Senior Member • Posts: 1,128
Closed lopp: Proof:

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.

Post (hide subjects) Posted by
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow