Why do lenses need AF calibration?

Started Oct 15, 2003 | Discussions thread
Miro Contributing Member • Posts: 648
No, here's why

If it was a closed loop system, it wouldn't have to be recalibrated. And, here's what happend a few weeks ago:

If you are familiar with the Sigma 15-30 you will know in order to engage/disengage the manual focus mechanism you have to move the focusing ring back and forth. Well, I didn't move it far enough, so auto focus motor would buzz, but lens didn't move. Now, here is a funny part: 10D thought it was in focus! And so did I The focusing motor made noise, green light came on, camera beeped and I took photos only later to realize that all of my photos taken with this lens were way out of focus. After that, I moved the focusing ring back and forth and rotated it a bit until I felt a 'click'. Focusing motor was now engaged. Too bad, really, because I lost one really good shot. But, it shows that the focusing system is open looped. And, futher more, in good lighting conditions (at least), camera doesn't even bother to recheck the focus after it thinks that proper focus has been achieved.

Cheers,
-Miro

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