Auto focus Micro adjust allows the user to more precisely control the point of autofocus. Autofocus systems in modern DSLRs have focus accuracy errors related to camera body and lens tolerance issues. This may cause a given lens body combination to focus slightly ahead or behind the intended target. Canon and Nikon have introduced a way for the AF of each lens to be calibrated separately and the calibration data to be stored in the camera body which can recognize a particular lens and or lens/teleconverter combination. In the past it was necessary to send a camera and lens to the camera manufacturer service people to tune the lens camera combination and this did not allow for different lens to be calibrated separately.
When I received the Canon 50D and was satisfied it was operating correctly I set about the task of calibrating all my lens. Toward this end I did a quick setup on all my lens and recorded the AF microadjustment offset numbers that I determined from the calibration for each lens. After one week of usage I then went back and retested and compared the numbers from the first run to be sure that the system was stable and repeatable. As the offsets were all identical I was reasonably convinced that the AF system was stable and I was getting repeatable results.
The question of what distance to calibrate the focus at is a good one. Logic dictates that calibrating at the distance you shoot at most often makes sense. Indoor distances limit this to 20-40 feet or so usually and limited Depth of Field suggests close distances are more critical. I performed calibration at 20 feet with my 500mm and x1.4 and I also checked it at 80 feet. When I compared the cal values they were identical!
After several weeks when the weather got
colder I decided to test the calibration values again as cold can effect
on opto electronic system and as I often shoot outdoors in the winter I
decided to check the cal values again and was glad to see that the calibration
values of my 500 and 300mm lens with and without a teleconverter had not
changed with a temperature of -2 degrees C.
created January 17, 2009
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Site and contents © 2009 Ken Newcombe