Canon’s Digital Learning Center published a long article about auto-focus microadjustement, a useful feature you find on Canon’s higher level DSLRs, and a feature that was thankfully re-introduced to the xxD line-up with the Canon EOS 70D.
From the introduction:
What does AF Microadjustment do?
It allows the user to command the camera to intentionally shift the sharpest focus either in front of or behind where it’s factory-set. The extremely precise AF system in a digital SLR is designed to read contrast at the subject, calculate how to drive the lens to focus sharply on the subject, and confirm sharp focus once the lens has stopped. With AF Microadjustment, the user is changing the data coming from the AF system, and asking it to move the lens farther in one direction or the other whenever it has to read and calculate sharp focus.
The adjustments applied using this control are based on the depth-of-field you’d have at a lens’s maximum aperture. They are not based on the lens’s focal length! When setting the Microadjustment, you’ll see a scale on the camera’s LCD monitor with up to + or – twenty steps. Each step is a very fine increment, equal to 1/8th of the depth-of-field you’d have with the current lens wide-open. And that 1/8th of the depth of field is only moving forward (toward the camera) or back (toward the background) from the sharpest plane of focus. The main thing to remember here is that these are very fine increments. Don’t expect radical shifts in focus with adjustments like plus 3 or minus 5.