Canon suggests using the procedures below to perform test shots, to see which of the AF Microadjustment settings is best for you. Always remember: you can return to the factory default AF settings in two ways, either by setting AF Microadjustment Custom Function back to option "zero" (this doesn't clear any of your Microadjustments from memory, it just ignores them until you re-activate the "Adjust all by same amount" or "Adjust by lens" options on the C.Fn menu), or alternatively by going into one of these AF Microadjustments, and dialing your setting back to zero on the +/- 20 step scale. This last action will clear your previous settings from the camera's memory.
To change an AF Microadjustment, you need to take test shots of a 3-dimensional subject, where you can precisely check whether focus is exactly where you want, or if it's occurring behind or in front of your subject. Simply shooting squarely into a flat wall won't tell you that; shooting at an angle might. Keep the following points in mind when taking these test shots:
• Shoot test pictures using the lens(es) you normally use, and at the distances you typically use them. In other words, if you're seeing a consistent focus shift when you shoot group pictures at weddings, don't take test shots of a ruler on your desk with a macro lens.
• Use Av mode, and always shoot the test shots with the lens aperture wide-open - regardless of whether you normally stop the aperture down or not. You'll be much better able to see any subtle focus changes wide-open, than with the lens stopped down.
• If you're using a zoom lens, zoom to its maximum telephoto focal length for test pictures. AF Microadjustment can only apply one correction to a zoom lens, so you cannot have one adjustment for the lens's wide-angle setting and another one for the same lens when it's zoomed to telephoto.
•Even if you're a full-time sports shooter, do not use AI Servo AF mode for these test shots. Shoot a completely stationary subject, with the camera set to One-Shot AF. There are far too many other variables involved in focus-tracking with AI Servo AF to get conclusive results for AF Microadjustment taking test shots this way.
•If at all possible, use a tripod to keep the camera absolutely positioned on one part of your intended subject, and also eliminate any potential camera movement from entering the mix.
•Manually select only the Center AF point (regardless of whether this is how you typically use the camera), and be certain that any AF point expansion is completely disabled. Do not, under any circumstances, use Automatic AF point selection mode - this can definitely lead to unpredictable results.
•Be sure the center AF point is solidly upon part of your subject with sufficient detail, and that there's adequate detail in front of and behind your target to assess whether focus is indeed occurring in front of or behind what the center AF point is seeing.
Since the values for each step on the AF Microadjustment scale are so fine (again, only 1/8th of the forward or backward depth-of-field!), it's best to start your test shots with major adjustments like plus or minus 20, and then work your way back to finer values if necessary. If you feel your camera focuses behind where it should, for example, you might take two initial test shots at minus 20 and minus 10, and see how your test subject appears when closely examined. If you start with very fine increments (such as plus 3 or plus 5, for example), the changes may be so subtle that you'll have trouble detecting them.
When you get test shots that seem to place the focus dead-on, note the adjustment value in place and be sure the camera is set there on the +/- 20-step scale. AF Microadjustment is complete. Go out and shoot some real subjects!