jcolwell
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Here's what the Canon 1DIII white paper says.
AF Micro-adjustment
AF precision is adjusted for the camera and lens to fall within the lens' maximum aperture's depth of focus. However, there are users who want to adjust it more minutely. They have had to go to a Canon Service Center to have it done. AF micro-adjustment is a feature developed for these users. The user himself can now finely adjust the AF focusing position. The adjustment range is ±20 steps in front of (-) or behind (+) the point of focus
The adjustment increment of one step differs depending on the maximum aperture of the lens. You should shoot, check the focus, and adjust it. Repeat this procedure to adjust the point of focus. When a lens registered with a point-of-focus adjustment is attached, the point of focus will be automatically shifted by the correction amount set. If you set 1 (Adjust all lenses by the same amount) or 2 (Adjust by lens) and press the INFO button, the adjustment screen will appear. The focus shift amount per step is calculated by multiplying the maximum aperture's single-side depth of focus by 1/8. If 1 is set, the focus shift amount will always be the same number of steps (but the actual amount will vary according to maximum aperture) for all lenses. If 2 is set, the focus shift amount will change for each different lens. You can register the focusing shift amount for up to 20 lenses. Then, when you use one of the registered lenses, the focus will shift by the set amount.
Note that since the camera does not recognize the unique ID of the lens, the same shift amount will be applied to the same lens model even if it has a different serial number. In the case of zoom lenses which have variable maximum apertures, the focus shift amount is technically different at the wide-angle end and telephoto end. However, since the focus shift amount cannot be adjusted individually for the wide and telephoto ends (there is only one shift amount per lens), adjusting it for the telephoto end is better. If an extender can be attached, the camera recognizes whether it is 1.4x or 2x and makes a different shift adjustment from when no extender is attached. When an extender is attached, the adjustment screen will display the lens name and extender name. To delete the registered lens settings, select 1 or 2, then press the Erase button. All the registered AF micro-adjustment settings will be cleared.
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. I avoided this site for almost 4 months and as soon as I come back I am tempted to buy the 70-200mkII and 5D2.