3 Custom Function Settings
C.Fn III
Normally, this adjustment is not required. Do this adjustment only if necessary. Note that doing this adjustment may prevent correct focusing from being achieved.
You can make fine adjustments for the AF’s point of focus. It can be adjusted in ±20 steps 
Forward / +: 
Backward).
The adjustment amount of one step varies depending on the maximum aperture of the lens. Adjust, shoot, and check the focus, then repeat to adjust the AF’s point of focus.
With setting 1 or 2 selected, press the <B> button to view the register screen. To cancel all the registered adjustments, press the <L> button.
0:Disable
1:Adjust all by same amount
The same adjustment amount is applied to all lenses.
2: Adjust by lens
An adjustment can be set individually for any particular lens. Adjustments for up to 20 lenses can be registered in the camera. When a lens whose focus adjustment has been registered is attached to the camera, its point of focus will be shifted accordingly.
If adjustments for 20 lenses have already been registered and you want to register an adjustment for another lens, select a lens whose adjustment can be overwritten or deleted.
When you adjust, shoot, and check the focus to make the adjustment, set the image size to JPEG Large and the JPEG quality (compression) to 8 or higher. It is best to make the adjustment at the actual place where you will shoot. This will make the adjustment more precise.
With setting 2, if an Extender is used, the adjustment will be registered for the lens and Extender combination.
AF adjustment cannot be done during Live View shooting in Live and u Live modes.
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