There are a couple of possible methods for AF Microadjustment. In all cases, you've got the camera on a tripod with good, even illumination of the focus target - same target for all testing. Use Av or manual mode and a low ISO, and adjust with the aperture wide open.


Here is Canon's recommended procedure (via Chuck Westfall's Tech Tips - scroll down to the second question). The 50x focal length distance to target would be 14 feet with this method, test target would be a sheet of text or something like that.


Personally, I use a LensAlign Pro, and follow their instructions. Their recommended distance to target is 25x focal length, 7 feet for an 85mm lens.


You can also blend the two procedures (and save $$) by using a printed chart like this one at a 45° angle to the camera - follow Chuck Westfall's steps, but instead of the flat target, tape the sheet to a door that you can open to a 45° angle. In choosing between the differentsuggested distances, you can use your normal shooting distance with the lens as a guide (e.g. if you usually shoot tight portraits with it, adjust at 7 feet).


Also, the 85mm f/1.2L II is pretty sharp wide open, but it does get noticeably sharper when stopped down to f/2 or so. That means it's important to differentiate between slightly soft due to wide open aperture, and soft due to missed focus. The focus testing will help sort that out.


After you do the AF adjustment, definitely shoot some 'real world' shots to make sure it works in practice.


I can tell you that almost all of my lenses have some amount of AF microadjustment applied, ranging from -5 to +6 depending on the lens.


--John