One thing I have noticed when taking pics at magnifications around 2:1 or higher with manual focus is that I have a tendency to backfocus the image.
I spent quite a lot of time checking the dioptre settings of the eye piece but the problem remains. Interestingly the problem only exists for 3D shapes and not 2-D flat surfaces such as a printed page.
I tend to correct for this by intentionally front focusing the object when shooting bugs.
I think I have just realised what is happening .
Obviously at higher magnifications the DOF is extremely narrow (especially with the lens wide open when focusing). I have a feeling that with a 3-D curved surface, when actually in focus at the nearest point, the brain perceives this as not being in focus because there is only a very small part of the object is actually in focus, so you tend to move the lens forward to get more of the image in focus. This results in the image being back focused. So my attempt at front focusing the image is actually giving me the correct focus.
It would be nice to use the DOF preview button but at high magnifications the image is very dull anyway.
Sorry for the ramble but thought it worth mentioning to stop you going mad calibrating the focus as I have been doing recently.