That's why god invented autofocus.
Autofocus is indeed a godsend in many situations, but one must remember that it is not exact but depends on depth of field to allow for minor errors. This
DPReview Thread discusses autofocus tolerances for Canon cameras, and gives an link to an excellent
Doug Kerr article discussing the matter in more detail. The higher accuracy Canon spec says that the tolerance is within 1/3 of the depth of focus, using the Canon specified Circle of Confusion (COC) for determining the depth of focus. See the Doug Kerr article for a discussion of how depth of focus relates to depth of field and how the depth of focus relates to various standard depth of field charts, which use a rather liberal COC which is too large for demanding situations.
One should also remember that autofocus focuses on the plane of the focus sensor, which may not coincide exactly with the plane of the iamging sensor of the camera due to manufacturing tolerances. Lens tolerances also come into play and the more advanced DSLRs allow fine tuning specific for the lens.
The advantage of live view is that it determines focus in the plane of the CCD or CMOS sensor, not the autofocus sensor and allows viewing of the image at high magnification. With liveview one can check for focus using interference (Moire) patterns and an expensive apparatus such as LensAlign is not needed, as explained
here.
My first experience with liveview was a revelation. I was attempting photomicrography using using my D3 attached to a Zeiss microscope using a 4x Planapochromat ojective, which has a very narrow depth of focus (unlike regular photography where wide angle lenses have a great depth of focus, depth of focus is least at low power in microscopy). In this setup, it is best to use live view with a program (Camera Control Pro 2) which allows viewing the live view on one's computer screen rather than on the LCD of the camera. I was able to obtain the best low power shots that I have ever been able to achieve.
A similar setup could be used for testing focus of regular lenses with an interference target as described in the above link. In the field, one would likely use the camera LCD as Bernard describes. Does the P65+ back allow liveview and fine tuning of autofocus?
