Here’s a little trick to get your lens focus-calibrated quickly. This discussion is only relevant for phase-detect focus. It’s well known that “Live View” focus is quite accurate, since the camera sensor itself is used, bypassing any mirrors and separate phase-detect sensors. This article doesn’t, of course, have any relevance to mirrorless cameras. There are some cameras that can focus-calibrate themselves, so this doesn’t apply to those cameras, either.
Pay attention to that focus distance!
The main requirement for this calibration trick is to have a lens with a focus distance scale on it. You should also use a tripod, if possible, to get reliable results. Pick a focus target that is easy for your camera to use, so that it will focus at the same distance each time.
Do yourself a favor, and make sure that you have sufficient illumination so that your camera focus system doesn’t have to struggle and hunt to find focus.
Set your camera’s aperture (typically wide-open), and then activate Live View. Now, focus your camera on the target. Note the precise distance on the lens focus scale. Repeat this exercise several times, in case your lens can only focus within a range of distances. Between each focus, manually change the focus distance, to force your camera to re-focus each time. It should only take seconds to find out the focus distance reading to use.
Now, you know the “real” focus distance setting that is accurate, since it was done using Live View. Next, turn off Live View, to switch over to phase-detect focus, ideally with single (versus continuous) auto-focus. Auto-focus the camera. Note the reading on the focus scale.
If the phase-detect focus distance value matches the Live View focus distance, then you’re done. If there’s a difference in the distance reading, however, then you’ll need to enter a focus calibration value into your camera. If the phase-detect reading was as a shorter distance than the Live View distance scale setting, then you’ll need to calibrate with a “+” setting, to push the focus further from your camera. If phase-detect focused farther than Live View did, then obviously enter a focus calibration value that’s smaller than what you had previously set for the calibration setting.
In this fashion, you can iterate on focus calibration settings that will quickly get your lens into perfect calibration. You don’t need to fuss with trying to view photos at high magnification to determine if you got the focus right; all you need to concern yourself with is to match the distance that Live View got. Simple.