How does lens calibration work

I've found that one of my lenses tends to miss focus quite a lot or focus very slightly behind the subject especially when comparing with manually focused images. It focuses on the subject but slightly behind or in front of where I would prefer using the smallest focusing point on my Olympus E-m1 mirrorless. Other lenses did not exhibit the same thing on the same body. The differences between autofocus and manual focus were present in more than half my tests. I did some tests that were not totally conclusive but enough to show a pattern so I sent it in for a check.

 

They said it was fine but suggested a "focus adjustment". I ask for clarification but they were not clear. They did say it was just an "electronic" fix rather than a physical one which I took for being some kind of firmware or software reset of the lens.

I understand if it's a minor thing and still operates within the tolerances set my the company for this type of thing. I'm not sure if their proposed "fix" will take care of the issue I'm seeing. I've already reset the firmware twice.

 

 

It's a mirrorless camera, Olympus E-m1 with a 12-40 lens.

 

**As the title suggests, I'm trying to find out about lens calibration and how it's done. I would like to keep it form going into other related topics like how to do further tests for my issue or how to calibrate the body instead. I've done my tests and searches and I'm trying to understand this particular aspect of it. Thanks.

This may not be much help, as I have no knowledge of Olympus kit, but on my Pentax K20D there is a way of adjusting focus for up to 20 lenses individually. I believe this may be what is meant by lens calibration.

HTH

Generally you shoot a pattern set at 45 degrees from horizontal facing the camera (like a ruler propped up) or one of the scaled ones you see for free on the net. You put your camera on a tripod and determine which spot on the measuring device you want to use as the zero point. Open the lens to its widest aperture, focus on the zero point, take a shot, and determine if focus is ahead or behind that spot. Most DSLR/mirrorless bodies have a focus adjustment function somewhere in their menu. So if your shot showed back focus, you dial in moving the focus forward. You keep repeating the process until you are dead on. This works great for prime lenses, not quite as well for zoom lenses.