Calibrating 70-200 II - some questions

Folks, I wanted to ask you how the calibration is done for the lens?
I have the 5D MKIII and started noticing that I lose focus....sometimes. I thought it might be me and was observing for a couple of months. Indeed, I do not always nail the focus in good light....not a user error.

I gave it to the official Canon service (the lens and the body) and they calibrated it. I wanted to ask they what they did and they cannot really say. I thought they do some diagnostics and then can say what was BEFORE and what is AFTER.....also what was done. They say they cannot really show anything...or even explain what they did.

Do you have any ideas/experience?

Thanks!

You say that you 'lose' focus. Does that mean you have your subject in focus - and then it slowly becomes out of focus? What focus mode do you use (AI Servo or One Shot)? What do you shoot?

To test the AF performance try shooting a flat target, a sheet of newspaper on a flat wall is good, with the camera on a tripod, in good light, using a remote release. Use AF and take a few shots, then switch to LiveView and take some more shots using the LiveView focus, then take a few more using manual focus.

If the shots with LiveView (auto and manual) are better focused than those with AF through the viewfinder then you'll want to look at doing Micro Focus Adjustment.

Your Canon 5D Mark III has got an autofocus fine adjustment in the camera menu. So if you can calibrate your autofocus with this function, there is no need to send your equipment to canon.

To do this correctly, you need a correct reference to see where your lens-camera combination is focussing (you need to do it for every combination).

The one I use is the lenscal. You'll find where in the camera menu you can set it when googling for it.

One of the 'tricks' I've discovered while doing Micro Focus Adjust on my 5Diii is to use a target that is at a distance about where your subject(s) is/are when using that lens. For zoom lenses, the 5Diii has settings for each end of the zoom.

I originally did MFA on my 24-105 using a well lit target about 20 feet away shortly after I bought the camera. My first shoot with that lens seemed just a bit softer than it was before I did the MFA. So I re-did it outdoors at about 50 feet away and that solved the problem as most of my subjects are typically 30-80 feet away when I use that lens. These days, I consider that lens to be as sharp as the 24-70 f2.8L mark i I briefly had a couple years ago.

Definitely adjust the lens at your typical working distance and with similar lighting! Your camera responds differently to fluorescent, daylight and tungsten lighting as far as autofocus tracking goes. My 5D3 and 7D back-focus noticeably under industrial fluorescents, but not in daylight.

Look at Dot-Tune for an easy, quick way to calibrate a lens to your camera.