Adjusting sigma lenses

Probably a stupid question, I'm micro adjusting my sigma 50mm ART at the weekend, and it gives distances to calibrate at, but where are these distances measured from? From the back of the camera to the target, the lens mount, from the front element? Can't seem to find this written down anywhere??

I did mine from the focal plane mark and it worked perfect. It's a circle with a line through it.

Check out this article.
http://petapixel.com …bol-on-your-camera-means/

Thank you! That makes perfect sense, I knew it couldn't be the front of the lens as that changes depending on what lens you've got on!

It would be at the focal plane mark (sensor) but really, it does not need to be that accurate. As long as you're at about 50x the FL, you'll be fine.

God, the sigma software is terrible! It repeatedly checks the firmware, lens settings, then does it again, and again, and again, eventually the customization buttons stops being greyed out, you click on it, then focus settings, then it kicks you out and starts over again, and again, been here 20m now, haven't managed to click the adjust focus button yet!