I recently sold a Rokkor 58mm 1.2 to a fellow FMer. I converted this lens with a brass Contax adapter with a Focus confirmation Chip. He has a 1d mark III and some how the red dot (looking at the lens/camera from the front) is at the 6 o clock
postion and can be moved from the 5 o clock to 7 o clock, but no further in either direction.
Does your adapter has stop screw preventing the lens to be "over rotate" during the mounting process?
Sounds like that might be the problem.
Since there is no photo of your modified mount, I can't tell if that person mounted the lens wrong way as the red dot did not aligned.
When I make the converted mount, I made sure that red dot aligned on both the red dot on camera body and on the lens barrel aligned before I mark the screw hole for drilling on the adapter.
Is the lens still stuck?
You can use a large monkey wrench to force it off. The camera mount is stronger so it should be okay. The "custom" mount would get damage and the mirror and the lens sensor on the body might get damage as well. I don't see any other way other than force it off.
Alternative is to yank it off with thin screw drivers and break your custom lens mount. This might protect the sensor and the mirror.
The question would be: Are you covering the damage cost?
I wouldn't recommend something that dramatic except as a very last resort. If he mounted the lens in the wrong position due to inexperience, I wonder how he got it on so easily? And if he mounted it in the right position, and it rotated to the wrong position, why won't it just rotate back? Something is missing in the description of the problem.
I have similar action once on 450D with my Minolta 35-80mm zoom converted to EF mount. One of my screw was not mounted correctly, and I think, it loosened once on the camera.
But again, as mentioned by "carstenw', the details are not plenty enough to provide the solution/views. Post some pics of camera-lens and details of lens.
If this is it: https://www.fredmiranda.com/forum/ufiles/39/291839.jpg
I don't see the stop screw. Looking from behind with the red dot at 12 o'clock, the stop screw should be at about 10 o'clock. If the lens goes past that, the AF chip goes past the camera pins and then they may spring up, preventing back turning.
Brut force will probably limit damage to the AF chip and more importantly the AF contact pins of the camera body. I don't see a risk free solution.
