I modified my EF-S 10-18 STM last night so it will mount on Full Frame cameras. This just involves unscrewing the plastic lens mount and cutting off the plastic part that extends too far into the mirror box area of the camera lens mount. I glued the pieces back together so that there is not a large opening in the back.
The mirror will hit at 10mm, vignettes badly at 10-13mm, but works jut fine at 14-18mm on a Full frame.
Today I was playing around with my 5D Mk III and have discovered a slight problem........
For some odd reason it will not do Phase Detection AF on my 5D3. It works fine in LiveView using Contrast detection AF. When I mount the lens, the "One Shot" message on the top LCD goes to "M Focus". When I switch to LiveView or Video, it goes to "One Shot", but as soon as I go back to using the viewfinder, it goes back to "M Focus". Not only will it not AF, but there is also no green dot focus confirmation in MF mode.
I also have a 18-135 STM that does the same thing when mounted on my 5D3. Both lenses work perfectly on my SL1/100D and also my old EOS 630 film camera. The EF 40mm STM also works perfectly on my 5D3.
I went down to my local "Henry's" camera store and they were nice enough to let me try my modded 10-18 on one of their 5D3's and sure enough it behaved exactly the same way.
The 10-18 works just fine with the 6D as reported by "Bright Colours", so I suspect this may have something to do with the advanced AF design of the 5D3. I also suspect that any other FF camera with advanced AF (like the 1Dx) might also have the same problem with these APS-C STM lenses.
Any thoughts as to what's going on with the 5D3's PD AF?
14mm on 5D3