Some m42 lenses have a pin that needs to be pushed in to use the aperture. My mamiya 50 is one. I hot glued it pushed in. All my takumar lenses have a manual/auto switch which overrides this. It's easy to check if you have the lenses by looking if turning the ring closes it down off camera without pushing the pin. Of course a m42 to canon might not matter anyway. You don't need a flush adapter because of Canon's shorter registration distance.
Edit
I failed to mention some have a tiny pin that needs depressing to switch from auto/manual when off camera where the lens meets the mount.
Last edited by swanlefitte; 03-26-2019 at 02:31 PM.