Intermittent Er20 code on 5D MkIV using M42 to EOS adapter
I am wondering if anyone has encountered this issue. I'm using Fotodiox Pro metal adapters to attach either Asahi Pentax Takumar lenses of various focal lengths OR several varieties of Helios 44 (or Carl Zeiss Jena Biotar) 58mm f/2 to my 5D MkIV. When I am using same lenses and adapters on my APS-C (crop-sensor) EOS 40D, no error issues are encountered.
No error messages occur on 5D when using EOS lenses or third party lenses with Canon mount.
After switching off the camera and turning it back on, the error message remains, ditto after removing/re-installing battery.
The best "clue" I've managed to get is that when I remove the M42+Adapter combination, I notice the 5D's mirror is "stuck" halfway. It re-positions correctly after recycling camera.
That's about as far as I've been able to diagnose this intermittent issue.