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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.

  • Never tried adapted lenses on EF mount cameras. But if the mirror is getting physically stuck only when using the adapters, then there is a physical clearance issue somewhere. If there's nothing physically obstructing the mirror's path, then something is causing the camera to believe there is. – Michael C Nov 30 '20 at 12:00
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I have used adapters for both Pentax M42 and Nikon lenses on my Canon EOS 5D Mk III. Some lenses will have mirror interference at infinity or long focus distances, where the rear element moves closer to the image sensor. This is common with all Full Frame cameras, but not a problem with APS-C Crop cameras due to the smaller mirror.

Using LiveView will eliminate this problem. The mirror still needs room to swing up out of the way, so before using LiveView, adjust the focus to a closer distance to move the rear element away from the sensor.

Some people will even go to the trouble of shaving the mirror to get the necessary clearance.

Here is a link to a compatibility list showing 401 different lenses that I think you will find very interesting: Compatibility list of M42 and manual lenses on Canon EOS 5D DSLR

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  • One caveat is that the 5D Mark IV mirror may or may not be precisely the same size as on the original 5D. Officially, the 5D Mark IV sensor is a fraction of a mm larger, so the mirror may be ever so slightly larger as well. At least one lens I used to own would hang at infinity on the 5D Mark IV, but didn't hang on the original 6D, which has the same sensor size as the original 5D. Either way, that list is probably close enough. :-) – dgatwood Dec 2 '20 at 17:02
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I had some of that gear a while back. On the 5D Mark IV, the Helios 44M-4 worked correctly without sticking; the 44 would snag the mirror at infinity. I seem to recall that it didn't stick on the 6D because the mirror was slightly smaller (or maybe it just didn't snag as often/badly), but I could be remembering wrong.

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