Why do some M42 lenses cause an Err 20 or mirror snag on a Canon 5D Mark IV with an adapter?
Asked 11/30/2020
5 views
2 answers
0
I’m adapting several M42 lenses to a Canon 5D Mark IV using metal M42-to-EF adapters. With some lenses, especially certain Helios 44/Biotar-type 58mm f/2 lenses, I intermittently get Err 20. When I remove the lens and adapter, the mirror sometimes appears stuck partway up until I power-cycle the camera. The same lenses and adapters work fine on an EOS 40D, and Canon EF-mount lenses do not cause any errors on the 5D Mark IV. Is this likely to be a mirror-clearance issue with full-frame bodies, and are there any safe workarounds?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
2 Answers
4
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
Originally by user39427. Source · Licensed CC BY-SA 4.0
user39427
5y ago
0
Generated from our catalog & community — verify before relying on it.
Yes — based on the reported behavior, this is most likely mirror interference from certain adapted M42 lenses on a full-frame EOS body, not a general adapter or camera fault. On full-frame cameras like the 5D series, some M42 lenses have rear elements that protrude far enough at infinity or longer focus distances to catch the larger mirror as it swings. That can trigger errors and leave the mirror stuck until the camera is reset. APS-C bodies such as the 40D usually avoid this because their mirrors are smaller.
This is lens-dependent: some Helios versions clear the mirror, while others do not. A practical workaround is to use Live View, but first focus the lens closer (not at infinity) so the rear element moves farther from the sensor and mirror path. The safer long-term approach is to check lens-by-lens compatibility on full-frame before use. Avoid forcing the shutter if a lens is known to snag the mirror. Some users modify equipment for clearance, but that carries risk.
Recommended products
UniqueBot
AI5y ago
Your Answer
Related Questions
Why does my Helios 44-3 bind on an M42-to-Canon EF adapter while my Takumar mounts crooked?
Will an EMF AF-confirm adapter work with a Helios 44-2 on a Canon 6D?
Can an M42-to-Nikon F adapter reach infinity focus without optics by recessing the lens into the mount?
What helicoid adapter setup is needed to mount a Kodak Vest Pocket lens on a Canon EF-M camera?
What are M42 lenses, and can I use them on a Canon T2i?