M42 and the Canon EOS 100/Eos Elan

A few weeks ago I gave my friend a great Pentax Super-Multi-Coated Takumar 55/1.8 lens. I've got 2 other similar M42 lenses, the SMC Tak 50/1.4 and the Zeiss Jena 50/1.8, so I figured he'd get more use out of than I would, especially after he mentioned that he intended to get a 50mm lens. He's also into shooting manual, so he would probably enjoy using the old M42 lenses.

So 2 weeks ago, I handed him the lens, and we went to Apliu Street and bought a M42 adapter for his old EOS film camera, and then he had somewhere to head off to. Last weekend he tried the lens and said it didn't work. The camera made a funny noise, locked up (he had to turn it off and on again) and the film wouldn't advance.

So I looked it up on the internet, and didn't see any indication that this setup shouldn't work. I was curious.

After meeting him on Monday night to look at the camera, I figured out exactly what the problem was. He had bought (while I was there, not knowing the difference)an M42-Eos adapter that looked like this:



There is a ring in the middle meant to push down the aperture pin for when lenses that are "auto only", meaning you can't switch them to manual aperture setting.

Unfortunately, it turns out that this ring interferes with the mirror of the film EOS body, which is larger and sticks out more when it flips than the mirrors on the digital bodies. While this adapter has a clear advantage over one without the ring for digital bodies, it is unfortunately incompatible with this model of EOS film camera, the EOS 100 or EOS Elan.

A normal M42 adapter without the stop-down ring should work fine. It would look like this:



I didn't find any other mention of this on the internet, so I thought I might post it here in case anyone is searching for a solution to the same problem.