Canon EF Mount to Filter Wheel Adapter

In the spring of 2024, a colleague asked if I could help him make an adapter that would allow the use of modern, bayonet (EF) mount Canon glass on a filter wheel—the total solar eclipse was coming up, and he wanted a setup that he could use to perform polarimetry with.

My colleague had purchased an off-the-shelf EF adapter that he wanted to modify in some fashion to allow use with the filter wheel. I decided that making an adapter with threads on one side (to thread into the filter wheel) into which the EF mount adapter could press would be the quickest and easiest option.

After coming up with a design, I started by turning one side of a slug of aluminum on the lathe. The off-the-shelf lens adapter that my colleague had purchased had a small knob that protruded from the side of it, so before flipping the part around in the lathe, I took it over to the mill to make a groove to account for the knob.

With that done, I could then flip the part around and start machining the other side.

Threading this part required some trickery. I needed to produce a 3″ thread very near a shoulder. The trick was to use a tool normally intended to produce internal threads (with the spindle turning in the usual counter-clockwise direction) to instead produce external threads by running the spindle backwards (clockwise). This would cause the tool to travel away from the chuck instead of towards it, eliminating the possibility of a crash.

With threading complete, I took the part out of the lathe to allow test fitting it into the filter wheel. I did end up needing to take another cut to get it to fit into the filter wheel nicely. To ensure that I would begin cutting at the right spot, I marked the part where the jaws of the chuck touched it, noting which jaw was #1. My marks didn't have to be perfectly precise as I'd be removing additional material, anyway.

Once that was done, I press fit the off-the-shelf Canon EF adapter into it:

And finally, a shot of the adapter mounted on the filter wheel, complete with lens:

Close up: