I have a set of Vivitar Extension Tubes with Canon FD mount. I'd like to use them to make some kind of DOF adapter.

The interesting thing that I didn't realize is that there are threads on the inside of these tubes which could provide me with some options where I don't have to take some of these apart.

But I have no idea what size the threads are. The hole is about 31mm across as far as my trusty ruler tells me, but the threads are rather coarse and do not seem to be nearly as small as the threads of something like a filter.

I've attached a photo. If anybody knows anything about the thread size or where I might find more info, I'm more than happy to hear it.
Thanks

Is that a thread or parallel rings turned into the inner cylindrical surface for anti-reflection? Anything you put in there may obscure the optical path and cause a vignette.

Is that a thread or parallel rings turned into the inner cylindrical surface for anti-reflection?

I used to think they were rings but they are, indeed, threads. In fact, I have a 2x telextender which seems to actually utilize the threads. The 2x magnifying element inside of the telextender appears to have been threaded into threads just like that.

Anything you put in there may obscure the optical path and cause a vignette.

I was actually hoping to just use it as a mounting method. Additionally, I was thinking I could use it to 'adjust the backfocus' by letting me set the extension tube and lens closer or further away from the focal plane in small increments.

Thanks