There’s two widely used standards for small lenses, C mount and CS mount. They can both screw into the same socket, and it can be tricky to figure out which is which.
If you get it wrong you’ll end up with a picture that’ll never be in focus, no matter how you try and twist it.
I spent a few days recently making an optical system, and I got fed up with confusing my lenses together. My original way to figure out the focal distance was to squat down, eyes level with the table and then use a ruler to measure the height to the flange when the image was in focus. But this gets pretty tiresome after a while.
After a few mintunes scribbling and laser cutting I came up with this:
This box allows you to check your lenses in under 5 seconds. Just hold the lens up to a hole, and see if an image is formed on the plate. If not, flip the box upside-down and try again.
It’s rather cool how clear the image turns out, even on a rough surface like the bamboo:
Files are up here: