Inspired by these instructions on making a macro lens, I decided to grab the lens from a slide projector I had lying around, and figure out how to take a picture with it.
I made a 3-foot long tube out of black bristol board, stuck the lens in one end, my Nikon D40 in the other, and took this picture of the Queen on the back of a quarter.
I could improve the setup a little, if I make it sturdier, maybe use some plumbing instead of cardboard, and bolt it to the camera rather than tape it down. The principal is pretty simple though, if you just think of how it’s a projector lens. Think of the quarter as the slide of film, and the camera as the wall you project the silde onto. So, the further away the lens is from the camera, the more magnification you get, but the more light you need.
It has a very narrow depth of field, and it would be bulky as hell to use in the field, so I’m probably not doing a lot with this, but it was nice to try out.