You've probably never heard of the Itorex Pan-Focus 50mm f40 - yes, really forty, not 4.0! It's basically a variant on the "lens in a body cap" idea, the difference being that it's much older and is designed to fit into a T-mount. As supplied by the original manufacturer they come with an F40 aperture, a 20mm metal disk with a teeny hole, which can be removed by unscrewing the rear element if you want to use the lens in low light and don't mind a ridiculously unfocused image. It doesn't actually focus at all, with the F40 stop it was supposed to be in focus from 6" to infinity.
One recently came my way in a job lot of filters etc. - the snag was that the stop was missing, and one test without it made it clear that it was not going to be much use. I decided to make my own; the snag was that I no longer have a good workshop available since I retired, so accurately making them out of metal was probably not going to be possible.
After a couple of trials making the disks out of cut-up soft drink cans I realised that the hardest part was making an exact 20mm disc. Fortunately there's a good source for 20mm black plastic disks - you can buy them in packs of 50 on eBay, sold as counters for games. This has the additional advantage that you can melt holes rather than drilling them, with was my preferred method for getting the pilot hole centred.
Since I had so many counters I decided to make a set of stops, like the ones sold for Lensbaby cameras. I was going to cover all apertures, but it became obvious that anything wider than F8 was just a fuzzy mess. In the end I made f40, f22, f16, f11, and f8 - since the plastic was about 1.5mm thick I used a countersink bit to give the openings a sharper edge.
The end result... well, it's still pretty horrible at all apertures, you get much better results with any real 50mm lens. But F16 and F22 were just about usable if you didn't mind the limitations of the crappy optics, and the softness of the wider apertures is actually quite interesting. Here are a few photos showing the lens, the stops I made, and some pictures I took with my D50 - I was hand-holding the camera on a day with very variable lighting and didn't have time to get exposure exactly right, but I hope these pictures show the general idea.
First the lens and the stops I made:
Next some photos of the view from my front door - not the most exciting vista, but there are some nice distant buildings with plenty of detail. These are on Photobucket, so reduced in size, but you're not missing much!
F40 - 1/30th second
F22 - 1/125th second
F16 - 1/250th second
F11 - 1/500th second
F8 - 1/800th second
No stop (f3.5-ish) - 1/4000th second
If anyone else has one of these lenses you might want to try experimenting with stop size - f40 is simply too slow to be useful for most purposes, and performance is about as good as a cheap Instamatic or similar at F22 and F16, which I suppose is more useful than a blank lens cap.