My "Pinhole Camera" post seems to be one of the more popular items on this blog. This morning, I tried something a little different. I placed a sheet of aluminum foil over a C-mount adapter, then poked a tiny hole in the middle of the foil. This turned the camera into a (rather expensive) pinhole camera. No lens required! Here's my old 10D with the setup:
After a little trial and error, I found that an exposure of about 13 seconds at ISO 800 produced a reasonable exposure.
You can see that the super-tiny aperture allowed for all the dust particles on the sensor to show up as dark gray blobs. It's been a while since I've cleaned the sensor (it's an old model that doesn't come with the auto-cleaning option). Also, the image is a little soft - if I had made the hole a little smaller, more round, and sanded the edges smooth it would have come out much sharper. But as a proof of concept, it worked wonderfully.