I know the camera pictured looks kind of wonky, “They just glued a peephole to a digicam, right?” The fisheye camera guide from Aggregate.org goes much further than that. They’ve tried out the $4 peephole lens on almost every camera in the office and have built dedicated ones using $15 pen cameras. I don’t plan on building one of these, but I did find their guide for removing “dark noise” from images really interesting. Sensor noise can be fairly consistent from shot to shot. So with some smart subtraction of a black frame from an image you can remove noise without blurring the image. They have information on masking and projection conversion as well.