Notes to someone who wants to do this:
- I modified the lens to be mounted to a toilet paper tube, so I could focus the image. I also cut a hole in the back of the box and taped some waxed paper over the hole so I could see if the image was in focus before taping the sunpaper to the back.
- Make sure your hands are dry before touching the sunpaper! You will remove the chemical if you do get it wet.
- You need bright sunlight for this to work. I just did a 1 1/2 hour exposure in bright (but cloudy) weather, and the image was severely underexposed.
- Lemon juice doesn't seem to make that big a difference to the end colors.