Matthew's suggestion of Surplus Shed is a great one. However, they don't seem to have quite the lens needed for Tempest's camera obscura. The 4000 mm lens is a bit too long, even for infinitely distant subjects. Their L 3855 is too short at 2600mm, but would focus on nearby scenes. It is larger and faster, and would form a much brighter image. It's depth of field would be much less. It could be used to focus on a screen a short distance from the far wall. Perhaps you can find a combination of two thrift store lenses of nearly the same power, one positive and one negative, to combine for the desired focal length.
I have set up several walk-in camera obscura rooms by blocking a window and inserting very long focal length lens in the opaque window screen.
Making a long focal length lens is easy and cheap and the image quality is ok because the large focal ratio minimises aberrations. I get two spectacle lenses, say +3 and -3 dioptre, from my local optometrist at about $20 each. When held together the powers cancel and the focal length is infinite. When separated, positive converging power (adjustable!) appears according to the distance between the lenses. The mathematical basis is given in Gullstrand's Equation.
Thank you, Maris! That's the best help yet on the subject. I suggest getting a variety of screw-in close-up lenses, all of one diopter rating and size, because of their cheapness and ease of mounting. Probably inconsistancies in their actual focal length will result in a combination with the desired focal length. I have several, and may experiment tomorrow if the weather is decent.
Excellent. I wondered what happened when they are separated. I knew that the method of adding dioptres I posted above was only for lenses next to each other or very close.
Originally Posted by Maris