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.
Great instructions; will do! I had already ordered a + 1 diopter to play with bringing in the lens to subject distance!
I have a brass barrel 48 inch APO Artar that is missing the aperture blades. I had intended to have it mounted in a shutter, but I will never use this lens for anything. If you are interested, make an offer.