Originally Posted by

**Maris**
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.