Make: Volume 91

For my bedroom obscura, the lens required

is 1/199 meters (window to roundabout, using

Google Earth) + 1/3.575 meters (window to

bedroom wall, measuring tape) = 0.28 dpt.

Unfortunately, standard optician lenses only

come in 0.25-diopter increments. Custom lenses

are expensive to make. However, an optician I

worked with had a great idea: he bought a pack of

cheap 65mm-diameter lens blanks (the circular

lenses from which shapes are cut to fit glasses

frames). He then measured their actual strength

using his focimeter lens-testing machine (aka

lensmeter). Sold as 0.25 dpt, their actual strength

varied due to production errors. One of the lenses ...