Infinitely Distant

A conundrum—you can estimate the focal length of a positive lens by using it to project the image of a distant scene onto the wall in a darkened room. But if you try to do it by placing the lens, a focal length away from an illuminated object and looking for the point where to produces a lens-filling collimated beam, you don’t get the correct length. When you’re figuring out the behavior of images in an optical system meant to be viewed with the human eye, don’t forget to include the optics of the eye itself.