For a double concave lens where the rays are diverged, the principal focal length is the distance at which the back-projected rays would come together and it is given a negative sign.

This use of light may support speculation among art historians that Vermeer used a mechanical optical device, such as a double concave lens mounted in a camera obscura, to help him achieve realistic light patterns in his paintings.

The fact is, however, that biconcave lenses are useful in correcting near- sightedness, and spectacles for that purpose began to be used about 1450.