In turn, the mirror lens reflects the light back through a hole in the concave mirror to a focus on the film. Mirror lenses have the advantage of long focal length, relatively short physical size, and large aperture.
For a spherically curved mirror in air, the magnitude of the focal length is equal to the radius of curvature of the mirror divided by two. The focal length is positive for a concave mirror, and negative for a convex mirror.
A special version was the Sellar finder which consisted just of a concave mirror with targeting aid. Old SLR cameras have a bright reflecting type finder with matte screen that uses the same lens as the camera uses for exposures.
 See also: Camera, Photograph, Image, Lens, Focus
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