A plano-concave lens converts a parallel beam of light into a diverging beam of light, which is back-focused at the imaginary focal point of the plano-concave lens with negative focal length, where one side is flat and the other side is concave. Flat-concave lens can be scattered from the imaginary focus of a collimated beam, usually used in Galileo type beam expander. When used, the concave side faces the direction of the incident light, and the flat side is oriented toward the focal plane to be dealt with. Flat concave lenses have a negative focal length and negative spherical aberration, and can be used to counteract the aberration of other lenses in the system.