A plano-concave lens diverges parallel input rays. It has a negative focal length. While the output rays do not actually meet to form a point, they do appear to be diverging from a virtual image located on the object side of the lens. At absolute conjugate ratio greater than 5:1 and less than 1:5, plano-concave lenses are nearly the best form lens to reduce spherical aberration, coma, and distortion. The curved surface of the lens should face the object in distance or infinite conjugate, except in a high energy laser system, this should be reversed to eliminate the possibility of a virtual focus.