The focal length of the plano-concave lens is negative, and the parallel beam will diverge after incident on the plano-concave lens. When the absolute conjugation ratio is greater than 5: 1 or less than 5: 1, the plano-concave lens is the best negative lens to reduce spherical aberration, coma aberration and distortion.

The concave surface of the plano-concave lens should be oriented towards the object, but when applied to high-energy laser systems, it should be placed opposite to reduce the possibility of focusing the residual reflected light on the surface.