Planoconvex lenses are usually used to concentrate parallel light. The front and rear surfaces of plano-convex lens are asymmetrical, so spherical aberration can be effectively reduced, especially in applications where the distance between objects and images is not equal. The best application is to place the object at infinity (i.e., parallel light incidence) and image it in focal plane. At this time, the object-image conjugate ratio (i.e., object distance/image distance) is infinite. The convex surface of the lens should be placed towards the object side.