Plano-convex lenses are mostly used for focusing parallel rays of light to a spot. The asymmetry of these lenses between the front and back surface minimizes spherical aberration effectively, particularly in the situations where the object and image are not at equal distance from the lens. The optimum case is where the object is placed at infinity (regarded as parallel rays entering lens) and the image is on the focus, which has infinite conjugate ratio, i.e. object distance/image distance. The curved surface of the lens should face the object direction.