a six - element cinema projection lens is described in front of which an aspherized plane - parallel plate is positioned on the screen side . this arrangement permits the cost - effective design of a cinema projection lens with a relative aperture of 1 : 1 . 7 having very good image - forming properties . the high relative aperture increases the light utilization and therefore leads to savings in energy and costs . furthermore , the projection lens can be used both for cinemascope format and for widescreen format , the differences in brightness being compensated by a variable diaphragm .