television optics, it is the effective diameter of the lens that
controls the amount of light reaching the photoconductive or photo
emitting image pickup sensor. The aperture of a lens controls the
amount of light which can reach the pickup device (image sensor when
relating to video). The size is controlled by the iris adjustment. By
increasing the F-number (F1.2, F1.8, etc), less light will be permitted
into the sensor.