System Aperture

The system aperture is the overall system F/#, Entrance Pupil Diameter, Numerical Aperture, or Stop Size. Any of these 4 quantities is sufficient to define the other 3 for a particular optical system. The system aperture is used to define the object space entrance pupil diameter, which in turn is used to launch all rays. The system aperture is always circular. Rays may be vignetted after being launched by various surface apertures. There is only one system aperture, although there may be many surface apertures.