ABSTRACT
The lens acts as a collector of radiant energy which is distributed over the detector area, with local variations in energy determining the information content of any non-uniform image. The area of the subject that can be ‘seen’ by the detector is termed the field, and lenses have a circular field so that the semi-field angle is the semi-apex angle of a right circular cone on the optical axis, with apex at the lens and whose base is the subject field. A bundle of rays passing through a multi-element lens will converge and diverge in their passage, but one element will determine the maximum bundle diameter and hence radiant energy transmitted. The problem of ramping or f-drop is the loss of aperture and light transmission as focal length increases, especially in extreme zoom lenses for outside broadcast work. The image of a focal plane field stop as given by the lens determines the entrance window of a system.
