In photography, what term is used to express the diameter of the entrance pupil in terms of the focal length of the lens?
Entrance pupil location on the optical axis may be behind, within or in front of the lens system; and even at infinite distance from the lens in the case of telecentric systems. In photography, the size of the entrance pupil (rather than the size of the physical aperture itself) is used to calibrate the opening and closing of the diaphragm aperture. The f-number ("relative aperture"), "N", is defined by "N = f/E", where "f" is the focal length and "E" is the diameter of the entrance pupil. Increasing the focal length of a lens (i.e., zooming in) will usually cause the f-number
Focal length lens, but the term is often used to describe any long focal length lens. Due to the popularity of the 35 mm standard, camera–lens combinations are often described in terms of their 35 mm-equivalent focal length, that is, the focal length of a lens that would have the same angle of view, or field of view, if used on a full-frame 35 mm camera. Use of a 35 mm-equivalent focal length is particularly common with digital cameras, which often use sensors smaller than 35 mm film, and so require correspondingly shorter focal lengths to achieve a given angle of view,