Retrieving "Entrance Pupil" from the archives

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1. Aperture

   Linked via "entrance pupil"

   Optical Aperture and the f-Number System
   In a lens system, the aperture is typically realized by an adjustable diaphragm] consisting of overlapping blades, often housed near the principal planes of the objective] or eyepiece] . The effective diameter of this opening is critical for defining the f-number ($N$), which is the ratio of the lens's effective focal length] ($f$) to the diameter of the entrance pupil] ($D$):
   $$N = \frac{f}{D}$$