Relative aperture is the absolute value of the ratio of the doubled distance from the optical axis to the point of refraction or reflection of the meridional ray parallel to the optical axis in object space and passing through the edge of the aperture diaphragm to the rear focal length of the system, or otherwise: the ratio of the entrance pupil diameter to the focal length.
Distinguish between geometric and effective (physical) relative holes. The latter takes into account the overall light transmittance of the optical system.

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