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glossary

Effective focal length: algebraic distance from the primary/secondary principal plane of an optical system to its front/back focal plane
Clear aperture: aperture diameter of an optical system
Image spot: spot described by the intersection of the observation plane with light rays coming from an object point and transmitted through an optical system
Primary principal plane: plane of the object space of an optical system whose conjugation by the said optical system occurs with a magnification of +1 and whose conjugate plane is called the secondary principal plane
Secondary principal plane: plane conjugate by an optical system of an object plane ( the primary principal plane ) with a transverse magnification of +1
Object: element (point, line, plane, or whatever,…) of the space linked to another element called image in a way that rays coming from the object and transmitted by an optical system focus on the image ; object and image are said to be conjugate by the optical system
Image: conjugate of an object by an optical system (see object)
Defocus: considering a focus plane conjugate of an object plane by an optical system, defines a shift of the focus plane from its best position (for which the image is the sharpest)