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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
Lens: optical component with two refractive surfaces axially symmetrical for redirecting the light in a controlled manner
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)
Object space: space considered before transformation of light rays by an optical system
Image space: space conjugate of the object space by an optical system
Transverse magnification: size ratio of an image and an object, the image being conjugated to the object by an optical system; for centered optical systems, the transverse magnification is defined with objects in a plane perpendicular to the optical axis - transverse magnification is then positive if object and image have the same orientation and negative otherwise
Aperture : diaphragm limiting the number of light rays that are able to propagate
Clear aperture: aperture diameter of an optical system
Numerical aperture ( optical system ): dimensionless number characterizing the aperture in the object space or in the image space and defined as the product of the refraction index by the sine of half the aperture angle