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A lens is placed in front of a plane circular source so as to produce a given magnification. A multimode fiber optic end is coinciding with the conjugate of the source by the lens. The page calculates the minimum clear aperture of the lens (so that the numerical aperture of the lens in the image space is equal to the numerical aperture of the fiber optic), the position of the source, the position of the fiber optic and the coupling efficiency as functions of the source diameter, the focal length, the magnification, the diameter and numerical aperture of the fiber optic. It is assumed that the light source is lambertian and emitting in a half space.