Definition: Optics designed to increase the effective focal length of a lens (which is then called the 'prime' lens). * Usually a compound optic the sum of whose powers is negative. The design may be complicated in order to preserve image quality of prime lens. * Mechanical precision of the mount is a major factor in final image quality. Give one, take other The increase in focal length with fixed maximum aperture forces the maximum aperture (and smaller apertures) to be decreased by a factor equal to the increase in focal length e.g. 2X increase causes a loss of 2 f/stops; a 1.4X increase loses about one stop. The lens aperture scale is obviously rendered inaccurate.

Related Terms: Barlow lens

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