Auxiliary Lens

(or lens attachment), a lens attached to an objective to change its focal length. A positive lens decreases the focal length, and a negative lens increases it. Positive lenses are used for large-scale photography (macrophotography) with still and motion-picture cameras, in which extension of the lens toward the object being photographed is limited. The focal length f’_c of the optical system consisting of the objective and the auxiliary lens is determined from the formula

where f’_Obj and f’_aux are the focal lengths of the objective and the auxiliary lens, respectively, and d is the distance between the second principal plane of the auxiliary lens and the first principal plane of the objective lens.

An auxiliary lens reduces the image quality because of increased aberrations. Reduction of the objective’s aperture (stopping down) and the use of yellow or yellow-green light filters to reduce the chromatic aberration effect are recommended to reduce the effects of the auxiliary lens on image quality.