PURPOSE: To obtain a sufficient back focus and to obtain excellent aberrations by composing a lens system of five lens elements in four groups and making lens various items meet specific requirements.

CONSTITUTION: The lens systems consists of a 1st positive lens, a 2nd negative lens, a 3rd negative lens, a 4th positive lens, and a 5th positive lens in order from the object side and the 3rd and the 4th lenses are cemented together to obtain four-group, five-element constitution. The various lens items meet requirements shown in a table, where (f) is the composite focal length of the whole system, FNO the aperture ratio, fB the back focus at infinite distance, ωa half field angle, ri the radius of curvature of the (i)th surface, di the gap between the (i)th surface and next surface, ni the refractive index of the (i)th lens, and vi the Abbe number. consequently, the lens system is reduced in external diameter and obtains the sufficient back focus and has sufficiently excellent aberrations.