PURPOSE: To obtain the lens system with excellent aberrations which consists of a small number of elements and has, specially, an F number of 4-5.6 and a half field angle of about 23°-27° and is suitable for a read of facsimile, etc., as to a lens system which consists of a positive front group lens, a stop, and a rear group lens.
CONSTITUTION: The image forming lens system consists of the front group lens which is made of optical glass and the rear group lens which is made of one plastic lens in a meniscus shape having its concave surface opposite the stop and has aspherical surfaces as both its surfaces, and satisfies conditional inequalities (1)-(5a). (1) f0.1/fR-1+1/FR-2<0.3, (2)-4<f0/rR-1<-0.7, (3) ΔXR-1/f0<0, (4) ΔXR-2/f0<0, and (5a) 0.1< dR/f0, where fR-1 and fR-2 are the focal lengths of the 1st and 2nd surfaces of the lens, fo the focal length of the whole system, rR-1 the radius of curvature of the 1st surface, ΔXR-1 and ΔXR-2 the quantities of asphericity of the 1st and 2nd surfaces corresponding to the effective radius of the lens, and dR the lens thickness.
ITO TAKAYUKI
IIZUKA TAKAYUKI