JPS57158609A - Wide-angle lens - Google Patents

Abstract

PURPOSE:To obtain a small-sized wide-angle lens which has a back facus of >=1.5 times as long as its focal distance with the compensated aberration, by forming an optical system with a front group of 7 lenses and a rear group of 6 lenses and selecting the optical beam properties to satisfy the prescribed conditions. CONSTITUTION:A retro-telephoto optical system is formed with a frond group of 7 lenses and a rear group of 6 lenses. The front group includes a positive lens L1, the negative meniscus lenses L2 and L3, a positive lens L4, a negative meniscus lens L5 plus positive lenses L6 and L7 which are set at the front side of diaphragm S; while the rear group includes the meniscus lenses L8 and L9, a negative lens L10 plus the positive lenses L11, L12 and L13 set at the back side of the diaphragm S respectively. Then the general focal distance 5, the F number, etc. are selected so as to satisfy the conditions of equations (1)-(3) respectively, and at the same time the lenses L8 and L9 are made of the nitric materials having a low and high degees of dispersion respectively. As a result, the sagittal flare and various aberrations are satisfactorily compensated for a small-sized wide-angle lens. (f1: the focal distance of the front group; phi: a reciprocal number of focal distances of lenses L8 and L9; Ra: the curvature radius of the rear surface of the lens L7)