JPS6466611A - F-theta lens - Google Patents

Abstract

PURPOSE:To minimize the curvature of image in a sagital direction even in case of long scanning length, and to improve the brightness and resolving power, by satisfy ing prescribed conditions in a lens system constituted of four groups and four pieces containing a negative meniscus lens. CONSTITUTION:In a lens system constituted of four groups and four pieces consisting of the first lens 1 of a positive meniscus lens (ML) whose concave surface is turned to an incident light beam image, the second lens 2 of a negative ML whose concave surface is turned to the light beam incident image, the third lens 3 of a positive ML whose concave surface is turned to the incident light beam side, and the fourth lens of a positive lens, the following conditions are satisfied. a) 0.85<¦r2/r3¦<1.1,2)-1.5f< r5<-0.3f, and 3)0.8f<f4<4f0. In this regard, r3, r5 and r2 and f4. and (f) denote radiuses of curvature of the surfaces of the light beam incident side of the second and the third lenses, a radius of curvature of the surface of the light beam emitting side of the first lens, a focal distance of the fourth lens, and a focal distance of the hole system, respectively. By forming the whole shape to a concentric shape against an entrance pupil, an astigmatism of high-order is minimized, and an (f).theta lens having an equal speed scanning property of high accuracy, being bright and having high resolution is obtained.