WIDE-ANGLE LENS WITH LONG BACK FOCUS

PURPOSE:To obtain a long back focus and to make excellent compensation for respective aberrations by composing the wide-angle lens of a 1st lens group consisting of one negative lens and a 2nd lens group consisting four lenses in combination and making the lens groups meet specific requirement. CONSTITUTION:This wide-angle lens consists of the 1st lens group consisting of one negative lens and the 2nd lens group consisting of four positive, negative, positive, and positive lenses from an object side. The incidence surface of the 1st lens group is an aspherical surface which decreases in negative refractive index toward the periphery and the respective lenses of the 2nd lens group meet the requirements shown by inequalities. Those inequalities prescribe the focal length of the 1st group, the composite focal length of the 2nd lens group, the thicknesses of the negative lenses, and the upper-limit and lower-limit values of the principal point interval of the lens groups. When those inequalities hold, the spherical aberration, astigmatism, distortion, and comatic aberration are compensated excellently and the variation range of the refractive index is limited, so that the long back focus is obtained.