Patent ID: 6028715
Filing Date: 2000-02-22
Classification: G02B

Abstract:
A variable magnification optical system comprising, from an enlargement side:a first lens unit having a positive optical power;a second lens unit having a negative optical power and including a first negative lens element, the second lens unit being adapted to move along an optical axis direction to accomplish varying the magnification of the optical system;a third lens unit having a negative optical power and including a second negative lens element, the third lens unit being adapted to move along the optical axis direction to accomplish varying of the magnification of the optical system; anda fourth lens unit having a positive optical power and including a first positive lens element, the fourth lens unit being fixed in the optical axis direction,wherein optical materials of said first negative lens element, the second negative lens element, and the first positive lens element fulfill the following conditions:0.015<.THETA.-(0.644-0.00168.multidot..nu.d)<0.0665<.nu.d<100where.THETA.=(ng-nF)/(nF-nC); and.nu.d=(nd-1)/(nF-nC);whereng represents a refractive index for g-line (whose wavelength is 435.84 nm);nF represents a refractive index for F-line (whose wavelength is 486.13 nm);nC represents a refractive index for C-line (whose wavelength is 656.28 nm); andnd represents a refractive index for d-line (whose wavelength is 587.56 nm).