Patent ID: 7075731

Claim:
A large aperture zoom lens comprising four groups of lenses, or first to fourth groups of lenses of positive, negative, positive and positive refractive powers, respectively, in the order on the closer-to-the-object first basis, the zoom lens having its first and second groups of lenses separated farther from each other to widen an air gap between them, its second and third groups of lenses contracted to straighten the air gap between them, and its third and fourth groups of lenses also contracted to straighten the air gap between them during varying the power of magnification from a wide-angle position to a telephoto position, and the groups of lenses being moved toward the object so as to satisfy the conditions as represented in the following formulae: 0.18 <|f 2 |/fT< 0.24â€ƒâ€ƒ(1) 1.1 <f 1 /fT< 1.5â€ƒâ€ƒ(2) 0.6 <f 4 /fT< 0.9â€ƒâ€ƒ(3) and also the following relation being satisfied relative to a variable ratio of the power of magnification of the whole system from its wide-angle position to its telephoto position: 0.57 <Z 2 /Z< 0.9â€ƒâ€ƒ(4) where f 1 is a focal length of the first group of lenses, f 2 is a focal length of the second group of lenses, f 4 is a focal length of the fourth group of lenses, fT is a focal length of the whole system at its telephoto position, Z 2 is equal to Î²2T/Î²2W or a focusing magnification of the second group of lenses at its wide-angle position divided by that of the second group of lenses at its telephoto position (Î²2T<0 and Î²2W<0), Z is equal to fT/fW or a focal length of the whole system at its telephoto position divided by that of the whole system at its wide-angle position.