Zoom lens system

Abstract

The invention provides a zoom lens system suitable for an image gathering system using a solid-state imaging device, having a zoom ratio of about three, a small lens length, and superb optical performance. The zoom lens system includes, in order from an object, a first lens group having negative refractive power, a second lens group having positive refractive power, and a third lens group having positive refractive power. The first lens group consists only of a negative lens and a positive lens. The second lens group includes at least two positive lenses and at least one negative lens. The third lens group consists of one lens. Zooming from a wide-angle end state to a telephoto end state is conducted by decreasing a distance between the first and second lens groups, increasing a distance between the second and third lens groups, and fixing the third lens group. Given conditions are satisfied.

1 . A zoom lens system comprising, in order from an object; a first lens group having negative refractive power; a second lens group having positive refractive power; and 
 a third lens group having positive refractive power;    the first lens group consisting only of a negative lens element and a positive lens element,    the second lens group comprising at least two positive lens elements and at least one negative lens element, and    the third lens group consists of one lens element; and    wherein when the state of lens group positions varies from a wide-angle end state to a telephoto end state, a distance between the first lens group and the second lens group decreases, a distance between the second lens group and the third lens group increases, and the third lens group is fixed; and    wherein the following conditional expression is satisfied:      2.5 <TL /( ft×fw ) 1/2 <4.5    where TL denotes the distance between the most object side lens surface of the zoom lens system and the image plane, fw denotes the focal length of the zoom lens system in a wide-angle end state, and ft denotes the focal length of the zoom lens system in a telephoto end state.    
   
   
       2 . The zoom lens system according to  claim 1 , wherein the first lens group includes at least one aspherical surface.  
   
   
       3 . The zoom lens system according to  claim 1 , wherein the second lens group includes at least one aspherical surface.  
   
   
       4 . The zoom lens system according to  claim 1 , wherein 
 the second lens group consists of, in order from the object, a positive lens element, a double convex positive lens element, and a negative lens element;    the double convex positive lens element is cemented with the negative lens element; and    the third lens group consists of one positive lens element.    
   
   
       5 . The zoom lens system according to  claim 4 , wherein 
 the most object side lens surface of the second lens group has a convex shape facing to the object side,    the most image side lens surface of the second lens group has a concave shape facing to the image side, and    the following conditional expression is satisfied:      −4.0<( G 2 r 1 +G 2 r 2)/(G2 r 2 −G 2 r 1)<−1.0    where G2r1 denotes the radius of curvature of the most object side lens surface of the second lens group, and G2r2 denotes the radius of curvature of the most image side lens surface of the second lens group.    
   
   
       6 . The zoom lens system according to  claim 4 , wherein 
 the following conditional expression is satisfied:      −0.5<( G 3 r 1 +G 3 r 2)/( G 3 r 2 −G 3 r 1)<0.5    where G3r1 denotes the radius of curvature of the most object side lens surface of the third lens group, and G3r2 denotes the radius of curvature of the most image side lens surface of the third lens group.    
   
   
       7 . The zoom lens system according to  claim 1 , wherein the one lens element composing the third lens group has positive refractive power and has at least one aspherical surface.  
   
   
       8 . The zoom lens system according to  claim 1 , wherein focusing from infinity to close object is conducted by moving the third lens group in the object direction.