Zoom lens

Abstract

The invention relates to a new mechanism for permitting a zoom lens to focus on a short distance subject by moving a focusing lens group in an amount that varies in an optical axis direction depending on a zooming position. The zoom lens comprises, in order from its object side, a positive first lens group, a negative second lens group and a positive third lens group. Zooming is implemented while the space between adjoining lens groups is varied, and focusing on a short distance subject is implemented by moving the negative second lens group in an amount that varies in the optical axis direction depending on a zooming position. The negative second lens group comprises a negative front unit G 2 F and a negative rear unit G 2 R. The space between the negative front unit G 2 F and the negative rear unit G 2 R is varied depending on a subject distance but that space remains constant wherever the zooming position lies.

1 . A zoom lens, capable of focusing on a short distance subject by moving two mutually adjoining lens groups with a change in a space between them yet in a varying amount in an optical axis direction depending on a zooming position, characterized in that the space between said two lens groups stays constant at a constant subject distance wherever the zooming position lies. 
   
   
       2 . A zoom lens, comprising, in order from an object side thereof, a positive first lens group, a negative second lens group and a positive third lens group in which zooming is implemented while varying a space between adjoining lens groups, and which focuses on a short distance subject by moving said negative lens group in an amount that varies in an optical axis direction depending on a zooming position, characterized in that said negative second lens group comprises a negative front unit and a negative rear unit, and a space between said negative front unit and said negative rear unit varies depending on a subject distance, but the space between said negative front unit and said negative rear unit stays constant wherever the zooming position lies. 
   
   
       3 . The zoom lens according to  claim 2 , characterized in that at least one lens surface in said front unit is defined by an aspheric surface having negative or positive power growing weak or strong with a distance from an optical axis, and said rear unit comprises, in order from an object side thereof, a negative lens and a positive lens. 
   
   
       4 . A lens barrel that is used with a zoom lens capable of focusing on a short distance subject by moving two mutually adjoining lens groups with a change in a space between them yet in a varying amount in an optical axis direction depending on a zooming position, characterized by comprising:
 a means for rotating and moving a zoom frame in an optical axis direction upon zooming,   a first lens group cam groove provided in said zoom frame,   a first lens group frame that is integral with a first lens group cam pin that moves in engagement within the first lens group cam groove,   a first keep means for keeping said first lens group frame against rotation about an optical axis,   a second lens group cam groove provided in said first lens group frame,   a second lens group frame that is integral with a second lens group cam pin that moves in engagement within said second lens group cam groove, and   a second keep means for keeping said second lens group frame against rotation about the optical axis, and   further comprising a focusing means that rotates said first keep means about the optical axis to rotate said first lens group frame about the optical axis.