Image Forming Optical System and Electronic Image Pickup Apparatus Using Image Forming Optical System

Abstract

An image forming optical system according to the present invention is characterized in that, in an image forming optical system having a positive lens group, a negative lens group, and an aperture stop, the positive lens group is disposed at an object side of the aperture stop, the positive lens group has a cemented lens which is formed by cementing a plurality of lenses, in a rectangular coordinate system in which a horizontal axis is let to be Nd and a vertical axis is let to be νd, when a straight line indicated by Nd=α×νd+β (where, α=−0.017) is set, Nd and νd of at least one lens forming the cemented lens is included in both of areas namely, an area which is determined by a line when a lower limit value is in a range of a following conditional expression (1a), and a line when an upper limit value is in a range of the following conditional expression (1a), and of an area determined by following conditional expressions (2a) and (3a). 1.45<β<2.15  (1a) 1.30<Nd<2.20  (2a) 3<νd<12  (3a) Here, Nd denotes a refractive index, and νd denotes an Abbe's number.

1 . (canceled) 
     
     
         2 . An image forming optical system comprising:
 a positive lens group;   a negative lens group; and   an aperture stop, wherein   the positive lens group is disposed at an image side of the aperture stop, and   the positive lens group has a cemented lens in which a plurality of lenses are cemented, and   in a rectangular coordinate system in which, a horizontal axis is let to be Nd and a vertical axis is let to be νd, when a straight line indicated by Nd=α×νd+β (where, α=−0.017) is set,   Nd and νd of at least one lens forming the cemented lens are included in both of areas namely, an area which is determined by a line when a lower limit value is in a range of a following conditional expression (1b), and a line when an upper limit value is in a range of the following conditional expressions (2b) and (3b)
   1.45<β<2.15  (1b)
 
   1.58<Nd<2.20  (2b)
 
   3<νd<40  (3b)
 
   where, Nd denotes a refractive index, and νd denotes an Abbe's number.   
     
     
         3 . The image forming optical system according to  claim 2 , wherein
 when the one lens of which Nd and νd are included in both the areas is let to be a predetermined lens, a center thickness of the predetermined lens along an optical axis of the predetermined lens is less than a center thickness of the predetermined lens along an optical axis of the other lens of the cemented lens.   
     
     
         4 . The image forming optical system according to  claim 3 , wherein
 the image forming optical system satisfies a following conditional expression
   0.22<t1<2.0 
   where, t 1  is the center thickness of the predetermined lens along an optical axis of the predetermined lens.   
     
     
         5 . The image forming optical system according to  claim 2 , wherein
 the cemented lens is a compound lens which is formed by closely adhering and hardening a resin on a lens surface of the one lens which forms the cemented lens.   
     
     
         6 . The image forming optical system according to  claim 2 , wherein
 the cemented lens is a compound lens which is formed by closely adhering and hardening a glass on a lens surface of the one lens which forms the cemented lens.   
     
     
         7 . The image forming optical system according to  claim 2 , wherein
 the image forming optical system is a zoom lens of which a closest side toward an object is a positive group.   
     
     
         8 . The image forming optical system according to  claim 2 , wherein
 the image forming optical system is a zoom lens of which a closest side toward an object is a negative group.   
     
     
         9 . The image forming optical system according to  claim 2 , further comprising:
 a prism for folding.   
     
     
         10 . The image forming optical system according to  claim 9 , wherein
 the prism is in a lens group on a closest side toward an object.   
     
     
         11 . An electronic image pickup apparatus comprising:
 an image forming optical system according to  claim 2 ;   an electronic image pickup element; and   an image processing means which processes image data obtained by image pickup by the electronic image pickup element an image which is formed through the image forming optical system, and outputs as image data in which a shape is changed upon processing, wherein   the image forming optical system is a zoom lens, and the zoom lens satisfies a following conditional expression at a time of infinite object point focusing
   0.7< y   07 /( fw ·tan ω 07w )<0.96
 
   
       where y 07  is indicated as y 07 =0.7y 10  when, in an effective image pickup surface (surface in which, image pickup is possible), a distance from a center up to a farther point (maximum image height) is let to be y 10 . Moreover, ω 07w  is an angle with respect to an optical axis in a direction of an object point corresponding to an image point connecting from a center on the image pickup surface in a wide angle end up to a position of y 07 .