Finder optical system and image pickup apparatus using the same

Abstract

A high-performance real-image finder optical system reduced in size, particularly in thickness, includes a positive objective optical system, an image-inverting optical system for erecting an intermediate image formed by the objective optical system, and a positive ocular optical system. The objective optical system has at least two movable units moving when zooming is performed. A prism is placed on the object side of the intermediate image. At least one reflecting surface of the prism has a rotationally asymmetric surface configuration. At least one reflecting surface of the image-inverting optical system is formed from a roof surface. The finder optical system satisfies the following condition: 1.0< d /( f W ·tan θ W ·Z )<2.5 where d is the distance from the entrance surface of the objective optical system to the first reflecting surface of the image-inverting optical system; f W is the focal length of the objective optical system at the wide-angle end; θ w is the maximum field angle of the objective optical system at the wide-angle end; and Z is a zoom ratio.

What we claim is:  
     
         1 . A finder optical system comprising, in order from an object side thereof: 
 an objective optical system having a positive refracting power;    an image-inverting optical system for erecting an intermediate image formed by said objective optical system; and    an ocular optical system having a positive refracting power;    wherein said objective optical system has: 
 at least two movable units moving when zooming is performed; and  
 a prism placed on an object side of the intermediate image formed by said objective optical system, said prism including at least one reflecting surface having a rotationally asymmetric surface configuration;  
   said image-inverting optical system having at least one reflecting surface formed from a roof surface;    said finder optical system satisfying the following condition:    1.0 <d /( f   W ·tan θ w   ·Z )<2.5  (1)    where d is a distance from an entrance surface of said objective optical system to a first reflecting surface of said image-inverting optical system; f W  is a focal length of said objective optical system at a wide-angle end; θ w  is a maximum field angle of said objective optical system at the wide-angle end; and Z is a zoom ratio.    
     
     
         2 . A finder optical system according to  claim 1 , wherein said roof surface is placed on a pupil side of said intermediate image.  
     
     
         3 . A finder optical system comprising, in order from an object side thereof: 
 an objective optical system having a positive refracting power;    an image-inverting optical system for erecting an intermediate image formed by said objective optical system; and    an ocular optical system having a positive refracting power;    wherein said objective optical system has: 
 at least two movable units moving when zooming is performed; and  
 a prism placed on an object side of the intermediate image formed by said objective optical system, said prism including at least one reflecting surface having a rotationally asymmetric surface configuration;  
   said image-inverting optical system including a Porro prism;    said finder optical system satisfying the following conditions:    1.0< d /( f   W ·tan θ w   ·Z )<2.5  (3) 0.5 <dp /( f   W ·tan θ w )<1.1  (4)    where d is a distance from an entrance surface of said objective optical system to a first reflecting surface of said image-inverting optical system; f W  is a focal length of said objective optical system at a wide-angle end; θ w  is a maximum field angle of said objective optical system at the wide-angle end; Z is a zoom ratio; and dp is a distance from an entrance surface of said image-inverting optical system placed on the object side of said intermediate image to said first reflecting surface.    
     
     
         4 . A finder optical system according to  claim 3 , wherein a second transmitting surface of said objective optical system has a power.  
     
     
         5 . A finder optical system according to  claim 4 , wherein said second transmitting surface has a rotationally asymmetric surface configuration.  
     
     
         6 . A finder optical system according to  claim 3 , wherein a first reflecting surface of said objective optical system has a power.  
     
     
         7 . A finder optical system according to  claim 1  or  3 , wherein the number of reflections in said prism is two or three.  
     
     
         8 . A finder optical system according to  claim 1  or  3 , wherein the number of said movable units is two.  
     
     
         9 . A finder optical system comprising, in order from an object side thereof: 
 an objective optical system having a positive refracting power;    an image-inverting optical system for erecting an intermediate image formed by said objective optical system; and    an ocular optical system having a positive refracting power;    wherein said objective optical system includes: 
 an optical system having at least two movable units moving when zooming is performed, said optical system having a positive composite power; and  
 a prism placed on a pupil side of said optical system, said prism including an image-inverting function, and said prism including at least one reflecting surface having a rotationally asymmetric surface configuration;  
   wherein at least either one of a first transmitting surface and first reflecting surface of said prism has a negative power, and a second transmitting surface of said prism has a positive power.    
     
     
         10 . A finder optical system according to  claim 9 , wherein either one of the first transmitting surface and the first reflecting surface has a positive power.  
     
     
         11 . A finder optical system according to  claim 9 , wherein the number of reflections in said prism is two or three.  
     
     
         12 . A finder optical system according to  claim 9 , wherein a composite focal length of said at least two movable units satisfies the following condition:  
       0.3 <f   move   /f   W <0.9  (7)  where f move  is the composite focal length of said at least two movable units at a wide-angle end, and f W  is a focal length of said objective optical system at the wide-angle end.    
     
     
         13 . A finder optical system comprising, in order from an object side thereof: 
 an objective optical system having a positive refracting power;    an image-inverting optical system for erecting an intermediate image formed by said objective optical system; and    an ocular optical system having a positive refracting power;    wherein said objective optical system has: 
 at least two movable units moving when zooming is performed; and  
 a prism placed on an object side of the intermediate image formed by said objective optical system, said prism having two reflecting surfaces, wherein at least one of said reflecting surfaces has a rotationally asymmetric surface configuration;  
   said finder optical system satisfying the following condition:    0.5 <dp /( f   W ·tan θ w )<1.1  (9)    where f W  is a focal length of said objective optical system at a wide-angle end; θ w  is a maximum field angle of said objective optical system at the wide-angle end; and dp is a distance from an entrance surface of said image-inverting optical system placed on the object side of the intermediate image to a first reflecting surface thereof.    
     
     
         14 . A finder optical system according to  claim 13 , wherein a first reflecting surface of said prism has a rotationally asymmetric surface configuration and is formed from an independent surface that is separate from other transmitting and reflecting surfaces.  
     
     
         15 . A finder optical system according to  claim 13 , wherein a second transmitting surface of said prism has a power.  
     
     
         16 . A finder optical system according to  claim 15 , wherein the second transmitting surface has a rotationally asymmetric surface configuration.  
     
     
         17 . A finder optical system according to  claim 13 , wherein an axial principal ray or a projective axial principal ray defined by projecting the axial principal ray onto a plane containing a part of the axial principal ray does not cross itself in said prism.  
     
     
         18 . A finder optical system according to  claim 13 , wherein either one of a first transmitting surface and first reflecting surface of said prism has a negative power.  
     
     
         19 . A finder optical system according to  claim 13 , wherein a composite focal length of said at least two movable units satisfies the following condition:  
       0.3 <f   move   /f   W <0.39  (11)  where f move  is the composite focal length of said at least two movable units at a wide-angle end, and f W  is a focal length of said objective optical system at the wide-angle end.    
     
     
         20 . A finder optical system comprising, in order from an object side thereof: 
 an objective optical system having a positive refracting power;    an image-inverting optical system for erecting an intermediate image formed by said objective optical system; and    an ocular optical system having a positive refracting power;    wherein said objective optical system has: 
 at least two movable units moving when zooming is performed; and  
 a prism placed on an object side of the intermediate image formed by said objective optical system, said prism having three reflecting surfaces, wherein at least one of said reflecting surfaces has a rotationally asymmetric surface configuration.  
   
     
     
         21 . A finder optical system according to  claim 20 , wherein at least two of the reflecting surfaces of said prism are formed from independent surfaces, respectively, which are separate from other transmitting and reflecting surfaces.  
     
     
         22 . A finder optical system according to  claim 20 , wherein a first reflecting surface of said prism has a rotationally asymmetric surface configuration and is formed from an independent surface that is separate from other transmitting and reflecting surfaces.  
     
     
         23 . A finder optical system according to  claim 20 , wherein a third reflecting surface of said prism has a rotationally asymmetric surface configuration and is formed from an independent surface that is separate from other transmitting and reflecting surfaces.  
     
     
         24 . A finder optical system according to  claim 20 , wherein a second transmitting surface of said prism has a power.  
     
     
         25 . A finder optical system according to  claim 24 , wherein the second transmitting surface has a rotationally asymmetric surface configuration.  
     
     
         26 . A finder optical system according to  claim 20 , wherein an axial principal ray or a projective axial principal ray defined by projecting the axial principal ray onto a plane containing a part of the axial principal ray does not cross itself in said prism.  
     
     
         27 . A finder optical system according to  claim 20 , wherein either one of a first transmitting surface and first reflecting surface of said prism has a negative power.  
     
     
         28 . A finder optical system according to  claim 20 , wherein a composite focal length of said at least two movable units satisfies the following condition:  
       0.3 <f   move   /f   W <0.9  (13)  where f move  is the composite focal length of said at least two movable units at a wide-angle end, and f W  is a focal length of said objective optical system at the wide-angle end.    
     
     
         29 . A finder optical system comprising, in order from an object side thereof: 
 an objective optical system having a positive refracting power;    an image-inverting optical system for erecting an intermediate image formed by said objective optical system; and    an ocular optical system having a positive refracting power;    wherein said objective optical system includes, in order from an object side thereof: 
 a negative first unit;  
 a positive second unit; and  
 a negative third unit;  
   wherein at least the first unit and the second unit are movable units moving when zooming is performed, and the third unit is formed from a prism including an image-inverting function, said prism including at least one reflecting surface having a rotationally asymmetric surface configuration.    
     
     
         30 . A finder optical system comprising, in order from an object side thereof: 
 an objective optical system having a positive refracting power;    an image-inverting optical system for erecting an intermediate image formed by said objective optical system; and    an ocular optical system having a positive refracting power;    wherein said objective optical system includes, in order from an object side thereof: 
 a negative first unit;  
 a positive second unit; and  
 a positive third unit;  
   wherein at least the first unit and the second unit are movable units moving when zooming is performed, and the third unit is formed from a prism including an image-inverting function, said prism including at least one reflecting surface having a rotationally asymmetric surface configuration, and said prism including at least one transmitting surface or reflecting surface that has a negative power, said third unit having a principal point positioned on a pupil side of a plane where the intermediate image is formed.    
     
     
         31 . A finder optical system according to  claim 29  or  30 , wherein a second transmitting surface of said prism has a power.  
     
     
         32 . A finder optical system according to  claim 31 , wherein said second transmitting surface has a rotationally asymmetric surface configuration.  
     
     
         33 . A finder optical system according to  claim 29  or  30 , wherein the number of reflections in said prism is two or three.  
     
     
         34 . A finder optical system according to  claim 29  or  30 , wherein a composite focal length of said movable units satisfies the following condition:  
       0.3 <f   move   /f   W <0.9  (15)  where f move  is the composite focal length of said movable units at a wide-angle end, and f W  is a focal length of said objective optical system at the wide-angle end.    
     
     
         35 . A finder optical system according to  claim 29  or  30 , wherein when zooming is performed, said third unit is stationary with respect to a plane where the intermediate image is formed.  
     
     
         36 . A finder optical system according to  claim 29  or  30 , wherein both said movable units are formed from refracting lenses.  
     
     
         37 . A finder optical system according to  claim 29  or  30 , wherein said movable units are each formed from a single refracting lens.  
     
     
         38 . A finder optical system comprising, in order from an object side thereof: 
 an objective optical system having a positive refracting power;    an image-inverting optical system for erecting an intermediate image formed by said objective optical system; and    an ocular optical system having a positive refracting power;    wherein said objective optical system has: 
 at least two movable units moving when zooming is performed; and  
 a prism placed on an object side of the intermediate image formed by said objective optical system, said prism including two reflecting surfaces, at least one of the reflecting surfaces having a rotationally asymmetric surface configuration;  
   wherein a first transmitting surface and second reflecting surface of said prism are formed from an identical surface having both transmitting and reflecting actions, and said prism has an optical path in which an axial principal ray or a projective axial principal ray defined by projecting the axial principal ray onto a plane containing a part of the axial principal ray bends in different directions from each other with respect to a travel direction of light rays at the two reflecting surfaces.    
     
     
         39 . A finder optical system according to  claim 38 , wherein a first reflecting surface of said prism has a negative power.  
     
     
         40 . A finder optical system according to  claim 38 , wherein a reflection angle at a first reflecting surface of said prism satisfies the following condition:  
       15°<θ A1 <40°  (17)  where θ A1  is the reflection angle at the first reflecting surface.    
     
     
         41 . A finder optical system according to  claim 38 , wherein an exit angle of said prism with respect to an optical axis entering said objective optical system satisfies the following condition:  
       25°<θ A <75°  (19)  where θ A  is the exit angle of said prism with respect to the optical axis entering said objective optical system.    
     
     
         42 . A finder optical system comprising, in order from an object side thereof: 
 an objective optical system having a positive refracting power;    an image-inverting optical system for erecting an intermediate image formed by said objective optical system; and    an ocular optical system having a positive refracting power;    wherein said objective optical system has: 
 at least two movable units moving when zooming is performed; and  
 a prism placed on an object side of the intermediate image formed by said objective optical system, said prism including two reflecting surfaces, both of said reflecting surfaces being independent of other transmitting and reflecting surfaces, at least one of said reflecting surfaces having a rotationally asymmetric surface configuration, said prism having an optical path in which an axial principal ray or a projective axial principal ray defined by projecting the axial principal ray onto a plane containing a part of the axial principal ray bends in a same direction with respect to a travel direction of light rays at said two reflecting surfaces;  
   said finder optical system satisfying the following condition:    0.5 <dp /( f   W ·tan θ w )<1.1  (21)    where dp is a distance from an entrance surface of said image-inverting optical system placed on the object side of the intermediate image to a first reflecting surface thereof; f W  is a focal length of said objective optical system at a wide-angle end; θ w  is a maximum field angle of said objective optical system at the wide-angle end.    
     
     
         43 . A finder optical system according to  claim 42 , wherein a first transmitting surface of said prism has a negative power.  
     
     
         44 . A finder optical system according to  claim 42 , wherein a second transmitting surface of said prism has a positive power.  
     
     
         45 . A finder optical system according to  claim 42 , wherein a reflection angle at a first reflecting surface of said prism satisfies the following condition:  
       30°<θ B1 <60°  (23)  where θ B1  is the reflection angle at the first reflecting surface.    
     
     
         46 . A finder optical system according to  claim 42 , wherein a reflection angle at a second reflecting surface of said prism satisfies the following condition:  
       30°<θ B2 <60°  (25)  where θ B2  is the reflection angle at the second reflecting surface.    
     
     
         47 . A finder optical system according to  claim 42 , wherein an exit angle of said prism with respect to an optical axis entering said objective optical system satisfies the following condition:  
       150°<φ B <210°  (27)  where φ B  is the exit angle of said prism with respect to the optical axis entering said objective optical system.    
     
     
         48 . A finder optical system according to  claim 42 , wherein a roof surface is disposed on a pupil side of the intermediate image.  
     
     
         49 . A finder optical system according to  claim 42 , wherein said image-inverting optical system is a Porro prism.  
     
     
         50 . A finder optical system comprising, in order from an object side thereof: 
 an objective optical system having a positive refracting power;    an image-inverting optical system for erecting an intermediate image formed by said objective optical system; and    an ocular optical system having a positive refracting power;    wherein said objective optical system has: 
 at least two movable units moving when zooming is performed; and  
 a prism placed on an object side of the intermediate image formed by said objective optical system, said prism including three reflecting surfaces, all of said reflecting surfaces being independent of other transmitting and reflecting surfaces, at least one of said reflecting surfaces having a rotationally asymmetric surface configuration, said prism having an optical path in which an axial principal ray or a projective axial principal ray defined by projecting the axial principal ray onto a plane containing a part of the axial principal ray bends in a same direction with respect to a travel direction of light rays at two consecutive reflecting surfaces and bends in a direction different from said same direction at the other reflecting surface.  
   
     
     
         51 . A finder optical system according to  claim 50 , wherein a first transmitting surface of said prism has a negative power.  
     
     
         52 . A finder optical system according to  claim 50 , wherein a reflection angle at a first reflecting surface of said prism satisfies the following condition:  
       30°<θ C1 <60 °  (29)  where θ C1  is the reflection angle at the first reflecting surface.    
     
     
         53 . A finder optical system according to  claim 50 , wherein a reflection angle at a third reflecting surface of said prism satisfies the following condition:  
       30°<θ C3 <60°  (31)  where θ C3  is the reflection angle at the third reflecting surface.    
     
     
         54 . A finder optical system according to  claim 50 , wherein an exit angle of said prism with respect to an optical axis entering said objective optical system satisfies the following condition:  
       70°<φ C <110°  (33)  where φ C  is the exit angle of said prism with respect to the optical axis entering said objective optical system.    
     
     
         55 . A finder optical system comprising, in order from an object side thereof: 
 an objective optical system having a positive refracting power;    an image-inverting optical system for erecting an intermediate image formed by said objective optical system; and    an ocular optical system having a positive refracting power;    wherein said objective optical system has: 
 at least two movable units moving when zooming is performed; and  
 a prism placed on an object side of the intermediate image formed by said objective optical system, said prism including three reflecting surfaces, at least one of said reflecting surfaces having a rotationally asymmetric surface configuration;  
   wherein a first reflecting surface and third reflecting surface of said prism are independent of other transmitting and reflecting surfaces, and a second reflecting surface of said prism is formed from an identical surface with a second transmitting surface of said prism;    said prism having an optical path in which an axial principal ray or a projective axial principal ray defined by projecting the axial principal ray onto a plane containing a part of the axial principal ray bends in a same direction with respect to a travel direction of light rays at two consecutive reflecting surfaces and bends in a direction different from said same direction at the other reflecting surface.    
     
     
         56 . A finder optical system according to  claim 55 , wherein a first transmitting surface of said prism has a negative power.  
     
     
         57 . A finder optical system according to  claim 55 , wherein a reflection angle at the first reflecting surface of said prism satisfies the following condition:  
       20°<θ D1 <60°  (35)  where θ D1  is the reflection angle at the first reflecting surface.    
     
     
         58 . A finder optical system according to  claim 55 , wherein a reflection angle at the third reflecting surface of said prism satisfies the following condition:  
       10°<θ D3 <50°  (37)  where θ D3  is the reflection angle at the third reflecting surface.    
     
     
         59 . A finder optical system according to  claim 55 , wherein an exit angle of said prism with respect to an optical axis entering said objective optical system satisfies the following condition:  
       20°<φ D <60°  (39)  where φ D  is the exit angle of said prism with respect to the optical axis entering said objective optical system.    
     
     
         60 . A finder optical system comprising, in order from an object side thereof: 
 an objective optical system having a positive refracting power;    an image-inverting optical system for erecting an intermediate image formed by said objective optical system; and    an ocular optical system having a positive refracting power;    wherein said objective optical system has: 
 at least two movable units moving when zooming is performed; and  
 a prism placed on an object side of the intermediate image formed by said objective optical system, said prism including three reflecting surfaces, at least one of said reflecting surfaces having a rotationally asymmetric surface configuration;  
   wherein a first reflecting surface and third reflecting surface of said prism are independent of other transmitting and reflecting surfaces, and a second reflecting surface of said prism is formed from an identical surface with first and second transmitting surfaces of said prism;    said prism having an optical path in which an axial principal ray or a projective axial principal ray defined by projecting the axial principal ray onto a plane containing a part of the axial principal ray bends at each of the three reflecting surfaces in a direction different from a direction of bending at a preceding reflecting surface with respect to a travel direction of light rays.    
     
     
         61 . A finder optical system according to  claim 60 , wherein the first reflecting surface of said prism has a negative power.  
     
     
         62 . A finder optical system according to  claim 60 , wherein a reflection angle at the first reflecting surface of said prism satisfies the following condition:  
       15°<θ E1 <45°  (41)  where θ E1  is the reflection angle at the first reflecting surface.    
     
     
         63 . A finder optical system according to  claim 60 , wherein a reflection angle at the third reflecting surface of said prism satisfies the following condition:  
       15°<θ E3 <45°  (43)  where θ E3  is the reflection angle at the third reflecting surface.    
     
     
         64 . A finder optical system according to  claim 60 , wherein an exit angle of said prism with respect to an optical axis entering said objective optical system satisfies the following condition:  
       160°<φ E <200°  (45)  where φ E  is the exit angle of said prism with respect to the optical axis entering said objective optical system.    
     
     
         65 . A finder optical system comprising, in order from an object side thereof: 
 an objective optical system having a positive refracting power;    an image-inverting optical system for erecting an intermediate image formed by said objective optical system; and    an ocular optical system having a positive refracting power;    wherein said objective optical system has: 
 at least two movable units moving when zooming is performed; and  
 a prism placed on an object side of the intermediate image formed by said objective optical system, said prism including three reflecting surfaces, at least one of said reflecting surfaces having a rotationally asymmetric surface configuration, said three reflecting surfaces being all independent of other transmitting and reflecting surfaces, said prism having an optical path in which an axial principal ray or a projective axial principal ray defined by projecting the axial principal ray onto a plane containing a part of the axial principal ray bends in a same direction with respect to a travel direction of light rays at a first reflecting surface and a second reflecting surface and is twisted by a third reflecting surface.  
   
     
     
         66 . A finder optical system according to  claim 65 , wherein a reflection angle at the first reflecting surface of said prism satisfies the following condition:  
       30°<θ F1 <60°  (47)  where θ F1  is the reflection angle at the first reflecting surface.    
     
     
         67 . A finder optical system according to  claim 65 , wherein a reflection angle at the third reflecting surface of said prism satisfies the following condition:  
       30°<θ F3 <60°  (49)  where θ F3  is the reflection angle at the third reflecting surface.    
     
     
         68 . A finder optical system according to  claim 65 , wherein an exit angle of said prism with respect to an optical axis entering said objective optical system satisfies the following condition:  
       70°<φ F <110°  (51)  where φ F  is the exit angle of said prism with respect to the optical axis entering said objective optical system.    
     
     
         69 . A finder optical system according to any one of claims  1 ,  3 ,  9 ,  13 ,  20 ,  38 ,  42 ,  50 ,  55 ,  60  and  65 , wherein said prism is placed closer to a pupil than any of said movable units.  
     
     
         70 . An image pickup apparatus comprising: 
 a photographic optical system having an optical path for photography; and    a finder optical system having an optical path for a finder;    wherein said finder optical system according to any one of claims  1 ,  3 ,  9 ,  13 ,  20 ,  38 ,  42 ,  50 ,  55 ,  60  and  65  is used as said finder optical system.