Changeable magnification optical system
BACKGROUND OF THE INVENTION 
     1. Field of the Invention: 
     This invention relates to optical systems of changeable magnification, and more particularly to such optical systems in which the image magnification is changed between different values by switching one of the constituent optical units between parallel light paths in cooperation with a path selector. 
     2. Description of the Related Art: 
     A wide variety of changeable magnification optical systems capable of selectively employing two lens arrangements for first and second image magnifications have been proposed. Particularly the selection method that is to releasably attach an auxiliary optical unit to either the front or the rear of a master optical unit to change the focal length of the entire system, or the so-called attachment type, because of its operating mechanism being simple in structure, has found many uses in photographic cameras and SV cameras. When the auxiliary optical unit was used as the front attachment, however, its diameter had to be very large. So, the size of the entire lens system tended to increase. For the rear attachment, though its diameter was smaller as compared with the front attachment, the number of lens elements had to be increased, or otherwise good correction of aberrations would have been difficult to perform. Thus, the physical length tended to increase. A common disadvantage of the front and rear attachments was that whenever necessity arose to change the focal length, the auxiliary optical unit had to be either detached or attached or interchanged with another. Because the swiftness to a shutter chance was very poor, snapshots were difficult to take. 
     In the field of cameras using 110 size film, it has been known to provide a changeable magnification optical system in which the auxiliary optical unit is arranged to slidingly move into and retract from axial alignment with the master optical unit at the front thereof when the image magnification is changed. There has been another proposal in which two lens systems, or the standard and telephoto lenses, are positioned stationary in parallel, and selectively operated by a movable mirror, in Japanese Patent Publication No. SHO 58-38778. The use of such a switch type in the design of changeable magnification optical systems enables a desired decrease in the bulk and size of the entire system to be relatively easily achieved. To focus the first changeable magnification optical system of the switch type, however, regardless of whether the master lens is moved alone, or along with the auxiliary lens, the total focusing movement is caused to differ with different focal length settings. This drawback was also encountered with the second changeable magnification optical system proposed in Japanese Patent Publication No. SHO 58-38778, as the standard and telephoto lenses had different total focusing movements from each other. 
     This implies that in application of the switch type to auto-focus cameras, for example the operating mechanism must be constructed so as to differentiate the speed of focusing movement of the lens system with different focal length settings, and, therefore that the complexity of the structure of the internal mechanisms of the camera is increased objectionably. 
     SUMMARY OF THE INVENTION 
     A general object of the present invention is to provide a changeable magnification optical system of high magnification change ratio with a minimum of the bulk and size. 
     A concomitant object is to provide a changeable magnification optical system with an operating mechanism of very simple form owing to a feature that the total focusing movement is maintained constant against change of the image magnification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1(A) and 1(B) are schematic views of an embodiment of the optical system according to the present invention. 
     FIG. 2 is a schematic view of another embodiment of the optical system according to the invention. 
     FIGS. 3 and 5 are longitudinal section views of an example 1 of a specific lens of the invention. 
     FIGS. 4 and 6 are graphic representations of the aberrations of the lens of FIGS. 3 and 5 in different focal length settings. 
     FIGS. 7 and 8 are longitudinal section views of another example 2 of a specific lens of the invention. 
     FIGS. 8 and 10 are graphic representations of the aberrations of the lens of FIGS. 7 and 8. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1(A) and 1(B) are schematic views of one embodiment of the optical system according to the invention. FIG. 1(A) illustrates a first optical arrangement of, for example, the standard image magnificantion, and FIG. 1(B) illustrates a second optical arrangement of, for example, the telephoto image magnification. In FIGS. 1(A) and 1(B), a master optical unit 1 of positive power in movable in a direction indicated by an arrow 11. An auxiliary optical unit of negative power 2 is held stationary. 3 is a fixed mirror; and 4 is a mirror pivotable about a shaft 4A, its directions of movement being indicated by arrows 12; 5-1 and 5-2 each are a diaphragm; 6-1 and 6-2 each are a shutter; 7 is a photosensitive surface of either silver halide film or a solid state image pickup element; S1 and S2 are respectively first and second optical axes. 
     In the standard focal length setting of FIG. 1(A), the movable mirrow 4 is retracted from the effective light beam of the master optical unit 1, and the shutter 6-1 is closed. Light from an object to be photographed is focused directly on the photosensitive surface 7 by the master optical unit 1 and focusing is performed by moving the master optical unit 1 along the optical axis S1. 
     In the telephoto focal length setting of FIG. 1(B), the master optical unit 1 is aligned with the auxiliary optical unit 2 on the optical axis S2 at the front thereof been moved in the direction of the arrow 11 from the position of FIG. 1(A), and the movable mirrow 4 is turned about the shaft 4A in the direction of the arrow 12 of FIG. 1(A) to such a position that the axis of the light beam from the fixed mirror 3 is reflected to the center of the area of a picture frame in the surface 7 and the shutter 6-2 is closed. In this figure, light from the object passes through the master optical unit 1 and the auxiliary optical unit 2 and is reflected from the fixed mirror 3 and the movable mirror 4 to form an image of the object on the photosensitive surface 7. Focusing is performed similarly to the standard focal length setting by moving the master optical unit 1 along the optical axis S2. 
     The use of the same unit or the master optical unit in focusing at both standard and telephoto focal length settings enables the total focusing movements in the standard and telephoto focal length settings to be equalized, thus permitting the focusing control mechanism to be constructed in a simple form. 
     For note, in this embodiment, the method of changing the position of the movable mirror 4 is not limited to the pivotal one, and may be the slide one instead. Additionally, an optical path selecting means which may take the form of a movable or tiltable mirror may be arranged in a position of convergence of the optical paths. 
     Also, another axially optical unit may be provided on the first optical axis S1, constituting part of the first optical arrangement. 
     FIG. 2 is a schematic view of another embodiment of the optical system according to the present invention. In this figure, dashed lines illustrate a first optical arrangement of the standard magnifying power, and solid lines illustrate a second optical arrangement of the telephoto magnifying power. Also, the same reference characters have been employed to denote parts similar to those shown in FIGS. 1(A) and 1(B). 
     In this embodiment, the auxiliary optical unit is divided into two parts 2A and 2B which are positioned on either side of an incident normal of the fixed mirror 3. As this allows for proper distribution of the negative power over the auxiliary optical unit, it is made possible to achieve good correction of aberrations. M is an interlinking mechanism by which the shifting of the main optical unit 1 is coordinated with the tilting of the movable mirror. 
     For note, in the embodiments shown in FIGS. 1 and 2, the second optical arrangement may be modified so that the auxiliary optical unit is moved partly or as a whole to make close-up shooting, or the so-called macro photography. 
     Next, numerical examples of the invention are shown. In the numerical examples, Ri is the radius of curvature of the i-th lens surface counting from the front, Di is the i-th lens thickness or air separation counting from the front. Ni and υi are respectively the refractive index and Abbe number of the glass of the i-th lens element counting from the front. 
     
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Numerical Example 1                                                       
The master optical unit:                                                  
F = 100 FNO = 1:2.8 2ω = 55.42°                              
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R1 =  33.30                                                               
           D1 = 11.25 N1 = 1.77250                                        
                                  ν1 = 49.6                            
R2 =  103.04                                                              
           D2 =  2.61                                                     
R3 = -167.04                                                              
           D3 =  4.25 N2 = 1.72151                                        
                                  ν2 = 29.2                            
R4 =  30.74                                                               
           D4 =  3.57                                                     
R5 =  298.96                                                              
           D5 =  1.86 N3 = 1.58144                                        
                                  ν3 = 40.7                            
R6 =   34.29                                                              
           D6 =  9.00 N4 = 1.80610                                        
                                  ν4 = 40.9                            
R7 =  -78.09                                                              
           D7 =  3.57                                                     
R8 =   Stop                                                               
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 b.f = 75.26                                                              
 
    
     The master optical unit and the auxiliary optical unit in combination: 
     
         ______________________________________                                    
F = 200 FNO = 1:5.6 2ω = 29.43°                              
______________________________________                                    
R1 =  33.30                                                               
           D1 = 11.25 N1 = 1.77250                                        
                                  ν1 = 49.6                            
R2 =  103.04                                                              
           D2 =  2.61                                                     
R3 = -167.04                                                              
           D3 =  4.25 N2 = 1.72151                                        
                                  ν2 = 29.2                            
R4 =  30.74                                                               
           D4 =  3.57                                                     
R5 =  298.96                                                              
           D5 =  1.86 N3 = 1.58144                                        
                                  ν3 = 40.7                            
R6 =  34.29                                                               
           D6 =  9.00 N4 = 1.80610                                        
                                  ν4 = 40.9                            
R7 =  -78.09                                                              
           D7 =  3.57                                                     
R8 =   Stop                                                               
           D8 =  5.36                                                     
R9 = -334.71                                                              
           D9 =  5.93 N5 = 1.69895                                        
                                  ν5 = 30.1                            
R10 =  -35.65                                                             
           D10 =  2.64                                                    
R11 =  -31.08                                                             
           D11 =  2.18                                                    
                      N6 = 1.88300                                        
                                  ν6 = 40.8                            
R12 =  92.89                                                              
           D12 =  0.29                                                    
R13 =  57.71                                                              
           D13 =  7.46                                                    
                      N7 = 1.53256                                        
                                  ν7 = 45.9                            
R14 = -349.81                                                             
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 b.f = 117.50                                                             
 
    
     
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Numerical Example 2                                                       
The master optical unit:                                                  
F = 100 FNO = 1:2.8 2ω = 63.00°                              
______________________________________                                    
R1 =  47.16                                                               
           D1 =  5.00 N1 = 1.51633                                        
                                  ν1 = 64.1                            
R2 =  30.25                                                               
           D2 = 15.95                                                     
R3 =  49.47                                                               
           D3 = 16.67 N2 = 1.80400                                        
                                  ν2 = 46.6                            
R4 =  -73.60                                                              
           D4 =  5.36 N3 = 1.60717                                        
                                  ν3 = 40.3                            
R5 = -641.62                                                              
           D5 =  2.67                                                     
R6 = -102.27                                                              
           D6 =  4.29 N4 = 1.74077                                        
                                  ν4 = 27.8                            
R7 =  58.87                                                               
           D7 =  3.54                                                     
R8 = -209.65                                                              
           D8 = 16.67 N5 = 1.80400                                        
                                  ν5 = 46.6                            
R9 =  -47.63                                                              
           D9 =  4.17                                                     
R10 =  Stop                                                               
______________________________________                                    
 b.f = 84.67                                                              
 
    
     The master and auxiliary optical units in combination: 
     
         ______________________________________                                    
F = 200 FNO = 1:5.6 2ω = 34.06°                              
______________________________________                                    
R1 =  47.16                                                               
           D1 =  5.00 N1 = 1.51633                                        
                                  ν1 = 64.1                            
R2 =  30.25                                                               
           D2 = 15.95                                                     
R3 =  49.47                                                               
           D3 = 16.67 N2 = 1.80400                                        
                                  ν2 = 46.6                            
R4 =  -73.60                                                              
           D4 =  5.36 N3 = 1.60717                                        
                                  ν3 = 40.3                            
R5 = -641.62                                                              
           D5 =  2.67                                                     
R6 = -102.27                                                              
           D6 =  4.29 N4 = 1.74077                                        
                                  ν4 = 27.8                            
R7 =  58.87                                                               
           D7 =  3.54                                                     
R8 = -209.65                                                              
           D8 = 16.67 N5 = 1.80400                                        
                                  ν5 = 46.6                            
R9 =  -47.63                                                              
           D9 =  4.17                                                     
R10 =   Stop                                                              
           D10 =  6.25                                                    
R11 = -221.73                                                             
           D11 =  7.92                                                    
                      N6 = 1.68893                                        
                                  ν6 =  31.1                           
R12 =  -45.20                                                             
           D12 =  4.37                                                    
R13 =  -38.74                                                             
           D13 =  3.33                                                    
                      N7 = 1.88300                                        
                                  ν7 = 40.8                            
R14 =  144.38                                                             
           D14 =  0.42                                                    
R15 =  80.97                                                              
           D15 = 10.42                                                    
                      N8 = 1.53256                                        
                                  ν8 = 45.9                            
R16 = -375.56                                                             
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 b.f =126.24                                                              
 
    
     The first specific optical system of the invention when set in the first optical arrangement, or the master optical unit alone, is shown in block form in FIG. 3 and its aberration curves are shown with an object at infinity in FIG. 4, and when set in the second optical arrangement, or the master optical unit plus the auxiliary optical unit, is shown in block form in FIG. 5 and its aberration curves are shown with an object at infinity in FIG. 6. 
     The second specific optical system of the invention when set in the first optical arrangement, or the master optical unit alone, is shown in block form in FIG. 7, and its aberration curves are shown with an object at infinity in FIG. 8, and when set in the second optical arrangement, or the combination of the master and auxiliary optical units, is shown in block form in FIG. 9 and its aberration curves are shown with an object at infinity in FIG. 10. 
     According to the present invention, the changeable magnification optical system having two optical arrangements selectively operated is made to be focused by the same unit in both arrangements, thereby giving an advantage that as the total focusing movement is maintained constant against change of the image magnification, the operating mechanism can be constructed in very simple form. Particularly when applied to autofocus cameras, a great advantage can be produced.