Retrofocus-type objective for endoscopes
This is a continuation of application Ser. No. 624,823, filed Oct. 22, 1975, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     a. Field of the invention 
     The present invention relates to an objective for endoscopes and, more particularly, to a retrofocus-type objective for endoscopes. 
     B. Description of the prior art 
     Known objectives for endoscopes are generally focused by advancing the lens system as a whole. Besides, a retrofocus-type lens system is known as a lens system by which the image plane can be made flat over a wide area. For the retrofocus-type lens system, however, the aperture stop is arranged in the inside of the lens system. Consequently, the position of the entrance pupil comes off the front lens surface of the lens system and, therefore, the front lens diameter becomes large. 
     Besides, for endoscopes, a cover glass for waterproof purpose is provided in front of the objective for endoscopes. When, therefore, the objective for endoscopes is to be focused by advancing the lens system as a whole in case of an endoscope employing a retrofocus-type objective with a large front lens diameter, the airspace between the cover glass and lens system is to be varied and, consequently, the cover glass diameter becomes very large. It is, however, not desirable to make the cover glass diameter large because pain of the patient will increase. 
     SUMMARY OF THE INVENTION 
     It is, therefore, a primary object of the present invention to provide a retrofocus-type objective for endoscopes comprising a front diverging lens group and rear converging lens group, said rear converging lens group being divided into two lens groups, said retrofocus-type objective for endoscopes being arranged to be focused by keeping the front lens fixed and varying the total focal length of the rear converging lens group by varying the airspace between the said two lens groups constituting the rear converging lens group. 
     When the lens system is focused by keeping the front lens fixed and varying magnification of the rear converging lens group by varying the airspace between two lens groups constituting the rear converging lens group as mentioned in the above, it becomes impossible when magnification of the rear lens group becomes close to ×l, to focus the lens system on an object at a distance shorter than the distance at that time. In this case, the lens system can be focused within the range which satisfies the formula (1) when reference symbol β represents magnification of the rear converging lens group when the object is at the infinite distance, reference symbol f B  represents the focal length of the rear converging lens group, and reference symbol Δ represents the amount of movement of the position of the image focused by the front diverging lens group when the distance to the object becomes shorter. 
     
         Δ ≦ f.sub.B (β + (1/β) - 2)         (1) 
    
     when the rear converging lens group is divided into two groups as mentioned in the above, i.e., into a first lens group and second lens group, relation between magnifications of said first and second lens groups is expressed the following formula (2) where reference symbol β 1  represents magnification of the first lens group when the object is at the infinite distance, reference symbol β 2  represents magnification of the second lens group when the object is also at the infinite distance, reference symbol f represents focal length of the endoscope objective as a whole, and reference symbol f F  represents focal length of the front diverging lens group. 
     
         β.sub.1 · β.sub.2 = β = f/f.sub.F = constant (2) 
    
     From the above formula (2), it becomes 
     
         |β.sub.1 | + |β.sub.2 | ≧ 2√|β|              (3) 
    
     That is, the sum of |β 1  | and |β 2  | may be an arbitrary value which is larger than 2√|β|. However, in order to make either |β 1  | or |β 2  | smaller than |β| and at the same time to make it possible to focus the lens system on an object at a short distance, it is necessary to satisfy the following condition. 
     
         |β.sub.1 | + |β.sub.2 | &gt; 1 + β 
    
     When the condition |β 1  | + |β 2  | &gt; 1 + |β| is satisfied, either ( β 1  + (1/β 1 ) ) or (β 2  + (1/β 2 ) ) becomes larger than (β+ (1/β), and it becomes possible to focus the lens system on an object at a short distance when it is arranged to focus by moving the lens group for which the above-mentioned value becomes larger than (β + (1/β) ). 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a sectional view of Embodiment 1 of the retrofocus-type objective for endoscopes according to the present invention; and 
     FIG. 2 shows a sectional view of Embodiment 2 of the retrofocus-type objective for endoscopes according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the retrofocus-type objective for endoscopes according to the present invention explained in the above are as shown below. 
     
         ______________________________________                                    
Embodiment 1                                                              
______________________________________                                    
r.sub.1 =                                                                 
      6.888                                                               
      d.sub.1 = 0.7                                                       
                   n.sub.1 = 1.51633                                      
                                ν.sub.1 = 64.1                         
r.sub.2 =                                                                 
      1.446                                                               
      d.sub.2 (Variable)                                                  
r.sub.3 =                                                                 
      -7.022                                                              
      d.sub.3 = 1.2                                                       
                   n.sub.2 = 1.69680                                      
                                ν.sub.2 = 55.7                         
r.sub.4 =                                                                 
      -3.565                                                              
      d.sub.4 (Variable)                                                  
r.sub.5 =                                                                 
      19.657                                                              
      d.sub.5 = 1.2                                                       
                   n.sub.3 = 1.69680                                      
                                ν.sub.3 = 55.7                         
r.sub.6 =                                                                 
      -5.829                                                              
      d.sub.6 = 0.2                                                       
r.sub.7 =                                                                 
      7.783                                                               
      d.sub.7 = 2  n.sub.4 = 1.69680                                      
                                ν.sub.4 = 55.7                         
r.sub.8 =                                                                 
      -2.831                                                              
      d.sub.8 = 0.7                                                       
                   n.sub.5 = 1.78472                                      
                                ν.sub.5 = 25.7                         
r.sub.9 =                                                                 
      43.3                                                                
f = 2.493, F.sub.NO = 3.0, 2ω = 70° (Diagonal);              
48°  30&#39; (between opposite sides)                                  
f.sub.F = -1.651, f.sub.B = 3.305                                         
β = -0.673, β.sub.1 = 6.896, β.sub.2 = -0.098              
______________________________________                                    
 
    
     The above Embodiment 1 has lens configuration as shown in FIG. 1 and comprises a front diverging lens group and rear converging lens group. The front diverging lens group comprises a first lens component L 1  and the rear coverging lens group comprises a second lens component L 2 , third lens component L 3  and fourth lens component L 4 . The rear converging lens group is divided into two groups, i.e., a first lens group which comprises the second lens component L 2  and a second lens group which comprises the third lens component L 3  and fourth lens component L 4 , and the lens system according to Embodiment 1 is arranged to be focused by varying the airspace d 4  between said first and second lens groups. For varying said airspace d 4 , there are two methods. One method is to move the second lens group and the other is to move the first lens group. Numerical values of airspaces d 2  and d 4  when the airspace d 4  is varied by these two methods are as shown below. 
     
         ______________________________________                                    
(1) When the second lens group is moved                                   
                   d.sub.2                                                
                          d.sub.4                                         
When distance to                                                          
object is ∞  2.51   1.5                                             
When distance to                                                          
object is 4 mm     2.51   0.43                                            
(2) When the first lens group is moved                                    
                   d.sub.2                                                
                          d.sub.4                                         
When distance to                                                          
object is ∞  2.5    1.7                                             
When distance to                                                          
object is 4 mm     4.15   0.05                                            
______________________________________                                    
 
    
     
         ______________________________________                                    
Embodiment 2                                                              
______________________________________                                    
r.sub.1 =                                                                 
      ∞                                                             
      d.sub.1 = 0.6                                                       
                   n.sub.1 = 1.51633                                      
                                ν.sub.1 = 64.1                         
r.sub.2 =                                                                 
      1.921                                                               
      d.sub.2 = 0.9                                                       
r.sub.3 =                                                                 
      ∞                                                             
      d.sub.3 = 6.9                                                       
                   n.sub.2 = 1.80610                                      
                                ν.sub.2 = 40.8 (Prism)                 
r.sub.4 =                                                                 
      ∞                                                             
      d.sub.4 (Variable)                                                  
r.sub.5 =                                                                 
      -49.257                                                             
      d.sub.5 = 1.0                                                       
                   n.sub.3 = 1.69680                                      
                                ν.sub.3 = 55.7                         
r.sub.6 =                                                                 
      -5.02                                                               
      d.sub.6 = 0.2                                                       
r.sub.7 =                                                                 
      4.254                                                               
      d.sub.7 = 1.6                                                       
                   n.sub.4 = 1.62041                                      
                                ν.sub.4 = 60.3                         
r.sub.8 =                                                                 
      -3.883                                                              
      d.sub.8 = 0.5                                                       
                   n.sub.5 = 1.84666                                      
                                ν.sub.5 = 23.9                         
r.sub.9 =                                                                 
      -7.972                                                              
      d.sub.9 (Variable)                                                  
r.sub.10 =                                                                
      -3.885                                                              
      d.sub.10 = 0.5                                                      
                   n.sub.6 = 1.84666                                      
                                ν.sub.6 = 23.9                         
r.sub.11 =                                                                
      -6.706                                                              
f = 2.178, F.sub.NO = 3.0, 2ω = 85° 28&#39; (Diagonal);          
57° 30&#39; (between opposite sides)                                   
f.sub.F = -1.935, f.sub.B = 2.703                                         
β = 0.825, β.sub.1 = -0.641, β.sub.2 = 1.286               
______________________________________                                    
 The above embodiment 2 has lens configuration as shown in FIG. 2 and
 comprises a front diverging lens group, prism P and rear converging lens
 group. The front diverging lens group comprises a first lens component
 L.sub.1 and the rear converging lens group comprises a second lens
 component L.sub.2, third lens component L.sub.3 and fourth lens component
 L.sub.4. The rear converging lens group is divided into two groups, i.e.,
 a first lens group which comprises the second lens component L.sub.2 and
 third lens component L.sub.3 and a second lens group which comprises the
 fourth lens component L.sub.4, and the lens system according to Embodiment
 2 is arranged to be focused by varying the airspace d.sub.9 between said
 first and second lens groups. Numerical values of airspaces d.sub.4 and
 d.sub.9 when the airspace d.sub.9 is varied are as shown below.
 
    
     
         ______________________________________                                    
                 d.sub.4                                                  
                      d.sub.9                                             
______________________________________                                    
When distance to                                                          
object is ∞  1.33   1.5                                             
When distance to                                                          
object is 5 mm     0.7    2.13                                            
______________________________________                                    
 
    
     As explained in the above, the retrofocus-type objective for endoscopes according to the present invention can be focused without advancing the lens system as a whole, i.e., by keeping the front lens fixed. Therefore, the front lens can be used also as a cover glass for waterproof purpose. Thus, the present invention provides a retrofocus-type objective for endoscopes which is small in size and which can be focused favourably.