Microscope objective lens system
BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a microscope objective lens system designed for low magnification on the order of 10x. 
     (B) Description of the Prior Art 
     Conventional microscope objective lens systems designed for low magnification, especially on the order of 10x and improved flatness of image comprise a large number of lens elements and have short working distance (W.D.). Further, lateral chromatic aberration is aggravated and can hardly be corrected in miscrocope objective lens systems for high magnification levels though such aberration can be favorably corrected relatively easily in microscope objective lens systems for low magnification levels. For this reason, lateral chromatic aberration is not corrected in high magnification microscope objective lens systems and is corrected later by eyepieces so that such aberration is favorably corrected in the entire optical systems of microscopes. When a low magnification objective lens system is used in combination with an eyepiece so designed as to correct lateral chromatic aberration produced by a high magnification microscope objective lens system, lateral chromatic aberration will unavoidably be produced in the reverse direction. 
     SUMMARY OF THE INVENTION 
     A general object of the present invention is to provide a low magnification microscope lens system comprising a small number of lens elements, assuring improved flatness of image and producing lateral chromatic aberration on the order similar to that in high magnification microscope objective lens systems. 
     The microscope objective lens system according to the present invention comprises a first positive lens, a second positive meniscus lens, a third negative lens, a fourth positive meniscus lens and a fifth positive lens. Said objective lens system satisfies the conditions defined below: 
     
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(1)         0.3F &lt; r.sub.3 &lt; 1.2f                                         
(2)         -0.5f &lt; r.sub.5 &lt; -0.15f                                      
(3)         0.15f &lt; d.sub.4 &lt; 0.5f                                        
(4)                                                                       
             ##STR1##                                                     
(5)         | ν.sub.1 - ν.sub.2 | &lt; 10 ,          
            | n.sub.1 - n.sub.2 | &gt; 0.05                
(6)         0.5f &lt; | r.sub.1 | &lt; 4f                     
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     Wherein the reference symbols represent as follows: r 1 , r 3 , r 5  : radii of curvature on the respective object side surfaces of the first, second and third lenses d 4  : airspace between the second and third lenses d 6  : airspace between the third and fourth lenses d 5 , d 7  : thicknesses of the third and fourth lenses respectively n 1 , n 2 , n 3 , n 4  : refractive indices of the first, second, third and fourth lenses respectively ν 1 , ν 2  : Abbe&#39;s numbers of the first and second lenses respectively f: focal length of the entire lens system as a whole 
     Now, the significance of the afore-mentioned conditions will be described below: The conditions (1) and (2) are adopted for favorably correcting spherical aberration and coma while maintaining curvature of field at a desirable level. That is to say, aforesaid aberrations are favorably corrected by selecting a meniscus lens as the second element and so designing the third lens as to have a strongly concave surface r 5  on the object side. If either one of r 3  and r 5  is smaller than the lower limit of the conditions (1) or (2), it will be difficult to correct curvature of field and it will be impossible to maintain favorable flatness of image. When r 3  or r 5  exceeds the upper limit of the condition (1) or (2), in contrast, it will be impossible to favorably correct spherical aberration and coma. 
     The condition (3) is effective mainly for correcting coma favorably. If d 4  is smaller than the lower limit defined by the condition (3), coma and astigmatic difference will be overcorrected. If d 4  is larger than the upper limit of the condition (3), however, proper balance will be lost among various aberrations. 
     The conditions (4) and (5) are adopted for favorably correcting spherical aberration while maintaining lateral chromatic aberration at a degree similar to that in high magnification microscope objective lens systems. The lens system according to the present invention is so designed as to have an OSC&#39; value larger than those of general objective lens systems. That is to say, C line has an OSC&#39; value of approx. 0.24˜0.35% and F line has an OSC&#39; value of approx. 0.56˜0.86. If d 5  /n 3  +d 6  +d 7  /n 4  has a value larger than the upper limit defined by the condition (4), it will be impossible to maintain proper balance in correction of spherical aberration. If aforesaid value is smaller than the lower limit of the condition (4), in contrast, it will be impossible to control lateral chromatic aberration to the level similar to that in high magnification objective lens systems. If |ν 1  -ν 2  | exceeds 10 in the condition (5), it will be impossible to increase lateral chromatic aberration to the level similar to that in high magnification objective lens systems. When |n 1  -n 2  | is smaller than 0.05 in the condition (4) in contrast, it will be impossible to favorably correct spherical aberration while maintaining lateral chromatic aberration to a level similar to that in high magnification objective lens systems. Finally, the condition (6) is required mainly for correcting spherical aberration favorably. If |r 1  | exceeds the upper limit of the condition (6), it will be difficult to correct spherical aberration. If |r 1  | is smaller than the lower limit of the condition (6), proper balance will be lost among various aberrations, especially aggravating coma and astigmatism. 
     Furthermore, it is preferable to select a glass material having a high refractive index and low dispersion for the third lens in order to maintain favorable flatness of image. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a sectional view illustrating the composition of the lens system accoreing to the present invention; 
     FIG. 2A through FIG. 2D show curves illustrating the aberration characteristics of the Embodiment 1; 
     FIG. 3A through FIG. 3D show curves illustrating the aberration characteristics of the Embodiment 2; 
     FIG. 4A through FIG. 4D illustrate graphs showing the aberration characteristics of the Embodiment 3; and 
     FIG. 5A through FIG. 5D illustrate graphs showing the abeeration characteristics of the Embodiment 4. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, some preferred embodiments of the present invention will be detailedly described below: 
     
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Embodiment 1                                                              
f = 1             NA = 0.25   β = -9.977                             
WD = 0.3980       Petzval sum 0.20                                        
r.sub.1 = -1.6154                                                         
           d.sub.1 = 0.0940                                               
                      n.sub.1 = 1.72916                                   
                                  ν.sub.1 = 54.7                       
r.sub.2 = -0.4890                                                         
           d.sub.2 = 0.0281                                               
r.sub.3 = 0.3617                                                          
           d.sub.3 = 0.1055                                               
                      n.sub.2 = 1.64                                      
                                  ν.sub.2 = 60.2                       
r.sub.4 = 1.0714                                                          
           d.sub.4 = 0.4012                                               
r.sub.5 = -0.1965                                                         
           d.sub.5 = 0.1307                                               
                      n.sub.3 = 1.80518                                   
                                  ν.sub.3 = 25.4                       
r.sub.6 = ∞                                                         
           d.sub.6 = 0.0075                                               
r.sub.7 = -1.3286                                                         
           d.sub.7 = 0.2132                                               
                      n.sub.4 = 1.62299                                   
                                  ν.sub.4 = 58.2                       
r.sub.8 = -0.4663                                                         
           d.sub.8 = 0.0344                                               
r.sub.9 = -12.9551                                                        
           d.sub.9 = 0.1399                                               
                      n.sub.5 = 1.64250                                   
                                  ν.sub.5 = 58.4                       
r.sub.10 = -0.6018                                                        
Embodiment 2                                                              
f = 1             NA = 0.25   β = -9.999                             
WD = 0.4083       Petzval sum 0.2                                         
r.sub.1 = -1.3336                                                         
           d.sub.1 = 0.0964                                               
                      n.sub.1 = 1.755                                     
                                  ν.sub.1 = 52.4                       
r.sub.2 = -0.5108                                                         
           d.sub.2 = 0.0296                                               
r.sub.3 = 0.3588                                                          
           d.sub.3 = 0.1079                                               
                      n.sub.2 = 1.64                                      
                                  ν.sub.2 = 60.2                       
r.sub.4 = 1.1284                                                          
           d.sub.4 = 0.3806                                               
r.sub.5 = -0.2050                                                         
           d.sub.5 = 0.1332                                               
                      n.sub.3 = 1.80518                                   
                                  ν.sub.3 = 25.4                       
r.sub.6 = ∞                                                         
           d.sub.6 = 0.0127                                               
r.sub.7 = -1.3245                                                         
           d.sub. 7 = 0.2177                                              
                      n.sub.4 = 1.62299                                   
                                  ν.sub.4 = 58.2                       
r.sub.8 = -0.4754                                                         
           d.sub.8 = 0.0289                                               
r.sub.9 = 36.3549                                                         
           d.sub.9 = 0.1387                                               
                      n.sub.5 = 1.6425                                    
                                  ν.sub.5 = 58.4                       
r.sub.10 = -0.6668                                                        
Embodiment 3                                                              
f = 1             NA = 0.25   β  = -10.0                             
WD = 0.4122       Petzual sum 0.25                                        
r.sub.1 = 0.7385                                                          
           d.sub.1 = 0.0976                                               
                      n.sub.1 = 1.7725                                    
                                  υ.sub.1 = 49.6                  
r.sub.2 = -0.8133                                                         
           d.sub.2 = 0.0296                                               
r.sub.3 = 1.1096                                                          
           d.sub.3 = 0.1065                                               
                      n.sub.2 = 1.691                                     
                                  ν.sub.2 = 54.8                       
r.sub.4 = 5.9537                                                          
           d.sub.4 = 0.2661                                               
r.sub.5 = -0.2321                                                         
           d.sub.5 = 0.1325                                               
                      n.sub.3 =  1.84666                                  
                                  ν.sub.3 = 23.9                       
r.sub.6 = ∞                                                         
           d.sub.6 = 0.0138                                               
r.sub.7 = -0.4858                                                         
           d.sub.7 = 0.2189                                               
                      n.sub.4 = 1.618                                     
                                  ν.sub.4 = 63.4                       
r.sub.8 = -0.3832                                                         
           d.sub.8 = 0.0849                                               
r.sub.9 = 2.8955                                                          
           d.sub.9 = 0.1159                                               
                      n.sub.5 = 1.618                                     
                                  ν.sub.5 = 63.4                       
r.sub.10 = -0.8358                                                        
Embodiment 4                                                              
f = 1             NA = 0.25   β = -10.0                              
WD = 0.4187       Petzval sum 0.25                                        
r.sub.1 = 3.6219                                                          
           d.sub.1 = 0.0988                                               
                      n.sub.1 = 1.7859                                    
                                  ν.sub.1 = 44.1                       
r.sub.2 = -0.4897                                                         
           d.sub.2 = 0.0305                                               
r.sub.3 = 0.3711                                                          
           d.sub.3 = 0.1109                                               
                      n.sub.2 = 1.6935                                    
                                  ν.sub.2 = 53.3                       
r.sub. 4 = 0.3933                                                         
           d.sub.4 = 0.2297                                               
r.sub.5 = -0.2429                                                         
           d.sub.5 = 0.1368                                               
                      n.sub.3 = 1.84666                                   
                                  ν.sub.3 = 23.9                       
r.sub.6 = ∞                                                         
           d.sub.6 = 0.0135                                               
r.sub.7 = -0.9266                                                         
           d.sub.7 = 0.2235                                               
                      n.sub.4 = 1.48749                                   
                                  ν.sub.4 = 70.1                       
r.sub.8 = -0.3829                                                         
           d.sub.8 = 0.0766                                               
r.sub.9 = 5.1828                                                          
           d.sub.9 = 0.1497                                               
                      n.sub.5 = 1.618                                     
                                  ν.sub.5 = 63.4                       
r.sub.10 = -0.7975                                                        
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     wherein the reference symbols r 1  through r 10  represent radii of curvature on the respective surfaces of the first through the fifth lenses, the reference symbols d 1  through d 9  designate thicknesses of the respective lenses and airsapaces therebetween and the reference symbols ν 1  through ν 5  denote Abbe&#39;s numbers of the respective lenses.