Abstract:
An objective lens system for endoscopes comprising a first negative meniscus lens component, a second positive meniscus lens component, a third biconvex lens component and a fourth positive cemented doublet component consisting of a positive lens element and a negative lens element, said lens system having a super-wide angle and favorably corrected aberrations, especially spherical aberration and lateral chromatic aberration.

Description:
BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a wide-angle retrofocus type of objective lens system for endoscopes. 
     (b) Description of the Prior Art 
     In clinical inspection with an endoscope, it is necessary to observe or photograph a wide range of inspection area at a time so as to locate abnormal portion within the sight. For this reason, objective lens systems having wide angles are desired for endoscopes. However, the conventional objective lens systems for endoscopes had field angles of about 80° at widest. Therefore, the endoscopes using these objective lens systems permitted observing or photographing narrow ranges only at a time, required long time for clinical inspections, made patients suffer from much pain or large burden, and had rather high possibility to make abnormal portions out of sight. 
     In order to widen field angle of an objective lens system for endoscopes, it is generally known to design the so-called retrofocus type of lens system in which a diverging lens group and a converging lens group are arranged as the front and rear lens groups respectively. This refrofocus type of lens system has excellent features to permit locating the rear focal points apart from the final lens surface, avoiding influences due to contamination and injury on lens surface and correcting curvature of field over a wide field angle owing to a concave lens component having strong power in the front lens group. 
     However, when the retrofocus type of lens system having the above-described excellent features is so designed as to have such a ultra-wide angle as is mentioned as the object of the present invention, it has drawbacks. Speaking concretely of the drawbacks, the lens system allows spherical aberration and lateral chromatic aberration to be produced by the negative refractive power which is too strong in the front lens group. 
     SUMMARY OF THE INVENTION 
     A general object of the present invention is to provide a ultra-wide-angle retrofocus type of objective lens system for endoscopes comprising relatively small number of lens elements, i.e., four components of five elements, and having favorably corrected aberrations, especially spherical aberration and lateral chromatic aberration. The lens system according to the present invention has such a composition as shown in FIG. 1. Speaking concretely, the lens system comprises a first negative meniscus lens component having a convex surface on the object side, a stop, a second positive meniscus lens component, a third biconvex lens component and a fourth positive cemented doublet component consisting of a positive lens element and negative lens element. Further, the lens system according to the present invention is so designed as to satisfy the following conditions (1) through (6): ##EQU1## wherein the reference symbols are defined as follows: f: focal length of the entire lens system as a whole 
     f 234  : total focal length of said second, third and fourth lens components as a whole 
     r 2 , r 5  and r 7  : radii of curvature on the image side surface of said first lens component, image side surface of said second lens component and image side surface of said third lens component respectively 
     n 1 , n 2 , n 3 , n 4  and n 5  : refractive indices of the respective lens elements 
     ν 1 , ν 2  and ν 3  : Abbe&#39;s numbers of said first, second and third lens components respectively 
     In the lens system according to the present invention, the first lens component arranged in the front lens group is so designed as to have the convex surface on the object side for the purpose of allowing rays to be incident even at wide angles and widening field angle of the lens system. Further, the image side surface of the first lens component functions to minimize the diameter of the flux of the rays emerging from said first lens component, thereby making it possible to widen field angle of the lens system without enlarging the entire lens system. Since the first lens component is designed as a negative meniscus lens component, it is possible to minimize Petzval&#39;s sum and correct curvature of field over the entire range of the wide field. Moreover, aberrations such as spherical aberration and chromatic aberration can be corrected more favorably by selecting material of the first lens component so as to satisfy the conditions (1) and (2). 
     The condition (1) is adopted for the purpose of minimizing Petzval&#39;s sum and suppress spherical aberration. If the refractive index of the first lens component is smaller than 1.51 defined as the lower limit of the condition (1), power of the first lens component must be increased to minimize Petzval&#39;s sum and, as a result, the spherical aberration produced by this lens component is aggravated too much to be corrected favorably by the rear lens group (i.e., the second, third and fourth lens components). 
     The condition (2) is adopted for the purpose of minimizing chromatic aberration to be produced by the first lens component. If the Abbe&#39;s number ν 1  of this lens component exceeds the lower limit of the condition (2), the chromatic aberration produced in this lens component cannot be corrected favorably by the rear lens group. 
     In the objective lens system according to the present invention, the aperture stop is arranged between the first lens component and the second lens component for the purpose of minimizing the angle of incidence for the rays which are incident on the image guide composed of optical fibers after passing through the fourth lens component. In other words, the aperture stop is arranged in the vicinity of the object side focal point of the rear lens group to make the rays passed through the rear lens group nearly parallel to the optical axis. In a lens system having such a composition as that of the lens system according to the present invention, the object side focal point of the rear lens group is located between the first lens component and the second lens component. Therefore, arranging the stop at the position described above makes it possible to minimize angle of incidence for the rays incident on the image guide composed of optical fibers, thereby assuring high transmission characteristic for the rays transmitted by the image guide. 
     The condition (3) is adopted mainly for correcting curvature of field and coma. If f 234  /f exceeds the upper limit of the condition (3), the distance between the conjugate points of the rear lens group is prolonged in proportion to focal length, thereby prolonging total length of the entire lens system. It is therefore impossible to satisfy the requirement of compact design necessary for an objective lens system for endoscopes. If f 234  /f is smaller than the lower limit of the condition (3), in contrast, the total focal length of the rear lens group will be shortened and, so far as height of the incident rays is kept constant, the rear lens group will have a larger field angle, thereby aggravating curvature of field and coma. 
     The condition (4) is adopted mainly for minimizing spherical aberration. Speaking concretely, in a wide-angle objective lens system as that according to the present invention, the image side surface of the first lens component must have strong refractive power for the purpose of thinning the flux of rays having passed through the rear lens group as already described above. When said surface has a strong refractive power, however, it will produce spherical aberration. The condition (4) is effective to correct the spherical aberration. If the lower limit of the condition (4) is exceeded, spherical aberration will be overcorrected by the rear lens group. If the upper limit of the condition (4) is exceeded, in contrast, the spherical aberration will be undercorrected. It is therefore impossible to balance the spherical aberration properly unless the condition (4) is satisfied. 
     The conditions (5) and (6) are necessary mainly for correcting lateral chromatic aberration. In a wide-angle objective lens system for endoscopes so as that according to the present invention, lateral chromatic aberration is produced remarkably. The condition (5) serves for suppressing production of lateral chromatic aberration in the second lens component and the third lens component which are designed as single-element lenses. If the condition (5) is not satisfied, it will be impossible to favorably correct the chromatic aberration produced remarkably in the second and third lens components. 
     Even when the lens system is so designed as to satisfy the conditions (2) and (5), it is difficult to suppress production of chromatic aberration. In order to correct the chromatic aberration, the fourth lens component is designed as a cemented doublet and the difference in refractive index between both the lens elements is selected as defined by the condition (6) so that lateral chromatic aberration will be minimized even at a wide angle. If the condition (6) is not satisfied, lateral chromatic aberration will be aggravated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a sectional view illustrating the composition of the objective lens system for endoscopes according to the present invention; and 
     FIG. 2 through FIG. 16 show graphs illustrating aberration characteristics of Embodiments 1 through 15 of the objective lens system according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, numerical data of the objective lens system for endoscopes will be described below as preferred embodiments of the present invention: 
     
         ______________________________________Embodiment 1r.sub.1 =   40.340   d.sub.1 = 0.509               n.sub.1 = 1.58913                          ν.sub.1 = 60.97r.sub.2 =   0.844   d.sub.2 = 1.253r.sub.3 =   ∞ (stop)   d.sub.3 = 0.372r.sub.4 =   -35.462   d.sub.4 = 1.067               n.sub.2 = 1.69680                          ν.sub.2 = 55.52r.sub.5 =   -2.318   d.sub.5 = 0.124r.sub.6 =   9.484   d.sub.6 = 1.129               n.sub.3 = 1.51728                          ν.sub.3 = 69.56r.sub.7 =   -2.955   d.sub.7 = 0.124r.sub.8 =   2.782   d.sub.8 = 1.948               n.sub.4 = 1.61800                          ν.sub.4 = 63.38r.sub.9 =   -1.608   d.sub.9 = 0.434               n.sub.5 = 1.84666                          ν.sub.5 = 23.90r.sub.10 =   -49.305f = 1.0, 1 = 15.0, S = 0.81IH = 0.881, 2ω = 115.84°, Σp = -0.014NA = 0.0129, f.sub.234 /f = 1.614 ##STR1##  Embodiment 2r.sub.1 =   28.7611   d.sub.1 = 0.4938               n.sub.1 = 1.58913                          ν.sub.1 = 60.97r.sub.2 =   0.8457   d.sub.2 = 0.9877r.sub.3 =   ∞ (stop)   d.sub.3 = 0.6173r.sub.4 =   -99.2330   d.sub.4 = 1.0494               n.sub.2 =  1.6968                          ν.sub.2 = 55.52r.sub.5 =   -2.1786   d.sub.5 = 0.1235r.sub.6 =   9.2568   d.sub.6 = 1.1111               n.sub.3 = 1.6968                          ν.sub.3 = 55.52r.sub.7 =   -4.4531   d.sub.7 = 0.1235r.sub.8 =   3.0441   d.sub.8 = 1.7284               n.sub.4 = 1.6180                          ν.sub.4 = 63.38r.sub.9 =   -1.6506   d.sub.9 = 0.4321               n.sub.5 = 1.84666                          ν.sub.5 = 23.90r.sub.10 =   -18.0296f = 1.0, 1 = 15.0, S = 0.863IH = 0.889, 2ω = 117.08°, ΣP = 0NA = 0.0129, f.sub.234 /f = 1.591 ##STR2##  Embodiment 3r.sub.1 =   20.6504   d.sub.1 = 0.5001               n.sub.1 = 1.58913                          ν.sub.1 = 60.97r.sub.2 =   0.8479   d.sub.2 = 1.2476r.sub.3 =   ∞ (stop)   d.sub.3 = 0.3770r.sub.4 =   -63.1598   d.sub.4 = 1.0635               n.sub.2 = 1.69680                          ν.sub.2 = 55.52r.sub.5 =   -2.4379   d.sub.5 = 0.1251r.sub.6 =   7.2310   d.sub.6 = 1.1253               n.sub.3 = 1.51728                          ν.sub.3 = 69.56r.sub.7 =   -3.0381   d.sub.7 = 0.1255r.sub.8 =   2.6453   d.sub.8 = 1.8743               n.sub.4 = 1.61800                          ν.sub.4  = 63.38r.sub.9 =   -1.6289   d.sub.9 = 0.4375               n.sub.5 = 1.84666                          ν.sub.5 = 23.90r.sub.10 =   -812.0951f = 1.0, 1 = 15.0, S = 0.773IH = 0.881, 2ω = 115.57°, ΣP = 0NA = 0.0129, f.sub.234 /f = 1.587 ##STR3##  Embodiment 4r.sub.1 =   21.3790   d.sub.1 = 0.5062               n.sub.1 = 1.58913                          ν.sub.1 = 60.97r.sub.2 =   0.8417   d.sub.2 = 1.2469r.sub.3 =   ∞ (stop)   d.sub.3 = 0.3704r.sub.4 =   -35.9354   d.sub.4 = 1.0617               n.sub.2 = 1.69680                          ν.sub.2 = 55.52r.sub.5 =   -2.2956   d.sub.5 = 0.1246r.sub.6 =   8.7810   d.sub.6 = 1.1235               n.sub.3 = 1.51728                          ν.sub.3 = 69.56r.sub.7 =   -3.0240   d.sub.7 = 0.1235r.sub.8 =   2.6730   d.sub.8 = 1.8765               n.sub.4 = 1.61800                          ν.sub.4 = 63.38r.sub.9 =   -1.6290   d.sub.9 = 0.4321               n.sub.5 = 1.84666                          ν.sub.5 = 23.90r.sub.10 =   -206.3722f = 1.0, 1 = 15.0, S = 0.794IH = 0.881, 2ω = 114.86°, ΣP = -0.006NA = 0.0129, f.sub.234 /f = 1.589 ##STR4##  Embodiment 5r.sub.1 =   17.6741   d.sub.1 = 0.4938               n.sub.1 = 1.58913                          ν.sub.1 = 60.97r.sub.2 =   0.8373   d.sub.2 = 0.9877r.sub.3 =   ∞ (stop)   d.sub.3 = 0.6173r.sub.4 =   -36.3478   d.sub.4 = 1.0494               n.sub.2 = 1.69680                          ν.sub.2 = 55.52r.sub.5 =   -2.4411   d.sub.5 = 0.1246r.sub.6 =   6.5303   d.sub.6 = 1.0864               n.sub.3 = 1.48749                          ν.sub.3 = 70.15r.sub.7 =   -2.8037   d.sub. 7 = 0.1235r.sub.8 =   2.5290   d.sub.8 = 1.7284               n.sub.4 = 1.61800                          ν.sub.4 = 63.38r.sub.9 =   -1.5909   d.sub.9 = 0.4321               n.sub.5 = 1.84666                          ν.sub.5 = 23.90r.sub.10 =   89.7642f = 1.0, 1 = 15.0, S = 0.863IH = 0.881, 2ω = 115.53°, ΣP = 0NA = 0.0129, f.sub.234 /f = 1.574 ##STR5##  Embodiment 6r.sub.1 =   20.3968   d.sub.1 = 0.4938               n.sub.1 =  1.58913                          ν.sub.1 = 60.97r.sub.2 =   0.8374   d.sub.2 = 0.9877r.sub.3 =   ∞ (stop)   d.sub.3 = 0.6173r.sub.4 =   -62.3951   d.sub.4 = 1.0494               n.sub.2 = 1.6968                          ν.sub.2 = 55.52r.sub.5 =   -2.4077   d.sub.5 = 0.1235r.sub.6 =   7.1414   d.sub.6 = 1.1110               n.sub.3 = 1.51728                          ν.sub.3 = 69.56r.sub.7 =   -3.0005   d.sub.7 = 0.1236r.sub.8 =   2.6126   d.sub.8 = 1.7284               n.sub.4 = 1.61800                          ν.sub.4 = 63.38r.sub.9 =   -1.6088   d.sub.9 = 0.4321               n.sub.5 = 1.84666                          ν.sub.5 = 23.90r.sub.10 =   -796.822f = 1, 1 = 15.0, S = 0.863IH = 0.0881, 2ω = 115.71°, ΣP = 0NA = 0.013, f.sub.234 /f = 1.584 ##STR6##  Embodiment 7r.sub.1 =   46.5568   d.sub.1 = 0.5043               n.sub.1 = 1.58913                          ν.sub.1 = 60.97r.sub.2 =   0.8358   d.sub.2 = 1.2423r.sub.3 =   ∞ (stop)   d.sub.3 = 0.3690r.sub.4 =   -39.4208   d.sub.4 = 1.0578               n.sub.2 = 1.6968                          ν.sub.2 = 55.52r.sub.5 =   -2.2932   d.sub.5 = 0.1230r.sub.6 =   8.9076   d.sub.6 = 1.1193               n.sub.3 = 1.5728                          ν.sub.3 = 69.56r.sub.7 =   -2.9702   d.sub.7 = 0.1230r.sub.8 =   2.7349   d.sub.8 = 1.8696               n.sub.4 = 1.61800                          ν.sub.4 = 63.38r.sub.9 =   -1.5664   d.sub.9 = 0.4305               n.sub.5 = 1.84666                          ν.sub.5 = 23.90r.sub.10 =   -363.0739f = 1.0, 1 = 15.0, S = 0.838IH = 0.881, 2ω = 115.646°, ΣP = -0.022NA = 0.0129, f.sub.234 /f = 1.53 ##STR7##  Embodiment 8r.sub.1 =   75.582   d.sub.1 = 0.523               n.sub.1 = 1.58913                          ν.sub.1 = 60.97r.sub.2 =   0.872   d.sub.2 = 1.264r.sub.3 =   ∞ (stop)   d.sub.3 = 0.376r.sub.4 =   -31.949   d.sub.4 = 1.076               n.sub.2 = 1.69680                          ν.sub.2 = 55.52r.sub.5 =   -2.309   d.sub.5 = 0.125r.sub.6 =   10.102   d.sub.6 = 1.139               n.sub.3 = 1.51728                          ν.sub.3 = 69.56r.sub.7 =   -2.963   d.sub.7 = 0.125r.sub.8 =   2.737   d.sub.8 = 1.965               n.sub.4 = 1.6180                          ν.sub.4 = 63.38r.sub.9 =   -1.602   d.sub.9 = 0.439               n.sub.5 = 1.84666                          ν.sub.5 = 23.90r.sub. 10 =   -72.703f = 1.0, 1 = 15.0, S = 0.78IH = 0.888, 2ω = 117.85°, ΣP = -0.009NA = 0.0132, f.sub.234 /f = 1.563 ##STR8##  Embodiment 9r.sub.1 =   76.346   d.sub.1 = 0.5277               n.sub.1 = 1.58913                          ν.sub.1 = 60.97r.sub.2 =   0.880   d.sub.2 = 1.2771r.sub.3 =   ∞ (stop)   d.sub.3 = 0.38r.sub.4 =   -31.030   d.sub.4 = 1.0872               n.sub.2 = 1.670                          ν.sub.2 = 57.33r.sub.5 =   -2.242   d.sub.5 = 0.1010r.sub.6 =   10.204   d.sub.6 = 1.1505               n.sub.3 = 1.51728                          ν.sub.3 = 69.56r.sub.7 =   -2.993   d.sub.7 = 0.1010r.sub.8 =   2.764   d.sub.8 = 2.0202               n.sub.4 = 1.618                          ν.sub.4 = 63.38r.sub.9 =   -1.618   d.sub.9 = 0.4433               n.sub.5 = 1.84666                          ν.sub.5 = 23.90r.sub.10 =   -73.4376f = 1.0, 1 = 15.0, S = 0.762IH = 0.899, 2ω = 120.19°, ΣP = -0.006NA = 0.013, f.sub.234 /f = 1.612 ##STR9##  Embodiment 10r.sub.1 =   75.582   d.sub.1 = 0.523               n.sub.1 = 1.58913                          ν.sub.1 = 60.97r.sub.2 =   0.872   d.sub.2 = 1.264r.sub.3 =   ∞ (stop)   d.sub.3 = 0.376r.sub.4 =   -31.949   d.sub.4 = 1.076               n.sub.2 = 1.6968                          ν.sub.2 = 55.52r.sub.5 =   -2.309   d.sub.5 = 0.125r.sub.6 =   10.102   d.sub.6 = 1.139               n.sub.3 = 1.51728                          ν.sub.3 = 69.56r.sub.7 =   -2.963   d.sub.7 = 0.125r.sub.8 =   3.074   d.sub.8 = 1.965               n.sub.4 = 1.69249                          ν.sub.4 =  58.56r.sub.9 =   -1.602   d.sub.9 = 0.439               n.sub.5 = 1.92286                          ν.sub.5 = 21.29r.sub.10 =   -36.333f = 1.0, 1 = 15.0, S = 0.853IH = 0.89, 2ω = 117.6°,ΣP = -0.005NA = 0.0129, f.sub.234 /f = 1.583 ##STR10##  Embodiment 11r.sub.1 =   80.059   d.sub.1 = 0.523               n.sub.1 = 1.726                          ν.sub.1 = 53.56r.sub.2 =   0.945   d.sub.2 = 1.264r.sub.3 =   ∞  (stop)   d.sub.3 = 0.376r.sub.4 =   -30.452   d.sub.4 = 1.076               n.sub.2 = 1.6968                          ν.sub.2 = 55.52r.sub.5 =   -2.227   d.sub.5 = 0.126r.sub.6 =   12.588   d.sub.6 = 1.139               n.sub.3 = 1.51728                          ν.sub.3 = 69.56r.sub.7 =   -2.878   d.sub.7 = 0.126r.sub.8 =   3.426   d.sub.8 = 1.965               n.sub.4 = 1.69249                          ν.sub.4 = 58.56r.sub.9 =   -1.557   d.sub.9 = 0.439               n.sub.5 = 1.92286                          ν.sub.5 = 21.29r.sub.10 =   -36.333f = 1.0, 1 = 15.0, S = 1.05IH = 0.89, 2ω = 116.69°, ΣP = -0.036NA = 0.0128, f.sub.234 /f = 1.666 ##STR11##  Embodiment 12r.sub.1 =   39.261   d.sub.1 = 0.45               n.sub.1 = 1.58913                          ν.sub.1 = 60.97r.sub.2 =   0.834   d.sub.2 = 1.28r.sub.3 =   ∞ (stop)   d.sub.3 = 0.38r.sub.4 =   -33.554   d.sub.4 = 1.05               n.sub.2 = 1.6968                          ν.sub.2 = 55.52r.sub.5 =   -2.325   d.sub.5 = 0.1r.sub.6 =   10.771   d.sub.6 = 1.06               n.sub.3 = 1.51728                          ν.sub.3 = 69.56r.sub.7 =   -2.962   d.sub. 7 = 0.1r.sub.8 =   2.907   d.sub.8 = 1.85               n.sub.4 = 1.6180                          ν.sub.4 = 63.38r.sub.9 =   -1.659   d.sub.9 = 0.44               n.sub.5 = 1.84666                          ν.sub.5 = 23.90r.sub.10 =   -22.0281f = 1.0, 1 = 15.0, S = 0.986IH = 0.9, 2ω = 118°, ΣP = -0.018NA = 0.0133, f.sub.234 /f = 1.651 ##STR12##  Embodiment 13r.sub.1 =   53.309   d.sub.1 = 0.444               n.sub. 1 = 1.58913                          ν.sub.1 = 60.97r.sub.2 =   0.833   d.sub.2 = 1.262r.sub.3 =   ∞ (stop)   d.sub.3 = 0.375r.sub.4 =   -31.892   d.sub.4 = 1.036               n.sub.2 = 1.6968                          ν.sub.2 = 55.52r.sub.5 =   -2.305   d.sub.5 = 0.099r.sub.6 =   10.489   d.sub.6 = 1.045               n.sub.3 = 1.51728                          ν.sub.3 = 69.56r.sub.7 =   -2.934   d.sub.7 = 0.099r.sub.8 =   2.852   d.sub.8 = 1.825               n.sub.4 = 1.6180                          ν.sub.4 = 63.38r.sub.9 =   -1.638   d.sub.9 = 0.434               n.sub.5 = 1.84666                          ν.sub.5 = 23.90r.sub.10 =   -22.031f = 1.0, 1 = 15.0, S = 0.974IH = 0.89, 2ω  = 118.0°, ΣP = -0.016NA = 0.0129, f.sub.234 /f = 1.630 ##STR13##  Embodiment 14r.sub.1 =   87.350   d.sub.1 = 0.523               n.sub.1 = 1.58913                          ν.sub.1 = 60.97r.sub.2 =   0.871   d.sub.2 = 1.264r.sub.3 =   ∞ (stop)   d.sub.3 = 0.376r.sub.4 =   -34.593   d.sub.4 = 1.076               n.sub.2 = 1.6968                          ν.sub.2 = 55.52r.sub.5 =   -2.281   d.sub.5 = 0.125r.sub.6 =   11.081   d.sub.6 = 1.080               n.sub.3 = 1.48749                          ν.sub.3 = 70.15r.sub.7 =   -2.839   d.sub.7 = 0.120r.sub.8 =   2.693   d.sub.8 = 1.965               n.sub.4 = 1.6180                          ν.sub.4 = 63.38r.sub.9 =   -1.604   d.sub.9 = 0.439               n.sub.5 = 1.84666                          ν.sub.5 = 23.90r.sub.10 =   -62.149f = 1.0, 1 = 15.0, S = 0.794IH = 0.89, 2ω = 118.58°, ΣP = -0.007NA = 0.0127, f.sub.234 /f = 1.610 ##STR14##  Embodiment 15r.sub.1 =   42.483   d.sub.1 = 0.423               n.sub.1 = 1.51633                          ν.sub.1 = 64.15r.sub.2 =   0.809   d.sub.2 = 1.259r.sub.3 =   ∞ (stop)   d.sub.3 = 0.374r.sub.4 =   -42.497   d.sub.4 = 1.032               n.sub.2 = 1.69680                          ν.sub.2 = 55.52r.sub.5 =   -2.358   d.sub.5 = 0.098r.sub.6 =   7.206   d.sub.6 = 1.032               n.sub.3 = 1.51728                          ν.sub.3 = 69.56r.sub.7 =   -3.096   d.sub.7 = 0.098r.sub.8 =   2.615   d.sub.8 = 1.770               n.sub.4 = 1.61800                          ν.sub.4 = 63.38r.sub.9 =   -1.658   d.sub.9 = 0.433               n.sub.5 = 1.84666                          ν.sub.5 = 23.90r.sub.10 =   -76.590f = 1.0, 1 = 15.0, S = 0.794IH = 0.885, 2ω = 118°, ΣP = 0.015NA = 0.0129, f.sub.234 /f = 1.562 ##STR15##______________________________________ 
    
     wherein the reference symbols r 1  through r 10  represent radii of curvature on the surfaces of the respective lens elements including the stop (r 3 ), the reference symbols d 1  through d 9   designate thicknesses of the respective lens elements and airspaces reserved therebetween, the reference symbols n 1  through n 5  denote refractive indices of the respective lens elements, the reference symbol 1 represents distance as measured from an object to be observed or photographed to the front surface of the lens system, the reference symbol IH designates height of image, the reference symbol 2ω denotes field angle, the reference symbol ΣP represents Petzval&#39;s sum and the reference symbol NA designates numerical aperture. Inclination angles θ of the principal rays at the individual field angles of the Embodiments described above are as listed in the following table: 
     
         ______________________________________Embodiment 1field angle (2ω)           θ115.84°  -4.514°74.44°   -3.610°51.55°   -2.733°Embodiment 22ω        θ117.08°  0.034°75.60°   -0.597°52.35°   -0.600°Embodiment 32ω        θ115.57°  -4.383°74.46°   -3.621°51.56°   -2.806°Embodiment 42ω        θ114.86°  -4.562°74.28°   -3.724°51.50°   -2.837°Embodiment 52ω        θ115.53°  0.020°75.114°  -0.645°51.95°   -0.739°Embodiment 62ω        θ115.71°  0.032°75.09°   -0.601°51.946°  -0.689°Embodiment 72ω        θ115.65°  - 5.253°74.35°   -4.103°51.50°   -3.080°Embodiment 82ω        θ117.85°  -4.510°75.26°   -3.623°52.04°   -2.749°Embodiment 92ω        θ120.19°  -4.564°76.35°   -3.683°52.73°   -2.799°Embodiment 102ω        θ117.6°   -4.141°75.34°   -3.293°52.16°   -2.495°Embodiment 112ω        θ116.69°  -5.351°74.87°   -3.963°51.88°   -2.914°Embodiment 122ω        θ118°     -4.5°75.43°   -3.608°52.2°    -2.729°Embodiment 132ω        θ118°     -4.5°75.2°    -3.61°52.02°   -2.77°Embodiment 142ω        θ118.58°  -4.57°75.496°  -3.686°52.168°  -2.799°Embodiment 152ω        θ118°     -4.5°75.1°    -3.88°51.9°    -2.97°______________________________________