Abstract:
An objective for an endoscope which is to be used in an endoscope employing relay lenses and comprises, in the order from the object side, a negative lens unit, a positive lens unit and a lens unit comprising a meniscus lens arranged to be concave toward the object side, the objective for an endoscope being arranged that the dispersion in the field angle is small and arranged to be capable of correcting curvature of field of the endoscope optical system as a whole satisfactorily favorably.

Description:
BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to an objective for an endoscope and, more particularly, to an objective for an endoscope employing relay lenses (endoscope arranged to transmit the image of object, which is formed by the objective, by using relay lenses). 
     (b) Description of the Prior Art 
     There is a known optical system for an endoscope employing relay lenses which is arranged that the image of object focused by an objective O is formed in the inside of field lens F as shown in FIG. 1 and aberrations thereof are corrected favourably. This type of endoscope optical system employing relay lenses has features as described in the following: (1) the exit pupil of the objective O is transmitted to the relay lens system comprising the relay lenses R 1 , R 2  and R 3  by using the field lens F, (2) the objective is arranged as a retrofocus type objective so that positive curvature of field is caused in the image formed by the objective so as to thereby cancel negative curvature of field caused by respective relay lenses constituting the relay lens system in order to thereby eliminate curvature of field of the endoscope optical system as a whole, and (3) a cemented doublet having a cemented surface R, which has negative refractive power, is provided as one of positive lens components constituting the objective O, which is arranged as a retrofocus type objective, so that coma of the endoscope optical system as a whole is corrected favourably. 
     However, the known endoscope optical system shown in the above has disadvantages described in the following. Firstly, dispersion in the field angle occurs in the course of manufacture of said endoscope optical system. Said dispersion in the field angle is caused by dispersion in the action to transmit the pupil by the field lens F. That is, dispersion in the action to transmit the pupil by the field lens F is caused by dispersion in the airspace between the objective O and field lens F, dispersion in the airspace between the field lens F and relay lens system, dispersion in the thickness of field lens, etc. and, consequently, dispersion is caused in the field angle of the endoscope optical system. 
     The second disadvantage of the known endoscope optical system employing relay lenses is that curvature of field is not yet corrected satisfactorily favourably. Curvature of field caused by the relay lens system is proportional to the number of relay lenses (R 1 , R 2 , R 3 , etc.) which constitute the relay lens system. Therefore, even when negative curvature of field caused by the relay lens system is favourably corrected by means of positive curvature of field caused by the objective in case that the number of relay lenses is three, for example, said negative curvature of field cannot be corrected satisfactorily and remains undercorrected in case that the number of relay lenses is five. 
     SUMMARY OF THE INVENTION 
     It is, therefore, a primary object of the present invention to provide an objective for an endoscope employing relay lenses which is free from dispersion in the field angle and which serves to correct curvature of field of the endoscope optical system as a whole satisfactorily favourably. 
     FIG. 2 shows an endoscope optical system employing relay lenses which is provided with the objective according to the present invention wherein reference symbol O represents the objective, and reference symbols R 1 , R 2 , R 3 , . . . respectively represent relay lenses. As shown in FIG. 2, the field lens F is not used in said endoscope optical system and, instead of the above, the objective is arranged to have such lens configuration that it comprises a meniscus lens which is concave toward the object side and located on the image side therein (in front of the image position). That is, the objective according to the present invention comprises, as shown in FIG. 2, a negative lens unit L 1 , a positive lens group L 2 , and lens unit L 3  comprising a meniscus lens which is concave toward the object side. 
     The objective having the above-mentioned lens configuration is arranged that the principal rays diverged or converged in front of the lens unit L 3 , which is a meniscus lens, are made approximately parallel with the optical axis by means of the lens unit L 3 , which is a meniscus lens, and the exit pupil of the objective is thereby transmitted to the relay lens system so that the exit pupil comes to a position approximately at the infinite distance. As the objective O is composed to form a so-called telecentric system which transmits the pupil to the relay lens system so tht the pupil comes to a position of infinite distance as explained in the above, variation in the airspace between the objective and relay lens system does not have any influence on the field angle of the endoscope optical system and, therefore, it is possible to obtain an endoscope optical system employing relay lenses for which the dispersion in the field angle is minimized. 
     Besides, as the objective according to the present invention comprises a meniscus lens (lens unit L 3 ) with Petzval&#39;s sum of negative value, Petzval&#39;s sum of the objective becomes a negative value with a larger absolute value compared with the known objective for an endoscope and, consequently, the objective causes positive curvature of field which is larger than that of the known objective. Therefore, it is possible to more favourably correct curvature of field of the endoscope optical system employing relay lenses as a whole. 
     Here, when the lens unit L 3  which is a meniscus lens is provided, Petzval&#39;s sum becomes negative when the following condition (1) is fulfilled: ##EQU1## where, reference symbols n a  and n b  respectively represent refractive indices of the front and rear lens elements in the lens unit L 3 , and reference symbols R a  and R b  respectively represent radii of curvature of the surface on the object side and surface on the image side of the lens unit L 3 . When the meniscus lens is arranged as a single lens, it becomes n a  =n b . 
     When the above-mentioned condition (1) is not fulfilled, curvature of field of the endoscope optical system as a whole cannot be corrected favourably. 
     When it is arranged that the above-mentioned radii of curvature R a  and R b  fulfill the condition (2) shown below, it is preferable for correcting curvature of field more favourably. ##EQU2## 
     When R a  /R b  becomes larger than the upper limit of the condition (2), the negative power of the meniscus lens (lens unit L 3 ) becomes weak. As a result, the action to correct Petzval&#39;s sum becomes weak and it is impossible to correct curvature of field satisfactorily. When R a  /R b  becomes smaller than the lower limit of the condition (2), the negative power of the meniscus lens (lens unit L 3 ) becomes strong, and this is advantageous for making Petzval&#39;s sum small. However, the diverging action of said meniscus lens for offaxial upper rays becomes strong, and coma will become unbalanced. 
     As described in the above, the objective according to the present invention enables to minimize the dispersion in the field angle of the endoscope optical system by providing the meniscus lens (lens unit L 3 ) and, at the same time, thereby enables to favourably correct curvature of field of the endoscope optical system. 
     By adopting the lens configuration as shown in FIG. 2, the objective according to the present invention also enables to favourably correct coma secondarily. That is, in case of the known objective shown in FIG. 1, coma is corrected in balanced state by means of only one surface, i.e., the cemented surface R. On the other hand, in case of the objective according to the present invention, it is possible to correct coma by means of two surfaces, i.e., the front surface of the meniscus lens (lens unit L 3 ) having negative refractive power and cemented surface R in the positive lens unit L 2 . Therefore, it is possible to make the radius of curvature of the above-mentioned cemented surface R large. As a result, it is also possible to improve the curve of coma and, consequently, to correct coma favourably. 
     For correction of coma, it is preferable to establish the conditions (3) and (4) shown below. By these conditions, it is possible to make the balance and curve of coma favourable. ##EQU3## In the conditions shown in the above, reference symbol f represents the focal length of the objective as a whole, reference symbol f 1  represents the focal length of the negative lens unit L 1 , and reference symbol R c  represents the radius of curvature of the cemented surface in the positive lens unit L 2 . 
     Coma will be undercorrected when |f 1  |/f and/or |R c  |/f become larger than the upper limits of the conditions (3) and/or (4) and will be overcorrected when |f 1  |/f and/or |R c  |/f become smaller than the lower limits of the conditions (3) and/or (4). Besides, coma willl become unbalanced when |f 1  |/f and/or |R c  |/f become larger than the upper limits or smaller than the lower limits of the conditions (3) and/or (4). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a sectional view of an endoscope optical system employing relay lenses and comprising a known objective; 
     FIG. 2 shows a sectional view of an endoscope optical system employing relay lenses and comprising the objective according to the present invention; 
     FIG. 3 shows a sectional view of Embodiments 1 through 6 and Embodiment 8 of the objective according to the present invention; 
     FIG. 4 shows a sectional view of Embodiment 7 of the objective according to the present invention; 
     FIG. 5 shows a sectional view of Embodiment 9 of the objective according to the present invention; 
     FIG. 6 shows a sectional view of Embodiment 10 of the objective according to the present invention; 
     FIG. 7 shows a sectional view of Embodiment 11 of the objective according to the present invention; 
     FIG. 8 shows a sectional view of Embodiments 12 and 13 of the objective according to the present invention; 
     FIG. 9 shows a sectional view of Embodiment 14 of the objective according to the present invention; 
     FIG. 10 shows a sectional view of Embodiment 15 of the objective accordiang to the present invention; 
     FIGS. 11 through 25 respectively show graphs illustrating aberration curves of Embodiment 1 through 15 of the objective according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, preferred embodiments of the objective for an endoscope according to the present invention are shown below. 
     
         ______________________________________Embodiment 1______________________________________r.sub.1 = ∞       d.sub.1 = 0.2000                 n.sub.1 = 1.78800                             ν.sub.1 = 47.43r.sub.2 = 0.6540       d.sub.2 = 0.3000r.sub.3 = ∞       d.sub.3 = 5.16                 n.sub.2 = 1.78800                             ν.sub.2 = 47.43r.sub.4 = -2.4390       d.sub.4 = 0.1000r.sub.5 = 3.6000       d.sub.5 = 1.7700                 n.sub.3 = 1.63854                             ν.sub.3 = 55.38r.sub.6 = -1.7490       d.sub.6 = 0.7000                 n.sub.4 = 1.84666                             ν.sub.4 = 23.88r.sub.7 = -9.0340       d.sub.7 = 1.2000r.sub.8 = -1.8270       d.sub.8 = 0.7000                 n.sub.5 = 1.72825                             ν.sub.5 = 28.46r.sub.9 = ∞       d.sub.9 = n.sub.6 = 1.77250                             ν.sub.6 = 49.66r.sub.10 = -2.5920 ##STR1## ##STR2## ##STR3## ##STR4##______________________________________ 
    
     
         ______________________________________Embodiment 2______________________________________r.sub.1 = ∞       d.sub.1 = 0.2000                 n.sub.1 = 1.78800                             ν.sub.1 = 47.43r.sub.2 = 0.9070       d.sub.2 = 0.3000r.sub.3 = ∞       d.sub.3 = 6.8                 n.sub.2 = 1.80610                             ν.sub.2 = 40.95r.sub.4 = -3.0700       d.sub.4 = 0.5000r.sub.5 = 5.6020       d.sub.5 = 1.7400                 n.sub.3 = 1.64000                             ν.sub.3 = 60.09r.sub.6 = -2.4830       d.sub.6 = 0.4300                 n.sub.4 = 1.84666                             ν.sub.4 = 23.88r.sub.7 = -17.4420       d.sub.7 = 2.9200r.sub.8 = -2.1630       d.sub.8 = 1.0000                 n.sub.5 = 1.62004                             ν.sub.5 = 36.25r.sub.9 = ∞       d.sub.9 = n.sub.6 = 1.78800                             ν.sub.6 = 47.43r.sub.10 = -3.1440 ##STR5## ##STR6## ##STR7## ##STR8##______________________________________ 
    
     
         ______________________________________Embodiment 3______________________________________r.sub.1 = ∞       d.sub.1 = 0.2000                 n.sub.1 = 1.78800                             ν.sub.1 = 47.43r.sub.2 = 0.6800       d.sub.2 = 0.3000r.sub.3 = ∞       d.sub.3 = 4.97                 n.sub.2 = 1.78800                             ν.sub.2 = 47.43r.sub.4 = -2.3820       d.sub.4 = 0.3100r.sub.5 = 4.6400       d.sub.5 = 1.8400                 n.sub.3 = 1.63854                             ν.sub.3 = 55.38r.sub.6 = -1.7990       d.sub.6 = 0.6000                 n.sub.4 = 1.84666                             ν.sub.4 = 23.88r.sub.7 = -8.0150       d.sub.7 = 1.8300r.sub.8 = -1.7750       d.sub.8 = 0.5000                 n.sub.5 = 1.72825                             ν.sub.5 = 28.46r.sub.9 = ∞       d.sub.9  = 2.2000                 n.sub.6 = 1.77250                             ν.sub.6 = 49.66r.sub.10 = -2.7600 ##STR9## ##STR10## ##STR11## ##STR12##______________________________________ 
    
     
         ______________________________________Embodiment 4______________________________________r.sub.1 = ∞       d.sub.1 = 0.2000                 n.sub.1 = 1.78800                             ν.sub.1 = 47.43r.sub.2 = 0.5900       d.sub.2 = 0.3000r.sub.3 = ∞       d.sub.3 = 3.35                 n.sub.2 = 1.78800                             ν.sub.2 = 47.43r.sub.4 = -1.9990       d.sub.4 = 0.3000r.sub.5 = 7.0690       d.sub.5 = 1.9435                 n.sub.3 = 1.64000                             ν.sub.3 = 60.09r.sub.6 = -1.4710       d.sub.6 = 0.5000                 n.sub.4 = 1.84666                             ν.sub.4 = 23.88r.sub.7 = -3.1560       d.sub.7 = 1.8785r.sub.8 = -1.4710       d.sub.8 = 0.5000                 n.sub.5 = 1.72825                             ν.sub.5 = 28.46r.sub.9 = ∞       d.sub.9 = n.sub.6 = 1.77250                             ν.sub.6 = 49.66r.sub.10 = -2.6380 ##STR13## ##STR14## ##STR15## ##STR16##______________________________________ 
    
     
         ______________________________________Embodiment 5______________________________________r.sub.1 = ∞       d.sub.1 = 0.2000                 n.sub.1 = 1.80610                             ν.sub.1 = 40.95r.sub.2 = 0.7330       d.sub.2 = 0.3000r.sub.3 = ∞       d.sub.3 = 4.93                 n.sub.2 = 1.80610                             ν.sub.2 = 40.95r.sub.4 = -2.2750       d.sub.4 = 0.1000r.sub.5 = 3.5180       d.sub.5 = 1.5900                 n.sub.3 = 1.64000                             ν.sub.3 = 60.09r.sub.6 = -1.8770       d.sub.6 = 0.3500                 n.sub.4 = 1.84666                             ν.sub.4 = 23.88r.sub.7 = -9.8460       d.sub.7 = 0.9800r.sub.8 = -1.6480       d.sub.8 = 0.5300                 n.sub.5 = 1.72825                             ν.sub.5 = 28.46r.sub.9 = ∞       d.sub.9 = n.sub.6 = 1.69680                             ν.sub.6 = 55.52r.sub.10 = -2.2060 ##STR17## ##STR18## ##STR19## ##STR20##______________________________________ 
    
     
         ______________________________________Embodiment 6______________________________________r.sub.1 = ∞       d.sub.1 = 0.2000                 n.sub.1 = 1.75700                             ν.sub.1 = 47.87r.sub.2 = 0.9130       d.sub.2 = 0.3000r.sub.3 = ∞       d.sub.3 = 6.09                 n.sub.2 = 1.80610                             ν.sub.2 = 40.95r.sub.4 = -2.7070       d.sub.4 = 0.5000r.sub.5 = 4.2870       d.sub.5 = 1.4300                 n.sub.3 = 1.51633                             ν.sub.3 = 64.15r.sub.6 = -2.3730       d.sub.6 = 0.4000                 n.sub.4 = 1.84666                             ν.sub.4 = 23.88r.sub.7 = -7.3700       d.sub.7 = 0.9200r.sub.8 = -2.0470       d.sub.8 = 0.8600                 n.sub.5 = 1.69895                             ν.sub.5 = 30.12r.sub.9 = ∞       d.sub.9 = n.sub.6 = 1.75700                             ν.sub.6 = 47.87r.sub.10 = -2.8310 ##STR21## ##STR22## ##STR23## ##STR24##______________________________________ 
    
     
         ______________________________________Embodiment 7______________________________________r.sub.0 = ∞       d.sub.0 = 0.3000                  n.sub.0 = 1.51633                             ν.sub.0 = 64.15r&#39;.sub.0 = ∞       d&#39;.sub.0 = 0.2000r.sub.1 = ∞       d.sub.1 = 0.2000                  n.sub.1 = 1.78800                             ν.sub.1 = 47.43r.sub.2 = 0.6800       d.sub.2 = 0.3000r.sub.3 = ∞       d.sub.3 = 3.5000                  n.sub.2 = 1.88300                             ν.sub.2 = 40.76r.sub.4 = ∞       d.sub.4 = 1.5400                  n.sub.3 = 1.78800                             ν.sub.3 = 47.43r.sub.5 = -2.3820       d.sub.5 = 0.7600r.sub.6 = 4.4630       d.sub.6 = 1.7900                  n.sub.4 = 1.62041                             ν.sub.4 = 60.27r.sub.7 = -1.7990       d.sub.7 = 0.4000                  n.sub.5 = 1.84666                             ν.sub.5 = 23.88r.sub.8 = -9.4030       d.sub.8 = 2.2600r.sub.9 = -1.7880       d.sub.9 = 0.7500                  n.sub.6 = 1.72342                             ν.sub.6 = 38.03r.sub.10 = ∞       d.sub.10 = 2.2000                  n.sub.7 = 1.78800                             ν.sub.7 = 47.43r.sub.11 = -2.8170 ##STR25## ##STR26## ##STR27## ##STR28##______________________________________ 
    
     
         ______________________________________Embodiment 8______________________________________r.sub.1 = ∞      d.sub.1 = 0.2000                 n.sub.1 = 1.78800                             ν.sub.1 = 47.43r.sub.2 = 0.6323      d.sub.2 = 0.3000r.sub.3 = ∞      d.sub.3 = 3.7153                 n.sub.2 = 1.78800                             ν.sub.2 = 47.43r.sub.4 = -2.2149      d.sub.4 = 0.6295r.sub.5 = 6.3945      d.sub.5 = 2.0000                 n.sub.3 = 1.64000                             ν.sub.3 = 60.09r.sub.6 = -1.7561      d.sub.6 = 0.5000                 n.sub.4 = 1.84666                             ν.sub.4 = 23.88r.sub.7 = -3.6986      d.sub.7 = 2.0353r.sub.8 = -1.5615      d.sub.8 = 0.5000                 n.sub.5 = 1.72825                             ν.sub.5 = 28.46r.sub.9 = ∞      d.sub.9  = 2.0000                 n.sub.6 = 1.77250                             ν.sub.6 = 49.66r.sub.10 = -2.9074 ##STR29## ##STR30## ##STR31## ##STR32##______________________________________ 
    
     
         ______________________________________Embodiment 9______________________________________r.sub.1 = ∞      d.sub.1 = 0.2000                 n.sub.1 = 1.78800                             ν.sub.1 = 47.43r.sub.2 = 0.6720      d.sub.2 = 0.3000r.sub.3 = ∞      d.sub.3 = 4.957                 n.sub.2 = 1.78800                             ν.sub.2 = 47.43r.sub.4 = -2.4390      d.sub.4 = 0.1000r.sub.5 = 4.3110      d.sub.5 = 1.4100                 n.sub.3 = 1.64000                             ν.sub.3 = 60.09r.sub.6 = -1.8420      d.sub.6 = 1.0300                 n.sub.4 = 1.84666                             ν.sub.4 = 23.88r.sub.7 = -7.0940      d.sub.7 = 1.7400r.sub.8 = -1.8770      d.sub.8 = 0.5900                 n.sub.5 = 1.72825                             ν.sub.5 = 28.46r.sub.9 = -6.9230      d.sub.9 = 1.0300                 n.sub.6 = 1.77250                             ν.sub.6 = 49.66r.sub.10 = -2.4390 ##STR33## ##STR34## ##STR35## ##STR36##______________________________________ 
    
     
         ______________________________________Embodiment 10______________________________________r.sub.1 = ∞      d.sub.1 = 0.2000                 n.sub.1 = 1.75700                             ν.sub.1 = 47.87r.sub.2 = 0.9050      d.sub.2 = 0.3000r.sub.3 = ∞      d.sub.3 = 5.6400                 n.sub.2 = 1.75700                             ν.sub.2 = 47.87r.sub.4 = -2.6190      d.sub.4 = 0.3000r.sub.5 = 4.2950      d.sub.5 = 2.0500                 n.sub.3 = 1.51633                             ν.sub.3 = 64.15r.sub.6 = -2.1660      d.sub.6 = 0.4000                 n.sub.4 = 1.84666                             ν.sub.4 = 23.88r.sub.7 = -7.9380      d.sub.7 = 2.1800r.sub.8 = -1.9880      d.sub.8 = 0.4100                 n.sub.5 = 1.67270                             ν.sub.5 = 32.10r.sub.9 = 5.8970      d.sub.9 = 1.1000                 n.sub.6 = 1.80610                             ν.sub.6 = 40.95r.sub.10 = -2.7420 ##STR37## ##STR38## ##STR39##______________________________________ 
    
     
         ______________________________________Embodiment 11______________________________________r.sub.1 = ∞      d.sub.1 = 0.2000                 n.sub.1 = 1.78800                             ν.sub.1 = 47.43r.sub.2 = 0.6133      d.sub.2 = 2.23r.sub.3 = -68.5068      d.sub.3 = 1.8626                 n.sub.2 = 1.78800                             ν.sub.2 = 47.43r.sub.4 = -2.3154      d.sub.4 = 0.3406r.sub.5 = 3.6741      d.sub.5 = 1.7700                 n.sub.3 = 1.63854                             ν.sub.3 = 55.38r.sub.6 = -1.4541      d.sub.6 = 0.7000                 n.sub.4 = 1.84666                             ν.sub.4 = 23.88r.sub.7 = -13.0093      d.sub.7 = 1.4797r.sub.8 = -1.7763      d.sub.8 = 0.7000                 n.sub.5 = 1.72825                             ν.sub.5 = 28.46r.sub.9 = ∞      d.sub.9 = 1.3900                 n.sub.6 = 1.77250                             ν.sub.6 = 49.66r.sub.10 = -2.4237 ##STR40## ##STR41## ##STR42##______________________________________ 
    
     
         ______________________________________Embodiment 12______________________________________r.sub.1 = ∞      d.sub.1 = 0.2000                 n.sub.1 = 1.78800                             ν.sub.1 = 47.43r.sub.2 = 0.6899      d.sub.2 = 0.3000r.sub.3 = 13.6272      d.sub.3 = 4.928                 n.sub.2 = 1.78800                             ν.sub.2 = 47.43r.sub.4 = -2.2510      d.sub.4 = 0.1000r.sub.5 = -164.3754      d.sub.5 = 1.0752                 n.sub.3 = 1.51633                             ν.sub.3 = 64.15r.sub.6 = -30.3152      d.sub.6 = 0.1000r.sub.7 = 3.0359      d.sub.7 = 1.7700                 n.sub.4 = 1.63854                             ν.sub.4 = 55.38r.sub.8 = -1.3278      d.sub.8 = 0.7000                 n.sub.5 = 1.84666                             ν.sub.5 = 23.88r.sub.9 = -10.2303      d.sub.9 = 0.5811r.sub.10 = -1.9987      d.sub.10 = 0.7000                 n.sub.6 = 1.72825                             ν.sub.6 = 28.46r.sub.11 = ∞      d.sub.11 = 1.3900                 n.sub.7 = 1.77250                             ν.sub.7 = 49.66r.sub.12 = -2.4285 ##STR43## ##STR44## ##STR45##______________________________________ 
    
     
         ______________________________________Embodiment 13______________________________________r.sub.1 = ∞      d.sub.1 = 0.2000                 n.sub.1 = 1.78800                             ν.sub.1 = 47.43r.sub.2 = 0.6240      d.sub.2 = 2.0663r.sub.3 = 80.4600      d.sub.3 = 1.5954                 n.sub.2 = 1.78800                             ν.sub.2 = 47.43r.sub.4 = -2.3135      d.sub.4 = 0.1205r.sub.5 = -112.1086      d.sub.5 = 1.4733                 n.sub.3 = 1.51633                             ν.sub.3 = 64.15r.sub.6 = -30.1167      d.sub.6 = 0.0929r.sub.7 = 3.1354      d.sub.7 = 1.7700                 n.sub.4 = 1.63854                             ν.sub.4 = 55.38r.sub.8 = -1.2528      d.sub.8 = 0.7000                 n.sub.5 = 1.84666                             ν.sub.5 = 23.88r.sub.9 = -9.2528      d.sub. 9 = 1.0722r.sub.10 = -1.9547      d.sub.10 = 0.7000                 n.sub.6 = 1.72825                             ν.sub.6 = 28.46r.sub.11 = ∞      d.sub.11 = 1.3900                 n.sub.7 = 1.77250                             ν.sub.7 = 49.66r.sub.12 = -2.4215 ##STR46## ##STR47## ##STR48##______________________________________ 
    
     
         ______________________________________Embodiment 14______________________________________r.sub.0 = ∞      d.sub.0 = 0.3000                 n.sub.0 = 1.51633                             ν.sub.0 = 64.15r&#39;.sub.0 = ∞      d&#39;.sub.0 = 0.2000r.sub.1 = ∞      d.sub.1 = 0.2000                 n.sub.1 = 1.80610                             ν.sub.1 = 40.95r.sub.2 = 0.7605      d.sub.2 = 0.3000r.sub.3 = ∞      d.sub.3 = 7.6000                 n.sub.2 = 1.78800                             ν.sub.2 = 47.43r.sub.4 = -3.3709      d.sub.4 = 0.1000r.sub.5 = 3.2819      d.sub.5 = 1.3089                 n.sub.3 = 1.60311                             ν.sub.3 = 60.70r.sub.6 = -3.2819      d.sub.6 = 1.8392                 n.sub.4 = 1.78472                             ν.sub.4 = 25.71r.sub.7 = ∞      d.sub.7 = 1.5075r.sub.8 = -2.0629      d.sub.8 = 1.3313                 n.sub.5 = 1.78472                             ν.sub.5 = 25.71r.sub.9 = ∞      d.sub.9 = 1.2847                 n.sub.6 = 1.78800                             ν.sub.6 = 47.43r.sub.10 = -3.3628 ##STR49## ##STR50## ##STR51## ##STR52##______________________________________ 
    
     
         ______________________________________Embodiment 15______________________________________r.sub.0 = ∞     d.sub.0 = 0.3000                 n.sub.0 = 1.51633                            ν.sub.0 = 64.15r&#39;.sub.0 = ∞     d&#39;.sub.0 = 0.2000r.sub.1 = ∞     d.sub.1 = 0.2000                 n.sub.1 = 1.80610                            ν.sub.1 = 40.95r.sub.2 = 0.7250     d.sub.2 = 0.3000r.sub.3 = ∞     d.sub.3 = 7.6000                 n.sub.2 = 1.78800                            ν.sub.2 = 47.43r.sub.4 = -3.2030     d.sub.4 = 0.1000r.sub.5 = 2.7600     d.sub.5 = 1.7800                 n.sub.3 = 1.62041                            ν.sub.3 = 60.27r.sub.6 = -3.1440     d.sub.6 = 0.7000                 n.sub.4 = 1.84666                            ν.sub.4 = 23.88r.sub.7 = 14.1130     d.sub.7 = 1.3100r.sub.8 = -1.8150     d.sub.8 = 1.5400                 n.sub.5 = 1.76182                            ν.sub.5 = 26.55r.sub.9 = ∞     d.sub.9 = 1.3900                 n.sub.6 = 1.78800                            ν.sub.6 = 47.43r.sub.10 = -3.2700 ##STR53## ##STR54## ##STR55## ##STR56##______________________________________ 
    
     In embodiments shown in the above, reference symbols r 1 , r 2 , . . . respectively represent radii of curvature of respective lens surfaces, reference symbols d 1 , d 2 , . . . respectively represent thicknesses of respective lenses and airspaces between respective lenses, reference symbols n 1 , n 2 , . . . respectively represent refractive indices of respective lenses, and reference symbols ν 1 , ν 2 , . . . respectively represent Abbe&#39;s numbers of respective lenses. 
     Out of respective embodiments shown in the above, Embodiments 1 through 6 and Embodiment 8 have lens configuration as shown in FIG. 3. Embodiment 7 has lens configuration as shown in FIG. 4 and is arranged that a cover glass is provided in front of the negative lens group L 1 , and the lens component on the object side in the positive lens group L 2  is arranged as a lens component consisting of a glass block and plano-convex lens which are centered together. Therefore, in the numerical data of Embodiment 7, reference symbols r 0 , r&#39; 0 , d 0 , d&#39; 0 , n 0  and ν 0  respectively represent the data of the cover glass. Besides, reference symbol r 4  represents the radius of curvature of the cemented surface of said cemented lens component (cemented lens component consisting of the glass block and plano-convex lens). Besides, reference symbol r 7  corresponds to R c , and r 4  is not included in R c . 
     Embodiment 9 has lens configuration as shown in FIG. 5. In Embodiment 9, the cemented surface in the lens unit L 3 , which is a meniscus lens, is not arranged as a planar surface but arranged as a spherical surface which is concave toward the object side. 
     Embodiment 10 has lens configuration as shown in FIG. 6. In Embodiment 10, the cemented surface in the lens unit L 3 , which is a meniscus lens, is arranged as a spherical surface which is convex toward the object side. 
     Embodiment 11 has lens configuration as shown in FIG. 7, and Embodiments 12 and 13 respectively have lens configuration as shown in FIG. 8. 
     Embodiments 14 and 15 respectively have lens configurations as shown in FIGS. 9 and 10 and are respectively arranged that a cover glass is provided in front of the negative lens unit L 1 . 
     The objective according to the present invention is arranged that the dispersion in the field angle is made small by providing the meniscus lens which is concave toward the object side and, moreover, it is thereby arranged that curvature of field of the endoscope as a whole including the objective is corrected favourably when said objective is used in an endoscope which employs relay lenses. Especially, by composing the lens system so as to fulfill the condition (1), it is possible to correct the above-mentioned curvature of field favourably. 
     Besides, by arranging the shape of the meniscus lens so as to fulfill the condition (2), it is possible to correct the above-mentioned curvature of field more favourably. 
     Furthermore, when the objective is arranged so as to fulfill the conditions (3) and (4). it is possible to obtain an objective of which coma is corrected favourably.