Abstract:
The disclosed wide angle objective has a smaller telephoto ratio than unity and includes, from front to rear, a positive meniscus lens of forward convexity, a bi-concave lens and a bi-convex lens followed, after a large space, by a negative meniscus lens of forward concavity with a diaphragm positioned between the bi-convex lens and the negative meniscus lens. During focusing, the positive meniscus lens, bi-concave lens and bi-convex lens are axially moved in unison. The bi-convex lens is formed by positive and negative lens elements cemented together, and the front surface of the negative meniscus lens is aspherical.

Description:
This application is a continuation of application Ser. No. 218,123, filed Dec. 19, 1980 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to photographic objectives, and more particularly to photographic objectives having shortened total lengths. 
     Recently, as the minimization of the bulk and size of photographic camera advances, there is an increasing demand for a compact wide angle photographic objective by shortening the total length. However, as far as the wide angle lens system is concerned, the shortening of the total length generally leads to extremely large astigmatism and distortion. It is, therefore, quite difficult to design a lens system of shortened total length while still preserving a high image quality which is uniform throughout the entire area of the format. 
     To overcome this drawback and to obtain a good optical performance, a method is proposed in Japanese Patent Publication No. Sho 44(1969)-10831 where the front lens group is provided with a negative meniscus lens with strong curvature toward the rear and where the front surface of this meniscus lens is made aspherical. It is also known later to reduce the number of lens elements in the front lens group with the intention of minimizing the bulk and size of the objective as disclosed in U.S. Pat. Nos. 3,998,527 and 3,951,523. 
     On the other hand, Japanese Patent Laid Open No. Sho 54(1979)-76148 (U.S. Pat. No. 4,204,747) discloses an objective comprising a positive meniscus lens, a bi-concave lens, a bi-convex cemented lens, a negative meniscus lens concave toward the front, and a positive meniscus lens, the last meniscus lens being provided instead of the aspherical surface to facilitate the correction of distortion. In this five-component photographic objective, if focusing is performed by moving the positive meniscus lens to the cemented lens in unison, the aberrations will deteriorate. It is therefore pointed out in the specification that focusing provision is made at the cemented lens. 
     To achieve a shortening of the total length of the lens system, on the other hand, use may be made of the telephoto type in which a lens group of positive power is arranged in front of a lens group of negative power. In the case of the wide angle objective, however, since the limitation of the outer diameter of the objective to an appropriate value requires that the diaphragm be positioned at the middle part of the lens system with the lens group of positive power arranged on the front side of the diaphragm and the lens group of negative power on the rear side, such lens system tends to increase the difficulty of correction of the off-axial aberrations such as astigmatism and distortion. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to shorten an objective of wide angular field of view in the longitudinal direction. 
     Another object of the invention is to achieve a good state of correction of the various aberrations for high grade imaging performance. 
     Still another object of the invention is to provide an efficient focusing method. 
     The present invention has the advantage of providing a four-component lens system with the first component of positive power followed in such adjacent relationship by the second component of negative power and with the lens configuration made suitable much that the off-axial aberrations which would be otherwise objectionably increased are as small as possible. Further, the third component is constructed in the cemented form and the fourth component in the aspherical form with the advantage that the residual off-axial aberrations particularly regarding the coma and astigmatism are well compensated for. Another feature is that the focusing movement is imparted into the front three components, so that the total axial movement is remarkably decreased for the equivalent focusing range while the range of variation of the aberrations is limited to a minimum. 
     In the latter connection, it should be pointed out that the lens system of the invention may be moved as a whole to effect focusing as is common in the art. The impartment of independent movement to the first component alone for focusing, or the so-called front focusing is, however, not suitable, since extremely large off-axial aberrations are produced as the first and second components cannot be positioned adjacent to each other in order to avoid the mechanical interference therebetween when focusing at infinity. The use of only the third component as the focusing member, or the so-called middle focusing gives rise to a problem that as the axial separation between the second and third components is narrowed, the distortion is increased objectionably when focused down to shorter object distances. 
     It is now found that the above-described difficult problems can be eliminated by imparting independent movement into the first three of the four components. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal section view of an embodiment of a wide angle objective lens according to the present invention. 
     FIGS. 2A-2C are aberration curves of Example 1 of a specific photographic objective when focused at infinity. 
     FIGS. 3A-3C are aberration curves of Example 1 objective when focused to a short object distance of 1/22 in image magnification. 
     FIGS. 4A-4C are aberration curves of Example 2 of a specific photographic objective when focused at infinity. 
     FIGS. 5A-5C are aberration curves of Example 3 of a specific photographic objective when focused at infinity. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, the general embodiment of the present invention comprises, from front to rear, a positive meniscus lens of forward convexity, a bi-concave lens, a bi-convex lens and, after the largest space, a negative meniscus lens of forward concavity, the positive meniscus lens, bi-convex lens and bi-concave lens moving forward as a whole to effect focusing down from infinity to shorter object distances. By this focusing provision, it is made possible to position the bi-concave lens so adjacent to the positive meniscus lens as to be in edge contact, and also to maintain the constant axial separation between the bi-concave lens and bi-convex lens. As is understandable from the comparison of FIGS. 2A-2C with FIGS. 3A-3C, though the aberrations are varied to some extent during focusing, the image quality can be maintained at the standard level. 
     The features of the invention are as follows: The bi-convex lens is constructed in the form of a doublet consisting of a bi-convex lens element and a negative meniscus lens cemented together at their adjoining surfaces, and the photographic objective fulfills the following requirements: 
     (1) 2.2&lt;r 2  /r 1  &lt;3.8 
     (2) 0.6&lt;|fA|/f&lt;1.0, fA&lt;0 
     (3) 0.6&lt;r 5  /r 4  &lt;1.2 
     (4) 0.08&lt;n 4  -n 3  &lt;0.3 
     where 
     f: the focal length of the entire system, 
     fA: the focal length of the air lens between the first and second lenses, 
     ri: the radius of curvature of the i-th lens surface counting from front, and 
     nj: the index of refraction of the glass of which the j-th lens element counting from front is made up 
     On the other hand, taking the axial direction as X-axis with the original point at the vertex of the front surface of the last or negative meniscus lens and the perpendicular direction thereto as Y-axis, and using an equation X=F(Y) in expressing the symmetric front surface of the above-cited last or 4th lens, this lens surface is an aspheric surface fulfilling the following requirements: 
     (5) 1×10 -3  &lt;ΔX(0.7 r 8 )/f&lt;2×10 -3   
     1×10 -4  &lt;ΔX(0.5 r 8 )/f&lt;5×10 -4   
     where ΔX(0.7 r 8 ) and ΔX(0.5 r 8 ) are the values which the amount of deviation of the above-described aspheric surface from the paraxial spherical surface having the radius of curvature r 8  takes when Y=0.7 r 8  and 0.5 r 8  respectively, the amount of deviation being expressed by: ##EQU1## 
     The above-described requirements will next be explained. 
     Requirements (1) and (2) represent conditions under which the off-axial aberrations that become serious when the telephoto type is employed can be prevented. When the upper limits are exceeded, extremely large positive distortion and negative astigmatism are produced. When the lower limits are exceeded, extremely large inward coma is produced, and moreover over-correction of spherical aberration results. Requirement (3) represents a condition under which coma is well corrected for without causing the production of appreciable spherical aberration. When the upper limit is exceeded, extremely large outward coma is produced, and when the lower limit is exceeded, extremely large inward coma is produced. Requirement (4) represents a condition under which coma is well corrected for. When the lower limit is exceeded, extremely large outward coma is produced. When the upper limit is exceeded, it becomes difficult to achieve a flatness of the image surface. 
     Requirement (5) represents conditions under which the off-axial aberrations that are difficult to correct in far as the system is spherical are compensated for in good balance. When the upper limits are exceeded, the outward coma is increased objectionably. When the lower limits are exceeded, the positive distortion is increased objectionably. Thus, the image quality cannot be made uniform over the entire area of the frame. 
     Recently, as the aspheric surface machining technique has achieved a great advance, aspheric surfaces can be formed with high surface precision and produced at a high efficiency. But in comparison with the spherical lenses, the aspherical lenses are far more expensive. In the present invention, the aspherical lens component is arranged near to the focal plane with the advantage that because of a small area across which the all image bearing light rays pass, the requirement for the surface precision is not so rigorous as compared with the Gauss type lens in which all the image bearing light rays spread fully in passing through the lens surfaces. Therefore, for the 4th or aspherical lens use may be made of a plastic lens manufactured by the injection-molding techniques without sacrificing an unduly large imaging performance, thus giving an advantage that the complete objective of the invention can be finished at a low cost. Further, if the residual aberrations of the objective even with the exclusion of the last lens are balanced in a relatively good state and minimized, the range of variation of the aberrations throughout the focusing range will be small and the total axial movement is small as compared with the whole focusing. For example, in Example 1, the total axial movement from the position for infinitely distant object to a position for the image magnification of 1/22 can be decreased to 0.4 times that of the bodily movable lens. 
     Three examples of specific objectives may be constructed in accordance with the numerical data given in the following tables where r is the radii of curvature, d is the axial thicknesses of the lens elements and the axial air separations, nd is the indices of refraction for the spectral d-line of the materials of which the lens elements are made up, and νd is the Abbe numbers, and where B, C and D are defined in the following equation: ##EQU2## 
     EXAMPLE 1 
     
         ______________________________________f = 100 b.f. (Back Focal Length) = 37.524r             d           nd      νd______________________________________1       32.565    9.719       1.7725                               49.62       104.005   2.932       1.3       -157.603  3.158       1.80518                               25.44       54.891    7.322       1.5       52.011    8.924       1.56732                               42.86       -30.23    2.632       1.80610                               40.97       -51.355   21.137      1.8       -19.187*  5.263       1.49171                               57.49       -40.559               1.______________________________________ B = 2.2526 × 10.sup.-4 C = 1.3571 × 10.sup.-7 D = 7.1264 × 10.sup.-9 
    
     EXAMPLE 2 
     
         ______________________________________f = 100 b.f. = 38.077r             d           nd      νd______________________________________1       32.935    9.983       1.7725                               49.62       119.814   2.497       1.3       -188.191  3.158       1.80518                               25.44       56.965    8.28        1.5       60.962    8.398       1.56732                               42.86       -33.217   2.632       1.66755                               41.97       -66.135   20.233      1.8       -18.806*  5.263       1.49171                               57.49       -36.015               1.______________________________________ B = 1.4265 × 10.sup.-4 C = 7.3818 × 10.sup.-7 D = 7.4476 × 10.sup.-9 
    
     EXAMPLE 3 
     
         ______________________________________f = 100 b.f. = 38.037r             d           nd      νd______________________________________1       32.46     9.61        1.7725                               49.62       83.094    3.683       1.3       -135.174  3.158       1.80518                               25.44       63.854    6.723       1.5       45.879    9.824       1.56732                               42.86       -28.817   2.632       1.7352                               41.17       -53.831   19.416      1.8       -18.348*  5.263       1.49171                               57.49       -40.757               1.______________________________________ B = 3.3024 × 10.sup.-4 C = 7.2842 × 10.sup.-7 D = 7.9985 × 10.sup.-9 
    
     Values of the factors in the various inequalities 
     
         ______________________________________   Example 1            Example 2  Example 3______________________________________r.sub.2 /r.sub.1     3.19       3.64       2.56|fA|/f     0.81       0.94       0.65r.sub.5 /r.sub.4     0.948      1.07       0.72n.sub.4 -n.sub.3     0.23878    0.10023    0.16788Δx(0.7 r.sub.8)/f     1.56 × 10.sup.-3                1.48 × 10.sup.-3                           1.82 × 10.sup.-3Δx(0.5 r.sub.8)/f     0.26 × 10.sup.-3                0.21 × 10.sup.-3                           0.32 × 10.sup.-3Telephoto 0.986      0.985      0.983Ratio______________________________________ 
    
     3rd-Order Aberration Coefficient 
     Example 1: 
     
         ______________________________________SA          CM       AS       PT     DS______________________________________1      7.1198   -0.5342  0.0401 1.3383 -0.10342      0.6685   -1.0895  1.7757 -0.4190                                  -2.21113      -5.4749  4.5987   -3.8628                           -0.2830                                  3.48234      -0.3591  -0.2469  -0.1697                           -0.8126                                  -0.67535      0.3948   0.3056   0.2365 0.6959 0.72166      -4.6123  1.5697   -0.5342                           -0.2790                                  0.27687      9.0722   -4.4654  2.1979 0.8691 -1.50968      -6.5325  0.0656   0.1085 -1.7180                                  0.05609      1.0501   -0.4526  0.1951 0.8127 -0.4344Total  1.3266   -0.2490  -0.0129                           0.2044 -0.3971______________________________________ SA: Spherical Aberration CM: Coma AS: Astigmatism PT: Petzval Sum DS: Distortion 
    
     Example 2: 
     
         ______________________________________SA          CM       AS       PT     DS______________________________________1      6.8328   -0.5938  0.0512 1.3233 -0.11862      1.0167   -1.4600  2.0967 -0.3638                                  -2.48863      -5.4784  4.7227   -4.0712                           -0.2370                                  3.71394      -0.1289  -0.1101  -0.0940                           -0.7830                                  -0.74925      0.0799   0.0951   0.1131 0.5938 0.84086      -1.7693  0.6618   -0.2475                           -0.1155                                  0.13587      5.7997   -3.3375  1.9206 0.6053 -1.45368      -6.7123  0.0812   0.0676 -1.7528                                  0.04289      1.2638   -0.4417  0.1543 0.9153 -0.3738Total  0.9540   -0.3823  -0.0092                           0.1856 -0.4505______________________________________ 
    
     Example 3: 
     
         ______________________________________SA          CM       AS       PT     DS______________________________________1      7.1891   -0.6500  0.0588 1.3426 -0.12672      0.1928   -0.4922  1.2563 -0.5245                                  -1.86793      -4.7436  4.1129   -3.5661                           -0.3300                                  3.37814      -0.2655  -0.2158  -0.1753                           -0.6985                                  -0.71005      0.7304   0.4454   0.2716 0.7890 0.64676      -3.9344  1.3138   -0.4387                           -0.2142                                  0.21807      8.5883   -4.3371  2.1902 0.7871 -1.50368      -7.5167  0.2573   0.1419 -1.7966                                  0.10259      1.0759   -0.5080  0.2398 0.8088 -0.4950Total  1.3164   -0.0736  -0.0215                           0.1637 -0.3579______________________________________