Abstract:
A large relative aperture objective lens having three lens groups of positive power with said three lens groups being all moved forward to effect focusing from an infinitely distant object to close objects. The three lens groups, being named from front to rear, 1st, 2nd and 3rd lens groups successively, have the amounts of forward movement of the 1st, 2nd and 3rd lens groups made progressively smaller. Accordingly, good stability of aberration correction throughout the extended focusing range is achieved.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to large relative aperture objective lenses, and, more particularly, to objective lenses of increased relative aperture while still permitting high grade imaging performance throughout an extended focusing range from infinity to very short object distances. 
     2. Description of the Prior Art 
     In the past, the photographic objectives called macro, or micro lenses which are adapted to be mainly used in closeup shooting and also capable of focusing up to infinity, when focused down to close object distances have suffered darkening of the effective F-number. This led to the production of a shade in the split prism area on the focusing screen. Therefore, the photographer had to turn to the surrounding or mat area for focusing purposes. Also, since the depth of field became shallow, it became very difficult to detect when the objective lens is focused to an optimum in-focus position. For this reason, there is an increasing demand for a larger relative aperture or fast objective lens. 
     However, since the range of variation of the image magnification is wide as compared with the normal photographic objective, aberrations vary to large degree with focusing. This aberration variation becomes prominent when the relative aperture is increased. Even in case, as usual, the start point for design on the image magnification is set in about 1/10 times, it is found that as the image magnification approaches 1/5 times or larger, the spherical aberration and curvature of the field becomes under-corrected, and asymmetric aberrations for off-axis rays are produced. Particularly regarding the outward coma in the intermediate zone to the marginal zone of the image format, for a subject of fine patterns, the image is largely blurred to impair the degree of sharpness, thus the photographer is obliged to use the objective with the diaphragm closed down. As it is also required to remove the distortion, it follows from these points that the symmetric lens type, for example, Gauss type with the inclusion of the modified Gauss type is suited for macro lenses of large relative aperture, and has found many uses. 
     With the ordinary Gauss type lens, however, in closeup shooting, the paraxial ray becomes stronger in divergence than when focused at infinity so that the height of incidence on the rear lens component becomes higher. The result of this is that the spherical aberration is largely under-corrected by the positive refractive power of the rear component. On the other hand, as regards the off-axis coma, because focusing down to shorter object distances is effected by moving the lens system forwards as a whole, the principal ray to the maximum image height makes a smaller angle with the optical axis so that the upper and lower portions of the image bearing beam are refracted more weakly by the rear lens component than when in shooting at an infinitely distant object, thereby outward coma is produced. 
     As a means for compensating for this drawback, it has already been known in the art, as disclosed in Japanese Laid Open Patent Sho Nos. 50-138823, 52-7723, 53-87728 and 53-10425 and U.S. Pat. Nos. 3,884,557 and 4,260,223, to change a certain air separation between lens members, as the entire lens system is moved forward, away from the image plane, with some advantage. However, this method, though being favorable to the closeup shooting of low image magnification, has drawbacks that, as the image magnification increases, the state of correction of the image aberrations cannot be maintained sufficiently acceptable. Further while the off-axis asymmetric aberrations are well corrected, the other various aberrations are somewhat sacrificed. The problem of achieving a large increase in the relative aperture while maintaining good stability of aberration correction throughout the extended focusing range has not been solved yet. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a photographic objective lens with means of relatively simple form making it possible to achieve a much-desired increase in the relative aperture while still permitting good correction of aberrations even at extraordinarily increased magnifications. 
     One of the features of the present invention is the arrangement of three lens groups each having a positive refractive power in such a way that, as the lens groups are numbered consecutively from front to rear, when focusing is effected down from infinity to shorter object distances, the three lens groups are moved axially forward, wherein the 2nd lens group is made to move faster than the 3rd lens group, and the 1st lens group is made to move faster than the 2nd lens group. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1-a and 1-b, FIGS. 2-a and 2-b, and FIGS. 3-a  and 3-b are lens block diagrams of embodiments of the present invention respectively. 
     FIGS. 4-a to d and 5-a to d are graphic representations of the various aberrations of the objective of FIG. 1. 
     FIGS. 6-a to d and 7-a to d are graphic representations of the various aberrations of the objective of FIG. 2. 
     FIGS. 8-a to d and 9-a to d are graphic representations of the various aberrations of the objective of FIG. 3. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will next be described in connection with embodiments thereof by reference to the drawings. 
     FIGS. 1 to 3 in longitudinal sectional views illustrate embodiments 1 to 3 of objective lenses according to the present invention wherein I, II and III denote the 1st, 2nd and 3rd lens groups respectively with (a) representing an operative position of the lens when focused at infinity and (b) when at an image magnification of 0.85. 
     The objective lens of the invention is constructed from three lens groups of positive power and operates in such a manner that, when focusing from an infinitely distant object to an object at a shorter distance, the speeds of forward movement of the 1st, 2nd and 3rd lens groups are made progressively slower as indicated by arrows in FIGS. 1 to 3. In this manner stability of aberration correction is maintained throughout the extended range of variation of the image magnification. And, a diaphragm is positioned in a space between the 1st and 2nd lens groups, and the 3rd lens group is constructed as including at least one positive lens, whereby the aberrations are well corrected. In the case where the 3rd lens group is in the form of a singlet lens, it is preferred to use a glass material of low dispersion for the purpose of correcting lateral chromatic aberration when in closeup shooting. To facilitate a further improvement of the lateral chromatic aberration at increased magnifications, it is recommended to construct the 3rd lens group from a plurality of lens elements. 
     In the present invention, though the above-described form of objective lens enables the image aberrations to be maintained stable throughout an extended range of variation of the image magnification, further assistance in the stabilization of the aberrations can be obtained when the following conditions are satisfied: 
     
         0.12&lt;Δd.sub.1 /ΔS&lt;0.21                         (1) 
    
     
         0.04&lt;Δd.sub.2 /ΔS&lt;0.21                         (2) 
    
     
         R.sub.2 &lt;0                                                 (3) 
    
     
         N1&lt;N2                                                      (4) 
    
     
         40&lt;νI&lt;44                                                (5) 
    
     wherein Δd 1  and Δd 2  are the amounts of variation of the intervals between the 1st and 2nd lens groups and between the 2nd and 3rd lens groups respectively when focusing from an infinitely distant object to close objects; ΔS is the amount of forward movement of the 3rd lens group; R2 is the radius of curvature of the 2nd lens surface counting from the front; N1 and N2 are the indices of refraction of the glasses from which the 1st and 2nd lens elements are made up respectively; and νI is the average Abbe number of the lens elements constituting the 1st lens component. 
     Among the above-cited conditions, conditions (1) and (2) are the fundamental ones concerning the present invention for good correction of outward coma and spherical aberration when focused to close objects. Condition (3) is to correct negative distortion due to the variation of the image magnification. Conditions (4) and (5) concern the Petzval sum and the achromatism of the 1st lens group. 
     Condition (1) represents a compromise between the requirements of correcting the outward coma and spherical aberration in the closeup shooting position. As the value of said interval increases, the off-axis ray of light arrives at the 2nd lens group at a higher height of incidence and, therefore, is refracted more strongly by the 2nd lens group to produce inward coma which can be well balanced with the outward coma resulting from the variation of the image magnification. On the other hand, however, the paraxial ray diverges in said space so that as this interval increases, under-correction of spherical aberration results. For this reason, when the upper limit of condition (1) is exceeded, though it is advantageous at the correction of outward coma, the spherical aberration is largely under-corrected. This also causes the focal length to be largely increased. When the lower limit is exceeded, though the range of variation of the spherical aberration is lessened, it is made more difficult to remove the outward coma. 
     Condition (2) cooperates with condition (1) to contribute a similar effect to the correction of outward coma and the correction of spherical aberration. Since expansion of said interval has a similar advantage to that described in connection with condition (1), its aim is to reduce the deterioration of the various aberrations which would be otherwise caused when the interval Δd 1  is varied in surplus to account for the variation of the interval Δd 2 , whereby even at extraordinarily high image magnifications the variation of aberrations is limited to a minimum. If a correction of the outward coma is attemped by imparting independent variation only to, for example, the interval Δd 2  while the interval Δd 1  of condition (1) is left unchanged during focusing, it will result that the required amount of variation of the interval Δd 2  becomes very large, and, therefore, that the under-correction of spherical aberration reaches an unacceptable level. Additional drawbacks wherein the outer diameter of the 3rd lens group is increased, and the focal length is considerably increased will also be involved. When the upper limit of condition (2) is exceeded, under-correction of spherical aberration results, and the focal length is increased. When the lower limit is exceeded, the advantage of correcting the outward coma will be lost. 
     Condition (3) is to limit the negative distortion in the closeup position to a minimum. When this condition is violated, it becomes difficult in any case to perform good correction of negative distortion. 
     Condition (4) cooperates with condition (5) to well correct the various aberrations. When this condition is violated, achromatization of the 1st lens group, avoidance of a positive large increase of the Petzval sum and removal of coma are difficult to perform. In particular, in order to limit the range of inclination of the image surface resulting from the variation of the image magnification to a minimum, it is required to take the N2 at a high value and to retain the Petzval sum to a small value. 
     Condition (5) is to achromatize the 1st lens group. In the present invention, every constituent lens element is made up from a relatively high Abbe number, and the satisfaction of the conditions with regard to achromatism is also taken into account. When the upper limit is exceeded, achromatism within the 1st lens group becomes difficult to achieve. On the other hand, the lower limit represents the average Abbe number of the glasses usable in the 1st lens group in view of the refractive indices of the glasses of the positive and negative lenses. 
     Three examples of specific objective lenses of the invention can be constructed in accordance with the numerical data given in the following tables for the radii of curvature, R, the axial thicknesses and air separations, D, and the indices of refraction, N, and the Abbe numbers, ν, of the glasses of the lens elements, all expressed with the usual subscripts numbered consecutively from front to rear. 
     
         ______________________________________Embodiment 1F = 1 FNO = 1:2 2ω = 45.5______________________________________R1 = 1.4306     D1 = 0.0638  N1 = 1.77250                              ν1 = 49.6R2 = -5.8384     D2 = 0.0030R3 = 0.4423     D3 = 0.0876  N2 = 1.79952                              ν2 = 42.2R4 = 0.8928     D4 = 0.0155R5 = 2.4975     D5 = 0.0597  N3 = 1.69895                              ν3 = 30.1R6 = 0.3489     D6 = VariableR7 = -0.3137     D7 = 0.0309  N4 = 1.67270                              ν4 = 32.1R8 = -0.8855     D8 = 0.0633  N5 = 1.77250                              ν5 = 49.6R9 = -0.4636     D9 = 0.0031R10 = -2.2632     D10 = 0.0453 N6 = 1.77250                              ν6 = 49.6R11 = -0.5787     D11 = VariableR12 = -1.6914     D12 = 0.0329 N7 = 1.56873                              ν7 = 63.1R13 = -0.7423     D13 =  0.0252                  N8 = 1.69895                              ν8 = 30.1R14 = -0.9305______________________________________ReproductionRatio            D6      D11______________________________________β= 0.0      0.1699  0.0031β= -0.85    0.2993  0.1248______________________________________ ##STR1## ##STR2## 
    
     
         ______________________________________Embodiment 2F = 1 FNO = 1:2 2ω = 45.5______________________________________R1 = 1.2261     D1 = 0.0638  N1 = 1.72600                              ν1 = 53.5R2 = -12.3375     D2 = 0.0030R3 = 0.4270     D3 = 0.0736  N2 = 1.83481                              ν2 = 42.7R4 = 0.9051     D4 = 0.0136R5 = 1.4972     D5 = 0.0504  N3 = 1.74950                              ν3 = 35.3R6 = 0.3177     D6 = VariableR7 = -0.3133     D7 = 0.0310  N4 = 1.64769                              ν4 = 33.8R8 = -1.0252     D8 = 0.0633  N5 = 1.77250                              ν5 = 49.6R9 = -0.5022     D9 = 0.0031R10 = -1.0510     D10 = 0.0453 N6 = 1.77250                              ν4 = 49.6R11 = -0.5850     D11 = 0.0031R12 = -1.3557     D12 = 0.0388 N7 = 1.69680                              ν7 = 55.5R13 = -0.7731     D13 =  VariableR14 = -1.5200     D14 = 0.388  N8 = 1.61800                              ν8 = 63.4R15 = -0.6047     D15 = 0.0213 N9 = 1.84666                              ν9 = 23.9R16 = -0.7460______________________________________ReproductionRatio            D6      D13______________________________________β = 0.0     0.2524  0.0031β = -0.85   0.4257  0.0464______________________________________ ##STR3## ##STR4## 
    
     
         ______________________________________Embodiment 3F = 1 FNO = 1:2 2ω= 45.5______________________________________R1 = 1.8432     D1 = 0.0638  N1 = 1.77250                              ν1 = 49.6R2 = -4.0060     D2 = 0.0030R3 = 0.4332     D3 = 0.0678  N2 = 1.79952                              ν2 = 42.2R4 = 0.9612     D4 = 0.0294R5 = 3.3725     D5 = 0.0484  N3 = 1.69895                              ν3 = 30.1R6 = 0.3621     D6 = VariableR7 = 0.3308     D7 = 0.0309  N4 = 1.67270                              ν4 = 32.1R8 = -1.4750     D8 = 0.0633  N5 = 1.77250                              ν5 = 49.6R9 = -0.4709     D9 = 0.0031R10 = -2.7852     D10 = 0.0581 N6 = 1.77250                              ν6 = 49.6R11 = -0.6416     D11 = VariableR12 = -1.7231     D12 = 0.0543 N7 = 1.56873                              ν7 = 63.1R13 = -1.0557______________________________________ReproductionRatio            D6      D11______________________________________β= 0.0      0.1659  0.002931 0.85          0.2652  0.1595______________________________________ ##STR5## ##STR6## 
    
     The various aberrations of the objective in embodiment 1 with an object at infinity and at a reproduction ratio of 0.85 are illustrated in FIGS. 4-a to d and FIGS. 5-a to d respectively. 
     The various aberrations of the objective in embodiment 2 with an object at infinity and at a reproduction ratio of 0.85 are illustrated in FIGS. 6-a to d and FIGS. 7-a to d respectively. 
     The various aberrations of the objective in embodiment 3 with an object at infinity and at a reproduction ratio of 0.85 are illustrated in FIGS. 8-a to d and FIGS. 9-a to d respectively. 
     In the drawings, S designates the sagittal image surface, M the meridional image surface and Y the image height.