Abstract:
An optical system has a convertible focal length. A positive lens system having a positive refractive power is supplemented by a negative lens system having a negative refractive power between said positive lens system and an image plane to convert the focal length. The front principal point of the aforesaid negative lens system thereby assumes a position rearwardly of the rear principal point thereof, and is made to lie rearwardly of the rear principal point of the aforesaid positive lens system. The overall length of the aforesaid negative lens system is longer than 0.1 times but shorter than 0.5 times the back focus of the aforesaid positive lens system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to optical systems of convertible focal length for increase of the focal length by putting a negative lens system in the rear of a positive lens system. 
     2. Description of the Prior Art 
     Conventionally, in order to convert the focal length, in most cases, it has been the common practice to put what is called the rear attachment lens, or rear conversion lens in the rear of a master lens. The principle of conversion of the focal length is depicted in FIG. 1. That is, when a negative lens system f&#39; having a negative refractive power is arranged in the rear of a positive lens system f having a positive refractive power, as the distance from the front principal point H to rear principal point H&#39; of the negative lens system, or the principal point interval HH&#39; is generally of positive value, the attachment of the negative lens system f&#39; causes image shift, from a position O for the positive lens system alone, rearwards to a position O&#39;. This is because the negative lens system f&#39; in most cases takes the form that the front lens group is divergent and the rear lens group is convergent so that the principal point interval HH&#39; is apt to be positive. For this reason, it has been very difficult to achieve the coincidence of the position of an image point resulting from the attachment of the negative lens system which has found its general use in the conventional focal length-convertible systems to the rear of the positive lens system with that of an image point resulting from the positve lens system only. 
     It is known to put the negative lens system in an intermediate space of the master lens system in converting the focal length of the entire system as in U.S. Pat. No. 4,240,697. As for the attachment of the negative lens system to the rear of the master lens system in converting the focal length of the entire system, mention is made of U.S. Pat. Nos. 4,129,359 and 4,154,508. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an optical system of convertible focal length in which the focal length is increased by arranging a negative lens system to the rear of a positive lens system, while maintaining the constant position of the image point before and after the addition of the negative lens system. 
     Another object of the present invention is to provide an optical system of convertible focal length which has achieved good stability of aberration correction against the conversion of the focal length. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram for explanation of the principle of focal length conversion of the conventional optical system. 
     FIG. 2 is a schematic diagram for explaining the principle of focal length conversion of the present invention. 
     FIG. 3 is a lens block diagram of an example of a specific master lens of the present invention. 
     FIGS. 4 to 7 are lens block diagrams of embodiments 1 to 4 of the present invention respectively. 
     FIGS. 8a to 8d are graphic representations of the various aberrations of the master lens of FIG. 3. 
     FIGS. 9a to 9d, to 12a to 12d are graphic representations of the various aberrations of the lenses of FIGS. 4 to 7 respectively when attached to the master lens of FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The principle of focal length conversion of the present invention is depicted in FIG. 2, where f is the master or positive lens system, and f&#39; is the negative lens system attached to effect focal length conversion. It is seen that the relation of the front and rear principal points H and H&#39; of the negative lens system f&#39; is inverse to that depicted in FIG. 1. In other words, the front principal plane H lies on the image side of the rear principal plane H&#39;. Therefore, the principal plane interval HH&#39; takes a negative value. By adopting such form, it is made possible to cause to coincide the position of an image point for the positive lens system alone with that of an image point O&#39; for the combined system with the negative lens system. We trace a parallel ray of light L incident upon the positive lens system f through the combined lens system. The arriving ray at the negative lens system f&#39; will emerge from the rear principal plane H&#39; at the same height from the optical axis as it aims at on the front principal plane, wherein the emerging ray from the negative lens system f&#39; makes a smaller angle with the optical axis than the angle of incidence of the arriving ray on the front principal plane of the negative lens system f&#39;, whereby the possibility of bringing the image point O&#39; into coincidence with the image point O can be achieved. 
     In order that the value of the principal plane interval HH&#39; of the negative lens system f&#39; is negative, as the negative lens system f&#39; is divided into two lens groups, it is required that the front group be of positive power, and the rear group of negative power, and that the following condition be satisfied. Now letting φ denote the refractive power of the negative lens system f&#39;, φ1 the refractive power of the front group, φ2 the refractive power of the rear group, and e&#39; the interval between the rear principal point of the front group and the front principal point of the rear group, and assuming the front and rear groups to be thin lens systems, the distance, Δ1, of the front principal point of the negative lens system from the front group may be expressed by: ##EQU1## and the distance, Δ2, of the rear principal point of the negative lens system from the rear group may be expressed by: ##EQU2## Therefore, in order to make a negative value of the principal point interval of the negative lens system, the satisfaction of Δ1-Δ2&gt;e&#39;  suffices. 
     Therefore, we have e&#39;(φ2+φ1)/φ&gt;e&#39; from this, ##EQU3## That is, what is required is to satisfy the above equation (1) 
     Next, in the specific embodiments of the present invention to be described later, the refractive power φ of the negative lens system, the refractive power φ1 of the front group, the refractive power φ2 of the rear group and the interval e&#39; between the rear principal point of the front group and the front principal point of the rear group take numerical values listed below: 
     
         ______________________________________Example 1  Example 2   Example 3 Example 4______________________________________-0.004901 -0.004529   -0.004732                              -0.0046831     0.005552  0.005774    0.005544                               0.0055492    -0.014194 -0.014076   -0.013895                              -0.013838e&#39;   47.458    46.434      46.986  46.961______________________________________ 
    
     When the negative lens system having the above various numerical values of factors is arranged in rear of the positve lens system, the focal length can be increased while maintaining the constant position of the image point. In practical examples, when the positive lens system as the master lens system to be described later exists alone, the distance from the 1st lens surface to the image plane is 803.45 m/m. With the addition of the negative lens system, the distance from the 1st lens surface to the image plane is from 803.41 m/m for Example 1 to be described later, 803.41 m/m for Example 2, 803.40 m/m for Example 3 and 803.43 m/m for Example 4, thus being substantially the same. 
     On the other hand, though the focal length can be increased by arranging the negative lens system having the aforesaid numerical values of the various factors in rear of the positive lens system, the holding of the image point position unchanged leads to the reduction of the telephoto ratio to an extremely small value. For example, in any of the embodiments to be described later, the telephoto ratio is about 0.49. 
     As the telephoto ratio decreases, the Petzval sum is generally increased to a negative direction, and further the secondary spectrum comes to increase. On this account, in the embodiments of the invention, the front and rear groups of the negative lens system are each provided with a cemented surface of diverging action to prevent the increase of the Petzval sum. This is achieved by using a glass of low refractive index in the positive lens, and a glass of high refractive index in the negative lens. Also the use of a lanthanum dense flint glass of strong abnormal dispersion in the negative lens well corrects the secondary spectrum. Further, the over-correction of spherical aberration resulting from the divergent cemented surface in the front group of the negative lens system is compensated for by constructing the front group from a meniscus doublet lens of forward convexity (in the embodiments, having a negative refractive power) and a bi-convex or positive lens. The above lens configuration can achieve an optical system of convertible focal length which is well corrected for aberrations. 
     On the other hand, in order to readily incorporate the negative lens system for focal length conversion in the interior of the mounting for the positive lens system which functions as the master lens, it is preferred from the mechanical standpoint that the physical length of the negative lens system lies between at least 1/10 and 1/2 times the back focus of the positive lens system. This is because, unless the physical length of the negative lens system is longer than 1/10 times the back focus of the positive lens system, it becomes difficult to obtain an optical system which preserves the good optical performance. Also because, unless smaller than 1/2 times, it becomes difficult to avoid mechanical interference with the lens mounting, and it will result in the size and bulk of the entire lens system being increased. 
     In the following, the specific embodiments of the present invention are shown. The attachment of one of the negative lens systems shown in Example 1 to Example 4 to the master or positive lens system enables the focal length to be increased about 1.41 times without causing a shift of the image point position. In the following tables of the numerical data, Ri is the radius of curvature of the i-th lens surface counting from the front; Di is the i-th lens thickness or air separation counting from the front, and Ni and νi are the refractive index and Abbe number of the glass of the i-th lens element counting from the front respectively. 
     
         ______________________________________Master Lens:F = 1160      FNo. = 1:5.7                    2ω = 2.137______________________________________R1 =  0.0        D1 = 15.00                      N1 =  1.48749                                   ν1 = 70.1R2 =  -602.340   D2 = 1.30R3 =  940.000    D3 = 25.00                      N2 =  1.49700                                   ν2 = 81.6R4 =  -461.600   D4 = 15.07R5 =  -410.140   D5 = 11.00                      N3 =  1.78590                                   ν3 = 44.2R6 =  -2259.200  D6 = 6.59R7 =  302.440    D7 = 22.50                      N4 =  1.49700                                   ν4 = 81.6R8 =  2074.800   D8 = 288.06R9 =  123.700    D9 = 10.13                      N5 =  1.51118                                   ν5 = 51.0R10 = 539.990   D10 = 3.94R11 = 311.730   D11 = 4.60 N6 =  1.81600                                   ν6 = 46.6R12 = 83.757    D12 = 10.16                      N7 =  1.50137                                   ν7 = 56.4R13 = 225.440   D13 = 17.08R14 = 236.720   D14 = 3.80 N8 =  1.61340                                   ν8 = 43.8R15 = 87.856    D15 = 7.68 N9 =  1.59270                                   ν9 = 35.3R16 = 206.420   D16 = 34.20R17 = 324.130   D17 = 4.83 N10 = 1.74000                                  ν10 = 28.3R18 = -2074.800 D18 = 3.50 N11 = 1.71300                                  ν11 = 53.8R19 = 139.590______________________________________ 
    
     EXAMPLE 1 
     
         ______________________________________Focal Length Magnification 1.41X______________________________________R1    75.621    D1      8.47 N1    1.48749                                    ν1                                         70.1R2    -75.621   D2      2.69 N2    1.883 ν2                                         40.8R3    151.077   D3      7.55R4    198.598   D4      6.08 N3    1.5927                                    ν3                                         35.3R5    -86.991   D5      41.13R6    -353.091  D6      2.0  N4    1.7725                                    ν4                                         49.6R7    44.061    D7      6.32 N5    1.5927                                    ν5                                         35.3R8    -76.101   D8      1.99R9    -82.307   D9      2.0  N6    1.7725                                    ν6                                         49.6 R10  59.973    D10     4.0  N7    1.48749                                    ν7                                         70.1 R11  372.075______________________________________Focal Length         f = -204.04Principal Point Interval                HH&#39;---- = -25.0Principal Point Distance                E = 1099.08with Master LensSurface interval     D = 102.8with Master Lens______________________________________ 
    
     EXAMPLE 2 
     
         ______________________________________Focal Length Magnification 1.41X______________________________________R1     79.875    D1      8.0  N1   1.48749                                    ν1                                         70.1R2     -73.755   D2      4.0  N2   1.883 ν2                                         40.8R3     151.577   D3      5.33R4     152.661   D4      7.0  N3   1.5927                                    ν3                                         35.3R5     -88.805   D5      39.95R6     1121.172  D6      4.0  N4   1.5927                                    ν4                                         35.3R7     -579.803  D7      2.5  N5   1.804 ν5                                         46.6R8     39.107    D8      1.59R9     40.541    D9      4.5  N6   1.5927                                    ν6                                         35.3 R10   -127.316  D10     2.0  N7   1.804 ν7                                         46.6 R11   120.177______________________________________       F = -220.82       HH&#39;---- = -26.91       E = 1094.28       D = 93.07______________________________________ 
    
     EXAMPLE 3 
     
         ______________________________________Focal Length Magnification 1.41X______________________________________R1     79.113     D1     8.5  N1   1.48749                                    ν1                                         70.1R2     -77.608    D2     2.95 N2   1.883 ν2                                         40.8R3     150.351    D3     6.61R4     164.209    D4     6.23 N3   1.5927                                    ν3                                         35.3R5     -92.557    D5     41.28R6     -2002.705  D6     1.71 N4   1.804 ν4                                         46.6R7     38.252     D7     7.0  N5   1.5927                                    ν5                                         35.3R8     -79.906    D8     1.0R9     -86.621    D9     1.7  N6   1.7859                                    ν6                                         44.2 R10   129.063______________________________________       f = -211.34       HH&#39;---- = -25.88       E = 1096.94       D = 101.3______________________________________ 
    
     EXAMPLE 4 
     
         ______________________________________Focal Length Magnification 1.41X______________________________________R1    77.605     D1     8.52 N1    1.48749                                    ν1                                         70.1R2    -77.605    D2     2.81 N2   1.883  ν2                                         40.8R3    139.818    D3     8.06R4    162.636    D4     6.15 N3   1.5927 ν3                                         35.3R5    -89.999    D5     43.94R6    -1867.407  D6     1.71 N4   1.804  ν4                                         46.6R7    38.561     D7     7.0  N5   1.5927 ν5                                         35.3R8    -100.778   D8     1.7  N6   1.804  ν6                                         46.6R9    143.98______________________________________       f = -213.54       HH&#39;---- = -26.18       E = 1096.24       D = 96.95______________________________________