Abstract:
An nonvariable transverse magnification focussing method and lens system for maintaining a transverse magnification at a fixed value even when the movement distance of each lens group is quite small by moving the basis of the first lens group and the second lens group while maintaining the relationship therebetween with a suitable paraxial arrangement in accordance with specific principles.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a focusing method and lens system for maintaining a constant transverse magnification in a lens system comprising a front lens group and a rear lens group by moving said anterior lens group and posterior lens group in accordance with changes in the object plane (position of the object). 
     2. Description of the Related Art 
     As shown in FIG. 2A, a lens system comprising a first lens group 1 having a focal length equivalent to an object distance f 1 , and a second lens group 2 having a focal length f 2  is considered. The object image on object point 3 is formed at image point 4. 
     As shown in FIG. 2B, when, for example, the object point 3 is moved a distance [a] only to the lens side, an a focal construction is formed between the first lens group 1 and the second lens group 2. If the first lens group 1 is moved a distance [a] only to the image side, the transverse magnification β(β=-f 2  /f 1 ) is nonvariable. That is, in the present instance, the moving distance of the first lens group 1 is large, and disadvantageously, the diameter of the moving group also becomes larger. 
     SUMMARY OF THE INVENTION 
     The inventors of the present invention have discovered that the transverse magnification can be rendered nonvariable even when the movement distance of each lens group is quite small by moving the basis of the first lens group 1 and the second lens group 2 while maintaining the relationship therebetween with a suitable paraxial arrangement in accordance with specific principles. 
     A main object of the present invention is to provide a focussing method that eliminates the disadvantages associated with conventional methods. 
     A further object of the present invention is to provide a focussing method which maintains a constant transverse magnification when the first lens group and the second lens group are moved. 
     A still further object of the present invention is to provide a compact lens system capable of focussing while maintaining a constant transverse magnification. 
     These and other objects are attained by a focussing method in a lens system comprising a first lens group and a second lens group each having limited focal lengths, wherein the first lens group and the second lens group are movable independently while maintaining a specific relation in accordance with the movement of the object point and without a change of transverse magnification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects or features of the present invention will become apparent from the following description of a preferred embodiment thereof taken in conjunction with the accompanying drawings, in which: 
     FIGS. 1A and 1B are paraxial optical path diagrams showing lens construction and thin lens power of the present invention; 
     FIGS. 2A and 2B are paraxial optical path diagrams showing conventional lens construction and thin lens power; 
     FIG. 3 is a section view showing details of the lens construction of the present invention; and 
     FIGS. 4A and 4B are paraxial optical path diagrams of an alternative embodiment of the invention. 
     In the following description, like parts are designated by like reference numbers throughout the several drawings. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiments of the present invention are described hereinafter with reference to the accompanying drawings. FIGS. 1A and 1B are paraxial optical path diagrams. The image forming lens system is divided into two groups, a first lens group 5 having a focal length f 1  and a second lens group 6 having a focal length f 2 . The first and seconds lens groups 5 and 6, while maintaining a specific relation therebetween, are independently movable in the direction of an optical axis so as to maintain a constant transverse magnification relative to a position change [a] of the position of an object point 3 to an object point 3&#39; (the coordinate system is such that the left side is always positive). 
     When the amount of movement of the object point 3 is designated [a] (upon the movement to an image side of object point 3: a&gt;0, upon the movement to an object side of object point 3: a&lt;0), the focal length of the first lens group 5 is designated f 1 , the focal length of the second lens group 6 is designated f 2 , the transverse magnification around the first lens group 5 is designated β1A in the state (a=0) described in FIG. 1A, the transverse magnification around the second lens group 6 is designated β1B in the state (a=0) described in FIG. 1A, the transverse magnification around the first lens group 5 is designated β2A in the state (a=a) described in FIG. 1B, the transverse magnification around the second lens group 6 is designated β2B in the state (a=a) described in FIG. 1B, the amount of movement of the first lens group 5 when changing from the FIG. 1A state to the FIG. 1B state (right side is positive) is designated [x], the amount of movement of the second lens group 6 when changing from the FIG. 1A state to the FIG. 1B state (right side is positive) is designated [y], the distance from the first lens group 5 to the object point 3 in the state described in FIG. 1A (with the first lens group as the origin point, the right side is positive) is designated S 1 , the distance from the first lens group 5 to the object point 4 in the state described in FIG. 1A (with the first lens group as the origin point, the right side is positive) is designated S 2 , the amount of movement of the image point of the first lens group 5 when changing from the FIG. 1A state to the FIG. 1B state (when the FIG. 1A state is the origin, the right side is positive) is designated S 3 , the transverse magnification in the states described in FIGS. 1A and 1B is nonvariable, such that the following equations may be obtained. 
     
         β1A·β1B=β2a·β2B      (1) 
    
     From the paraxial relation around the first lens group 5, the following equations 2 through 5 may be derived. 
     From the state described in FIG. 1A we obtain the following equations. ##EQU1## 
     From the state described in FIG. 1B we obtain the following equations. ##EQU2## 
     From the paraxial relation around the second lens group 6, the following equations 6 and 7 may be derived. ##EQU3## 
     From the aforesaid Eqs. 2, 3, 4, and 5 we can eliminate the factors S 1  and S 2 , and by solving for the factors of [x] and S 3  we can derive Eqs. 8 and 9. ##EQU4## 
     From the aforesaid Eqs. 6 and 9, we can eliminate the factor S 3 , and can derive the following Eq. 10. 
     When we eliminate the factor β2B from the aforesaid Eqs. 1 and 10, we can adjust the factor ##EQU5## β2A and thereby derive Eq. 11. 
     
         Aβ2A.sup.2 +Bβ2A+C=0                             (11) 
    
     where the factors A, B and C express the following: ##EQU6## 
     Accordingly, when f 1  &lt;0 the factor β2A&gt;0 can be solved and when f 1  &gt;0 the factor β2A&lt;0 can be solved to derive the following equation. ##EQU7## 
     Although, in the aforesaid example the first lens group 5 is a negative lens group, and the second lens group 6 is a positive lens group, it should be clear to those skilled in the art that, alternatively, said first lens group 5 may be a positive lens group and said second lens group 6 may be a negative lens group. 
     Concrete examples of a lens system adapted for the nonvariable transverse magnification focussing method is described below in Table 1. 
     
                       TABLE 1______________________________________Radius                   Refrac-of                       tive        AbbeCurva-       Spacing     Index       No.ture         on Axis     (Na)        (νd)______________________________________Lens Group 1r1   -178.168         d1     1.000  N1   1.78831                                   ν1                                        47.32r2     15.006         d2     1.600r3     16.244         d3     2.700  N2   1.68300                                   ν2                                        31.52r4     50.962         d4     2.050˜                2.691r5     18.260         d5     2.200  N3   1.67003                                   ν3                                        47.15Lens Group 2r6    288.268         d6     0.400r7     10.960         d7     2.500  N4   1.72000                                   ν4                                        52.14r8     23.753         d8     0.550r9    701.399         d9     2.200  N5   1.80518                                   ν5                                        25.43r10    9.248         d10    2.500r11    18.960         d11    2.800  N6   1.56567                                   ν6                                        43.02r12   -23.961         d12    0.0S DIAPHRAGM         d13    13.500r13   -13.376         d14    1.700  N7   1.49310                                   ν7                                        83.58r14   -24.209         d15    22.796˜                21.877r15  ∞         d16    3.00   N8   1.51680                                   ν8                                        64.20r16  ∞______________________________________ Σd = 61.496˜61.219 
    
     In the above table, the radii of curvature from the enlarging side of the screen (left side) are sequentially designated r1, r2, . . . r16, the spacings on the axis are sequentially designated d1, d2 . . . d16, the refractive indices on the glass d-line are sequentially designated N1, N2, . . . N8, and the Abbe numbers are sequentially designated ν1, ν2, . . . ν8. The aforesaid are examples of numerical values of an embodiment including the film holder. 
     The previously described lens system comprises a negative first lens group and a positive second lens group, and has a transverse magnification β such that β=-1/18 (approximate). This lens system may be used in conjunction with an image scanner, or microfilm projector. In regards to the change [a] in object distance described below, this lens system maintains a constant transverse magnification via the movement amount x and y for the first lens group and the second lens group, respectively. 
     
         ______________________________________Anterior focus        Posterior focus______________________________________f1 = -38.469              x = 0.277f2 = 21.394   a = 30      y = 0.9185β1A = 0.04352        β2A = 0.04504β1B = -1.2766        β2B = -0.05556β = -0.05556         β = -0.05556______________________________________ 
    
     In conventional tandem-type autofocussing methods, an afocal state (no magnification power, parallel luminous flux) is produced between the first lens group 1 and the second lens group 2. Therefore, when, for example, the distance from the object to the first lens group 1 is a distance [a] only, a focussed state cannot be maintained unless the first lens group 1 is always moved a distance [a]. 
     On the other hand, the present invention provides that a focussed state can be maintained by moving the first lens group and the second lens group a distance less than distance [a] because an afocal state is not produced between said first lens group 1 and said second lens group 2. Furthermore, the present invention allows that the diameter of either one or the other of the first lens group 1 or the second lens group 2 may be smaller because an afocal state is not produced (no parallel luminous flux) between said first lens group 1 and said second lens group 2. 
     In accordance with the foregoing description, the nonvariable transverse magnification focussing method of the present invention decreases the moving distance of the first lens group and second lens group and provides a nonvariable transverse magnification optical system by providing a suitable paraxial arrangement of said lens system and moving the first lens group and second lens group while maintaining a relation therebetween in accordance with specific principles. 
     Furthermore, when the aforesaid nonvariable transverse magnification focussing method is applied to the optical system of a copying apparatus, said method is capable of simultaneously correcting the focussing for distortion produced by the curvature (change of object position) of an original document such as a thick book or the like placed on the document platen of a copying apparatus. 
     The preferred other embodiment of the present invention is described hereinafter with reference to the accompanying drawings. FIGS. 4A and 4B are paraxial optical path diagrams. The image forming lens system is divided into two groups, a first lens group 9 having a focal length f 3  and a second lens group 10 having a focal length f 4 . The first and seconds lens groups 9 and 10, while maintaining a specific relation therebetween, are independently movable in the direction of an optical axis so as to maintain a constant transverse magnification relative to a position change [b] of the position of an object point 8 to an image point 8&#39; (the coordinate system is such that the left side is always positive). 
     When the amount of movement of the image point 8 is designated [b] (upon the movement to an object side of image point 8: b&gt;0, upon the movement to an image side of image point 8: b&lt;0), the focal length of the first lens group 9 is designated f 3 , the focal length of the second lens group 10 is designated f 4 , the transverse magnification around the first lens group 9 is designated β1C in the state (b=0) described in FIG. 4A, the transverse magnification around the second lens group 10 is designated β1D in the state (b=0) described in FIG. 4A, the transverse magnification around the first lens group 9 is designated β2C in the state (b=b) described in FIG. 4B, the transverse magnification around the second lens group 10 is designated β2D in the state (b=b) described in FIG. 4B, the amount of movement of the first lens group 9 when changing from the FIG. 4A state to the FIG. 4B state (right side is positive) is designated [x], the amount of movement of the second lens group 10 when changing from the FIG. 4A state to the FIG. 4B state (right side is positive) is designated [y], the distance from the first lens group 9 to the image point 8 in the state described in FIG. 4A (with the first lens group as the origin point, the right side is positive) is designated S 4 , the distance from the first lens group 9 to the object point in the state described in FIG. 4A (with the first lens group as the origin point, the right side is positive) is designated S 5 , the amount of movement of the object point of the first lens group 9 when changing from the FIG. 4A state to the FIG. 4B state (when the FIG. 4A state is the origin, the right side is positive) is designated S 6 , the transverse magnification in the states described in FIGS. 4A and 4B is nonvariable, such that the following equations may be obtained. 
     
         β1C·β1D=β2C·β2D      (14) 
    
     From the paraxial relation around the first lens group 5, the following equations 15 through 18 may be derived. 
     From the state described in FIG. 4A we obtain the following equations. ##EQU8## 
     From the state described in FIG. 4B we obtain the following equations. ##EQU9## 
     From the paraxial relation around the first lens group 9, the following equations 19 and 20 may be derived. ##EQU10## 
     From the aforesaid Eqs. 15, 16, 17 and 18 we can eliminate the factors S 4  and S 5 , and by solving for the factors of [y] and S 6  we can derive Eqs. 21 and 22. ##EQU11## 
     From the aforesaid Eqs. 19, 21 and 22, we can eliminate the factor S 6  and [y], and can derive the following Eq. 23. ##EQU12## 
     When we eliminate the factor β2D from the aforesaid Eqs. 14 and 23, we can adjust the factor β2C and thereby derive Eq. 24. ##EQU13## 
     Although, in the aforesaid example the first lens group 9 is a positive lens group, and the second lens group 10 is a negative lens group, it should be clear to those skilled in the art that, alternatively, said first lens group 9 may be a negative lens group and said second lens group 10 may be a positive lens group. 
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.