Abstract:
A wide-conversion lens for attachment to a master lens system consists of a bi-negative first lens in a object side and a bi-positive second lens in a master lens side, and in the second lens its radii of curvature of both surfaces are selected the same.

Description:
FIELD OF THE INVENTION AND RELATED ART STATEMENT 
     1. Field of the Invention 
     The present invention relates to a wide-conversion lens for making a focal length of the combined whole of a lens system short by attaching it in front of a lens system, particularly to a wide-conversion lens for the use of taking still pictures as well as of taking moving pictures for a video camera, which has a high optical performance and is Compact and light weight by reducing number of component lenses to two. 
     2. Description of the Related Art 
     Heretofore, there has been a demand of converting an apparatus such as a video movie or a still camera which includes lenses into apparatus having a wide-angled lens. It has been proposed to attach a wide-conversion lens on the objective side of the lens for making the focal length of a lens system shorter. And at the same time, to make a lens system per se compact is also an inevitable matter, since it is also required to make the apparatus including lenses compact. 
     For instance, as an example of the wide-conversion lens of this kind, such as Japanese Unexamined published patent application Sho (hereafter Tokkai Sho) 63-253319 or Tokkai Hei 4-70616 can be cited. However, in Tokkai Sho 63-253319, the number of lenses constructing the lens system is three or four and hence it is rather complex. Whereas in Tokkai Hei 4-70616, although the number of lenses is two, its first lens is a bi-negative and its second lens is a bi-positive, and the radius of curvature of the objective side surface of the second lens and that of the image side surface have values that are close to each other. It was not satisfactory, because its handling in the assembling stage became difficult, and also its aberration characteristics was sufficient. 
     As has been described above, in wide-conversion lenses of prior art, there has been a problem that, when the lens number was reduced, a sufficient aberration compensation became unattainable, and the optical performance was degraded. 
     OBJECT AND SUMMARY OFT THE INVENTION 
     The purpose of the present invention is to remove the drawback described above, and to offer a compact wide-conversion lens of high performance with an afocal magnification of about 0.8. 
     In order to attain the above-mentioned purpose, the wide-conversion lens of the present invention is of a lens system that is to be attached on the front side of the camera lens. And it is characterized in that it comprises a bi-negative first lens and a bi-positive second lens, each having the same radius of curvature on its object side surface and on its image side surface for the convenience at its assembling stage. 
     Practically, it is desired that the above-mentioned wide-conversion lens satisfies the following conditions: 
     
         ν1&gt;50                                                   (1) 
    
     
         ν2&gt;55                                                   (2) 
    
     
         1.5&lt;ν2-ν1&lt;10                                         (3) 
    
     where ν1 is an Abbe number of the first lens, and ν2 is an Abbe number of the second lens. 
     And, a zoom lens of the present invention that is to accomplish the above-mentioned purpose is a lens system which is constructed at least by attaching the above-mentioned wide-conversion lens in front of a zoom lens which is expected to serve as a master lens. 
     According to the above constitution, the lens system is configured by two lenses in a manner that the above-mentioned first lens to be a hi-negative lens and the above-mentioned second lens to be a bi-positive lens having the same radius of curvature on its objective side surface and its image side surface. As a result of such configuration, a compact wide-conversion lens having an afocal magnification of about 0.8 can be offered by a relatively simple constitution, and that, assembling becomes easy since there is no need of confirming which side of the negative lens must be adhered to which side of the positive lens. 
     And, by satisfying the afore-mentioned conditions (1)-(3), a high performance wide-conversion lens whose aberration is well compensated can be presented. 
     Also, by using the wide-conversion lens of the present invention, a compact and high performance zoom lens can be realized. 
     As has been described above, the advantageous feature present invention is that, by making the focal length of whole of the camera system short by attaching the wide-conversion lens of the present invention in front of a camera lens followed by constituting it using two lenses that satisfy the above-mentioned conditions, a light-weight and compact wide-conversion lens having a high optical performance of an afocal magnification of about 0.8 can be offered. And that, when it is attached on a zoom lens, a compact and light-weight zoom lens can be realized with maintaining its high optical performance over whole zooming range. 
     Furthermore, by making the radius of curvature of surfaces on the object side and on the image side of the second lens to be the same value, possible misplacement of the side of the lens, that lens, placement of wrong side of front and rear of the lens, In the assembling stage can be avoided. 
     Hereupon, although the above explanation has been given mainly on examples that the wide-conversion lens of the present invention was attached on a zoom lens, it is needless to mention that it can also be applied on a single focal length lens. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view when the wide-conversion lens of the first embodiment of the present invention is attached on the front plane of a zoom lens. 
     FIG. 2(a)-(c) is an aberration diagram at the wide-angled extreme of the zooming action when the wide-conversion lens of the first embodiment of the present invention is attached on the front plane of a zoom lens. 
     FIG. 3(a)-(c) is an aberration diagram at the telescopic extreme of the zooming action when the wide-conversion lens of the first embodiment of the present invention is attached on the front plane of a zoom lens. 
     FIG. 4(a)-(c) is an aberration diagram at the wide-angled extreme of the zooming action when the wide-conversion lens of the second embodiment of the present invention is attached on the front plane of a zoom lens. 
     FIG. 5(a)-(c) is an aberration diagram at the telescopic extreme of the zooming action when the wide-conversion lens of the second embodiment of the present invention is attached on the front plane of a zoom lens. 
     FIG. 6(a)-(c) is an aberration diagram at the wide-angled extreme of the zooming action when the wide-conversion lens of the third embodiment of the present invention is attached on the front plane of a zoom lens. 
     FIG. 7(a)-(c) is an aberration diagram at the telescopic extreme of the zooming action when the wide-conversion lens of the third embodiment of the present invention is attached on the front plane of a zoom lens. 
     FIG. 8(a)-(c) is an aberration diagram at the wide-angled extreme of the zooming action when the wide-conversion lens of the fourth embodiment of the present invention is attached on the front plane of a zoom lens. 
     FIG. 9(a)-(c) is an aberration diagram at the telescopic extreme of the zooming action when the wide-conversion lens of the fourth embodiment of the present invention is attached on the front plane of a zoom lens. 
     FIG. 10(a)-(c) is an aberration diagram at the wide-angled extreme of the zooming action when the wide-conversion lens of the fifth embodiment of the present invention is attached on the front plane of a zoom lens. 
     FIG. 11(a)-(c) is an aberration diagram at the telescopic extreme of the zooming action when the wide-conversion lens of the fifth embodiment of the present invention is attached on the front plane of a zoom lens. 
    
    
     It will be recognized that some or all of the Figures are schematic representations for purposes of illustration and do not necessarily depict the actual relative sizes or locations of the elements shown. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Now, an explanation is given on the embodiments of the present invention referring to the drawings. 
     FIG. 1 shows a cross-sectional view of the present embodiment when the wide-conversion lens in the first embodiment of the present invention is attached on the front plane of a zoom lens that is a master lens. 
     Ri generally represents a radius of curvature of an i-th face of lens from the object side, Ti generally represents a thickness on axis of a lens having an i-th surface and an (i+1)-th surface, Di represents distance on axis of an inter-lens space between i-th surface of a lens and (i+1)-th surface of the next lens toward the master lens, and Ni and νi are a refractive index and an Abbe number of an i-th lens from the object side, respectively. 
     In general, the master lens is normally aberration-compensated within it because it is used usually by itself. Therefore, in order to obtain a satisfactory aberration characteristic as the whole when the wide-conversion lens is attached on the master lens, it is also necessary to compensate the aberration of the wide-conversion lens itself. 
     Thus, in the present invention, by making the first lens to be bi-negative, an excessive compensation for the spherical aberration possibly occurring at the telescopic extreme in the zooming action when the wide-conversion lens is attached on a zoom lens is prevented. 
     And by fulfilling the conditions (1), (2), and (3), chromatic aberration that is one of the cause of the picture quality degradation is compensated. 
     Furthermore, by making the radius of curvature of surfaces on the object side and on the image side of the second lens to be the same value, discrimination of the face sides of the lens in the assembling stage becomes unnecessary. Thus it provides a large advantage in the lens manufacturing. 
     In the following, numerical examples in the embodiment are shown. 
     
         ______________________________________Numerical Example 1Afocal magnification m = 0.87R1 = -54.854      T1 = 1.50  N1 = 1.603112                              ν1 = 60.7R2 = 38.089      D2 = 3.25R3 = 43.000      T3 = 5.64  N2 = 1.516330                              ν1 = 64.1R4 = -43.000Numerical Example 2Afocal magnification m = 0.82R1 = -65.338      T1 = 1.50  N1 = 1.603112                              ν1 = 60.7R2 = 35.556      D2 = 6.23R3 = 46.864      T3 = 5.10  N2 = 1.516330                              ν1 = 64.1R4 = -46.864Numerical Example 3Afocal magnification m = 0.82R1 = -61.490      T1 = 1.50  N1 = 1.603112                              ν1 = 60.7R2 = 34.808      D2 = 5.90R3 = 45.196      T3 = 5.30  N2 = 1.516330                              ν1 = 64.1R4 = -45.196Numerical Example 4Afocal magnification m = 0.85R1 = -76.673      T1 = 1.50  N1 = 1.638539                              ν1 = 55.4R2 = 39.128      D2 = 5.90R3 = 49.196      T3 = 5.30  N2 = 1.5616329                              ν1 = 64.1R4 = -49.196Numerical Example 5Afocal magnification m = 0.82R1 = -69.551      T1 = 1.50  N1 = 1.638539                              ν1 = 55.4R2 = 36.747      D2 = 6.23R3 = 46.864      T3 = 5.10  N2 = 1.522491                              ν1 = 59.8R4 = -46.864______________________________________ 
    
     Numerical example of the master lens, to which the above-mentioned respective wide-conversion lenses are to be attached, is shown below. 
     
         ______________________________________Numerical example of the master lens______________________________________f-number = 1.85  Focal length: f = 4.7-46.8R5 = 41.099      T5 = 0.90   N3 = 1.805177                              ν3 = 25.4R6 = 18.520      T6 = 5.10   N4 = 1.589130                              ν4 = 61.2R7 = -62.450      D7 = 0.12R8 = 14.908      T8 = 2.70   N5 = 1.603112                              ν5 = 60.7R9 = 38.640      D9 = variableR10 = 38.640      T10 = 0.60  N6 = 1.772499                              ν6 = 49.6R11 = 5.694      T11 = 2.13R12 = -6.668      T12 = 0.80  N7 = 1.665470                              ν7 = 55.2R13 = 6.668      T13 = 1.90  N8 = 1.799250                              ν8 = 24.5R14 = 82.608      D14 = variableR15 = (stop)      D15 = 1.00R16 = 7.805      T16 = 3.00  N9 = 1.606020                              ν9 = 57.4R17 = -21.520      D17 = 1.39R18 = 18.622      T18 = 0.70  N10 = 1.846660                              ν10 = 23.9R19 = 8.379      D19 = variableR20 = 9.200      T20 = 0.80  N11 = 1.846660                              ν11 = 23.9R21 = 6.120      T21 = 2.90  N12 = 1.606020                              ν12 = 57.4R22 = -29.003      D22 = variableR23 = ∞      T23 = 4.00  N13 = 1.516330                              ν13 = 64.1R24 = ∞16th aspherical       17th aspherical                     22nd asphericalplane       plane         planeaspherical  aspherical    asphericalcoefficient coefficient   coefficientK = -9.14068E-1       K = -8.60167E-1                     K = -1.19338AD = -8.76743E-5       AD = 1.20432F-4                     AD = 2.424495E-4AE = -6.30037E-7       AE = -2.01710E-6                     AE = 1.10516E-6AF = 8.86838E-8       AF = 8.72027E-8                     AF = 1.51740E-7AG = -3.26086E-9       AG = -3.17966E-9                     AG = -5.90709E-9______________________________________ 
    
     
                       TABLE 1______________________________________Variable         Focal LengthDistance         3.92   38.37______________________________________D9               0.80   13.36D14              13.54  0.98D19              5.71   4.60D22              2.01   3.12______________________________________ 
    
     TABLE 1 represents variable distance of inter-lens space or zoom distance at the wide-angled extreme and the telescopic extreme. And the aspheric shape is defined by the following equation: ##EQU1## where y: height from the optic axis of a point on the aspheric surface 
     Z: distance of a point on the aspheric surface, height of the point from the optical axis being y and being measured from a tangential plane on the aspheric surface at its apex 
     c: radius of curvature of the aspheric surface at its apex 
     k: conical constant 
     AD, AE, AF, and AG: aspheric coefficients 
     FIG. 2(a)-(c) through FIG. 11(a)-(c) are aberration diagrams when the wide-conversion lenses of numerical example 1 through numerical example 5 of the present invention are attached respectively on the front plane of the master lens in the above-mentioned embodiment. In the aberration diagrams, d, F, and c represents aberrations at the d-line, F-line, and c-line, respectively. And in the astigmatic diagrams, ΔM represents the aberration on the meridional image plane, and ΔS represents the aberration on the sagital image plane. 
     Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that such disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the true spirit and scope of the invention.