Wide angle converter for a zoom lens

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 (R3, R4) 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-concave and its second lens is a bi-convex, 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 OF 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. The wide-conversion lens comprises a bi-concave first spherical lens and a bi-convex spherical second lens. The bi-convex second lens 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 > 50 , ν2 > 55 , 1.5 < ν2 - ν1 < 10 , 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 bi-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 is, 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 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 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 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 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 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 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 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 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 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 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 scopic 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 1 Afocal magnification m=0.87 R1 = -54.854 T1 = 1.50 N1 = 1.603112 ν1 = 60.7 R2 = 38.089 D2 = 3.25 R3 = 43.000 T3 = 5.64 N2 = 1.516330 ν1 = 64.1 R4 = -43.000 Numerical Example 2 Numerical Example 3 Afocal magnification m=0.82 R1 = -61.490 T1 = 1.50 N1 = 1.603112 ν1 = 60.7 R2 = 34.808 D2 = 5.90 R3 = 45.196 T3 = 5.30 N2 = 1.516330 ν1 = 64.1 R4 = -45.196 Numerical Example 4 Afocal magnification m=0.85 R1 = -76.673 T1 = 1.50 N1 = 1.638539 ν1 = 55.4 R2 = 39.128 D2 = 5.90 R3 = 49.196 T3 = 5.30 N2 = 1.5616329 ν1 = 64.1 R4 = -49.196 Numerical Example 5 Afocal magnification m=0.82 R1 = -69.551 T1 = 1.50 N1 = 1.638539 ν1 = 55.4 R2 = 36.747 D2 = 6.23 R3 = 46.864 T3 = 5.10 N2 = 1.522491 ν1 = 59.8 R4 = -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 - 46.8 R 5= 41.099 T 5= 0.90 N 3= 1.805177 ν 3= 25.4 R 6= 18.520 T 6= 5.10 N 4= 1.589130 ν 4= 61.2 R 7= -62.450 D 7= 0.12 R 8= 14.908 T 8= 2.70 N 5= 1.603112 ν 5= 60.7 R 9= 38.640 D 9=variable R10= 38.640 T10= 0.60 N 6= 1.772499 ν 6= 49.6 R11= 5.694 T11= 2.13 R12= -6.668 T12= 0.80 N 7= 1.665470 ν 7= 55.2 R13= 6.668 T13= 1.90 N 8= 1.799250 ν 8= 24.5 R14= 82.608 D14=variable R15= (stop) D15= 1.00 R16= 7.805 T16= 3.00 N 9= 1.606020 ν 9= 57.4 R17= -21.520 D17= 1.39 R18= 18.622 T18= 0.70 N10= 1.846660 ν10= 23.9 R19= 8.379 D19=variable R20= 9.200 T20= 0.80 N11= 1.846660 ν11= 23.9 R21= 6.120 T21= 2.90 N12= 1.606020 ν12= 57.4 R22= -29.003 D22=variable R23= ∞ T23= 4.00 N13 = 1.516330 ν13= 64.1 R24= ∞ 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: Z = cy 2 1+ 1-(1+k)c 2 y 2 +AD·y 4 +AE·y 6 +AF·y 8 +AG·y 10 , 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 through FIG.11 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 scope of the invention.

Claims: A wide-conversion lens for making a focal length of the whole of a lens system short by attaching it in front of a master lens system, wherein the conversion lens consists of a bi-concave first spherical lens and a bi-convex second spherical lens in the order starting from the object side, characterised in that the radius (R3, R4) of curvature of the surface of the second lens on its object side and that on its image side are the same. A wide-conversion lens in accordance with claim 1 wherein the following conditions are satisfied; ν 1 > 50; ν 2 > 55; and 1,5 < ν 2 - ν 1 < 10, where ν 1 is the Abbe number of the first lens, and ν 2 is the Abbe number of the second lens. A zoom lens which comprises the wide-conversion lens of claim 1.

Inventors: ONO SHUSUKE, YAMADA KATSU, ONO, SHUSUKE, YAMADA, KATSU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1993
Main classification: G02B15
IPC codes: G02B15