JPH0545001B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a compact zoom lens, and more particularly to a compact zoom lens with a short overall lens length (distance from the first lens surface to the image plane) that is effective for lens shutter cameras, video cameras, and the like. In recent years, with the miniaturization of lens shutter cameras, video cameras, etc., there has been a demand for zoom lenses with short overall lens lengths. Additionally, zoom lens devices are required in the field of cameras that do not require interchangeable lenses, such as lens-shutter cameras, and small zoom lenses with a length comparable to conventional single-focal-length lenses are required. . In Japanese Patent Application Laid-Open No. 57-201213, the applicant
The first lens group has a positive refractive power and the second lens group has a negative refractive power.
We have proposed a compact two-group zoom lens that is composed of two lens groups and that changes the magnification by changing the distance between both lens groups. In this proposal, by configuring lens groups with positive and negative refractive powers in order from the object side, a compact zoom lens with a short back focus and a mechanically simple configuration is achieved. In addition, in Japanese Patent Application Laid-Open No. 58-184916, first, second, and third refractive powers of positive, positive, and negative refractive powers are arranged in order from the object side.
A three-group zoom lens has been proposed, which is composed of three lens groups and whose magnification is changed by moving the three lens groups. However, in the aforementioned two-group zoom lens, since there are only two lens groups that move during zooming, it is difficult to obtain a predetermined zoom ratio without moving the lens groups relatively often. The lens tends to be mechanically complex because three lens groups are moved to change the magnification. Furthermore, since both zoom lenses also move the aperture when changing the magnification, the movement mechanism tends to become complicated. SUMMARY OF THE INVENTION An object of the present invention is to provide a compact zoom lens that can easily obtain a predetermined variable power ratio, is mechanically simple, and has a short overall lens length. A further object of the present invention is to provide a compact zoom lens suitable for a lens shutter camera with a variable magnification ratio of about 1.5. The main features of a compact zoom lens for achieving the purpose of the present invention are, in order from the object side, a first lens group with positive refractive power, a second lens group with positive refractive power, and a third lens group with negative refractive power. The zoom position is changed from the wide-angle end to the telephoto end by fixing the second lens group and moving the first lens group and the third lens group monotonically toward the object side. The zoom lens performs magnification, and the first lens group is composed of three lenses each having positive, negative, and positive refractive powers, and the first lens group, the third lens group, and
The focal lengths of the lens groups are respectively f ₁ and f ₃ , the focal length of the entire system at the wide-angle end zoom position is f _W , and the first lens group and the second lens group at the wide-angle end zoom position are respectively f 1 and f 3 .
When the distance between the principal points of the lens groups is e _12W and the distance between the principal points of the second lens group and the third lens group at the telephoto end zoom position is e _23T , -0.3f _W < f ₁ < 5f _W ...... (1) −2.5f _W ＜f ₃ ＜−0.5f _W ……(2) e _12W ＞−0.5f _W ……(3) e _23T ＞−0.4f _W ……(4) Satisfy the following conditions. It is. In this way, in the present invention, magnification is changed by monotonically moving two of the three lens groups, the first and third lens groups, toward the object side, and moving the second lens group, which often includes an aperture, to the object side. is fixed to simplify the mechanism. In addition, the first lens group has a triplet type lens configuration consisting of lenses with positive, negative, and positive refractive powers in order from the object side, which reduces the occurrence of aberrations and further reduces aberration fluctuations due to zooming. ing. Next, the technical meaning of the above-mentioned conditions will be explained. Conditional expression (1) relates to the refractive power of the first lens group, and if the upper limit is exceeded, the rate of change in the imaging magnification of the second lens group due to movement of the first lens group becomes small, so it is important to maintain a predetermined zoom ratio. In order to achieve this, the amount of movement of the first lens group must be increased, and as a result, the total length of the lens increases at the telephoto end.
Moreover, if the lower limit is exceeded, the refractive power of the first lens group becomes too strong, and high-order aberrations tend to occur at the telephoto end, making it difficult to obtain a sufficient zoom ratio. Conditional expression (2) relates to the refractive power of the third lens group; when the upper limit is exceeded, the negative refractive power becomes large and the Petzval sum increases in the negative direction, so the curvature of field increases in the positive direction. Moreover, if the lower limit is exceeded, the amount of movement of the third lens group becomes large when trying to obtain a sufficient zoom ratio, which is not preferable because the total length of the lens at the telephoto end becomes long. Conditional expressions (3) and (4) are related to the distance between each lens group at the zoom position where the distance between each lens group is the minimum, and the overall length of the lens can be shortened by appropriately setting the distance between each lens group. If the range of conditional expressions (3) and (4) is exceeded, the distance between the lens groups at the wide-angle end and the telephoto end will become too narrow, causing interference between the lens groups, which is not desirable in terms of lens construction. . The object of the present invention is achieved by satisfying the above conditions, but more preferably, the radius of curvature of the i-th lens surface from the object side of the first lens group is adjusted in consideration of image quality. When is Ri, 0.3f _W <R1<2f _W ……(5) −5f _W ＜R3＜−0.3f _W ……(6) −5f _W ＜R6＜−0.3f _W ……(7) It is to satisfy the conditions. Conditional expressions (5), (6), and (7) each relate to the radius of curvature of the lens surface in the main part of the lens configuration of the first lens group, and the upper limit of conditional expression (5) and conditional expression (6) , (7), positive spherical aberration will occur, and if the lower limit of conditional expression (5) and the upper limit of conditional expressions (6) and (7) are exceeded, higher-order aberrations will occur. This is not desirable because coma aberration occurs frequently, especially at the zoom position at the telephoto end. In the present invention, the focal length f ₂ of the second lens group, which is fixed during zooming, is set as f _W < f ₂ < 5f _W (8). This is preferable in order to promote the effect of zooming by moving the group, and to better correct aberrations of the entire screen during zooming. When the upper limit of conditional expression (8) is exceeded, the refractive power of the second lens group becomes weaker, and in order to obtain a predetermined variable power ratio, the principal point distance e ₂ between the second and third lens groups must be increased. In addition, the negative refractive power of the third lens group also becomes weaker, and the amount of movement becomes larger. If the lower limit is exceeded, the refractive power of the second lens group becomes strong, the negative refractive power of the third lens group becomes excessive, and the distance between the principal points of the second lens group and the third lens group becomes small, resulting in a narrow movement space. It's gone and it's not appropriate. In the present invention, in order to efficiently change the magnification, the imaging magnification β ₂ of the second lens group and the imaging magnification β ₃ of the third lens group are required when changing the magnification from the wide-angle end to the telephoto end. It is preferable that both of them are continuously multiplied. In order to continuously increase the imaging magnification β ₂ , since both the first lens group and the second lens group have positive refractive power, the lens distance between the first lens group and the second lens group must be adjusted. is to continuously increase the value.
In addition, in order to continuously increase the imaging magnification β ₃ , the combined refractive power of the first and second lens groups is always positive, so the combination of the second and third lens groups is The goal is to continuously reduce the spacing. In addition, in a zoom lens in which the third lens group moves as the power is changed, as in the present invention, the third lens group is composed of a meniscus-shaped lens with a negative refractive power with a convex surface facing the image plane, but it is completely variable. This is preferable for correcting the image plane characteristics of the entire screen well over the magnification range. In the present invention, focusing is preferably performed by extending the entire lens system, but it may also be done by extending the first lens group or the second lens group, or by retracting the third lens group. The lens group and the second lens group may be extended integrally. As described above, according to the present invention, it is possible to achieve a compact zoom lens that is mechanically simple and has high zooming efficiency. Next, numerical examples of the present invention will be shown. In the numerical examples, Ri is the radius of curvature of the i-th lens surface from the object side, D _i is the thickness and air gap of the i-th lens from the object side, and N _i and ν _i are the curvature radius of the i-th lens surface from the object side, respectively. These are the refractive index and Atsube number of the glass of the i-th lens. Further, Table 1 shows the relationship between each of the above-mentioned conditional expressions and various numerical values in the numerical examples.

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【table】 [Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an optical system according to an embodiment of the present invention;
2 and 3 are lens sectional views of numerical examples 1 and 2 of the present invention, respectively, and FIGS. 4 and 5 are various aberration diagrams of numerical examples 1 and 2 of the present invention, respectively. In the figure, A, B, and C are various aberration diagrams at the wide-angle end, intermediate, and telephoto end zoom positions, ΔS is the sagittal image plane, and ΔM is the meridional image plane.

Claims (1)

[Claims] 1. A first lens group having positive refractive power in order from the object side,
It has three lens groups, a second lens group with positive refractive power and a third lens group with negative refractive power, and the second lens group is fixed and the first lens group and the third lens group are monotonically arranged. A zoom lens that changes magnification from a wide-angle end to a telephoto end by moving toward the object side, the first lens group being composed of three lenses each having positive, negative, and positive refractive powers. The focal lengths of the first and third lens groups are respectively f ₁ and f ₃ , the focal length of the entire system at the wide-angle end zoom position is f _W , and the first lens group and the The distance between the principal points of the second lens group is e _12W , and the distance between the principal points of the second lens group at the telephoto end zoom position is
When the principal point distance between the lens group and the third lens group is e _23T , −0.3f _W <f ₁ <5f _W −2.5f _W <f ₃ <−0.5f _W e _12W >−0.5f _W e _23T A compact zoom lens that satisfies the condition of > -0.4f _W. 2 When the radius of curvature of the i-th lens surface from the object side of the first lens group is R _i , 0.3f _W <R ₁ <2f _W −5f _W <R ₃ <−0.3f _W −5f _W < A small zoom lens according to claim 1, characterized in that it satisfies the following condition: R ₆ <-0.3f _W.