JP2568266B2 - Zoom lens - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a zoom lens, and more particularly to a negative lens group closest to the object side, which is suitable for a photographing lens such as a single-lens reflex camera and a still video that requires a relatively long back focus. Related to the zoom lens.

<Prior Art> Conventionally, in a single-lens reflex camera, a rotary reflecting mirror is provided in the rear of the taking lens to reflect the light flux from the taking lens and guide it to a finder system.

Therefore, the taking lens used in a single-lens reflex camera can easily obtain a back focus that is long enough to arrange the above-mentioned reflecting mirror, and a zoom lens having a high zoom ratio while maintaining high optical performance is required. ing. Further, along with the downsizing of the camera body, downsizing and weight saving of the taking lens itself are desired.

As a zoom lens which is comparatively smaller than the conventional one and is suitable for a single-lens reflex camera, a zoom lens having a two-group structure having a negative refractive power and a positive refractive power in order from the object side is disclosed in, for example, Japanese Patent Laid-Open No. 49-2545. Japanese Patent Publication No. 52-26
It is proposed in Japanese Patent Laid-Open No. 236 and Japanese Patent Laid-Open No. 59-64811.

Further, a zoom lens in which a fixed negative lens is arranged on the image side is a developed type of the above-mentioned two-group zoom lens for promoting further miniaturization, which is disclosed in JP-A-58-111013 or JP-A-61-183613. Japanese Patent Laid-Open No. 62-112115. Japanese Patent Laid-Open Nos. 61-183613 and 62-112115 disclose zoom lenses in which aspherical surfaces are used in some lens groups to achieve miniaturization.

<Problems to be Solved by the Present Invention> The zoom lens described above is basically a reverse-telescopic wide-angle lens, and is suitable for a zoom lens that covers the focal length on the wide-angle side.

By the way, as a product, it is widely used as a standard zoom of 35 mm to 70 mm. On the other hand, recently, it has been desired to increase the focal length of the standard zoom lens on the telephoto side. For example, JP-A-58-75108 discloses 28 mm to 85 mm and JP-B-55-1.
Although a zoom lens having a high magnification of 35 mm to 140 mm is proposed in Japanese Patent No. 4403, etc., the number of lenses to be configured is large and the number of moving lens groups is also large. Therefore, the structure of the lens barrel is complicated, but it is expensive, and the total length of the lens is large, so that it cannot be said to be sufficient as a standard zoom that is always used.

It is an object of the present invention to provide a zoom lens in which the focal length on the telephoto side is expanded while realizing the two problems of the standard zoom described above, that is, compactness and cost reduction, while maintaining the optical performance.

<Means and Structure for Solving Problems> First object group having negative refracting power and moving along the optical axis during zooming in order from the object side, optical axis having positive refracting power during zooming A second lens unit that moves along with a third lens unit that has a negative refracting power and is fixed during zooming, and the first lens unit has a negative refracting power and a strong concave surface on the image side. A lens that has a positive refracting power and an aspherical surface with a meniscus shape with a convex surface facing the object side.
The third lens group includes 12 lenses, the second lens group includes 21 lenses having positive refracting power, 22 lenses having positive refracting power, 23 lenses having biconcave surfaces, and 24 lenses having biconvex lenses in order. The group has 31 meniscus lenses each having a concave surface facing the image side, and satisfies the following conditions.

_{That, (1) -0.65 <f 1} / f T <-0.45 (2) 0.43 <r 2 / r 3 <0.63 (3) r 9 / r 10 <-15 (4) n 5> 1.8.

Where r ₂ ; the radius of curvature of the 11 lens on the image side, r ₃ ; the radius of curvature of the 12 lens on the object side, r ₉ ; the radius of curvature of the 23 lens on the object side, r ₁₀ ; the image side of the 23 lens Radius of curvature, f ₁ ; focal length of the first lens group, f _T ; focal length of the entire system at the telephoto end, n ₅ ; refractive index of the Franhofer d-line of the 23 lenses.

<Embodiment> FIGS. 2, 4, 6, and 8 are lens cross-sectional views of Numerical Embodiments 1, 2, 3, and 4 relating to the present invention. The first lens group I has a negative refracting power and moves along the locus indicated by the arrow during zooming from the telephoto end to the wide-angle end, and Π has positive refraction and has a locus indicated by the arrow during zooming. The second lens group III moves in the same way, and the third lens group III has a negative refractive power and is fixed during zooming. The focusing is performed by moving the first lens group along the optical axis.

By the way, in order to increase the zoom ratio, it is general to increase the amount of movement of the first lens group and the second lens group, but the distance between the first lens group and the second lens group at the wide-angle end is large. Also, the distance from the second lens group to the image plane at the telephoto end becomes large, and the total lens length becomes long. On the other hand, in order to obtain a desired zoom ratio while making the total lens length relatively short, it is necessary to strengthen the refractive power of each lens group, but in the case of the two-group configuration, the solution is almost uniquely determined and as a result There is a disadvantage that the total lens length on the wide-angle side is long and the amount of focusing movement is increased. Therefore, it is advisable to adopt a zoom type having a three-group structure as described in Japanese Patent Laid-Open No. 58-111013.

Further, in the present invention, the above-mentioned object is achieved by further improving the refractive power arrangement and lens shape of each lens unit as described above.

 Next, the significance of the numerical range of each conditional expression will be described.

If the lower limit of conditional expression (1) is exceeded and the refractive power of the first lens group is weakened, the lens length on the wide-angle side will increase, and the amount of focusing movement will increase to increase the lens barrel overall length. The outer diameter of the lens becomes large in order to secure the peripheral light amount, which is not preferable. If the refracting power is increased beyond the upper limit, distortion and astigmatism will occur particularly on the wide angle side, and the correction limit will be exceeded even if an aspherical surface is used.

Conditional expression (2) is a condition for satisfactorily correcting spherical aberration and coma generated by increasing the refractive power of the first lens group. Table 1 shown for reference is Numerical Example 1
Is the Seidel aberration value of each surface. As can be seen from the table, the spherical aberrations generated at r ₂ and r ₃ increase remarkably on the telephoto side, and when the focal length at the telephoto end is enlarged, the amount of increase also increases. On the other hand, the coma aberration has a sign that the signs are reversed between the wide-angle side and the telephoto side,
If the upper limit or the lower limit of conditional expression (2) is exceeded, spherical aberration and coma cannot be corrected simultaneously.

Conditional expression (3) is an expression regarding the sharing of the refractive powers of the two surfaces of the 23 lenses. When the refractive power of the first lens group is strengthened according to the conditional expression (1), the total lens length is shortened by strengthening the refractive power of the second lens group at the same time. However, in order to correct the intra-group aberration of strong positive refractive power, A negative lens surface is required, and an appropriate shape of one biconcave lens is required to realize with a small number of constituents. higher-order aberration occurs and strengthen the r ₉ side,
Off-axis aberrations worsen when the r ₁₀ side is strengthened. A high refractive index glass material is used as in conditional expression (4), and a stronger curvature is given to the r ₁₀ side as in conditional expression (3). By using a negative meniscus lens having a strong concave surface on the image side for the 31 lenses of the 3 lens groups, good aberration correction is possible. As can be seen from Table 1, the 31 lens has the effect of correcting astigmatism to the over side on the wide angle side and to the under side on the telephoto side.

More preferably, the image side surface of 12 lenses is made aspherical,
By giving the shape such that the curvature becomes weaker toward the periphery, that is, the negative refractive power is weakened at the periphery, excellent aberration correction can be performed. Generally speaking, in a wide-angle zoom lens, focusing on correcting astigmatism and distortion, there is no great difference in effect even if any surface of the first lens group is made aspherical. However, it is explained by the conditional expression (2). As described above, the situation is different when the zoom area is expanded to the telephoto side, and it is preferable to make the image side of the 12 lenses aspherical in addition to the condition of Expression (2). Further, the refracting power of the first lens group is strengthened according to the conditional expression (2), and moreover, on the surface closest to the stop in the first lens group, that is, the surface which is low in aberration theory, the amount of aspherical surface is increased and the shape is extremely small. It is necessary to give a shape such that the local radius of curvature is reversed in the peripheral portion, that is, the radius of curvature is positive in the lens central portion and negative in the peripheral portion. FIG. 1 is a diagram in which an aspherical shape is expressed by default and has a convex shape in the periphery.

In the present invention, it is more preferable to use a low-dispersion glass material having 31 lenses with an Abbe number of more than 50 (ν> 50). The minimum configuration is a single lens, and if the refracting power is increased in order to shorten the overall length, zoom fluctuation of lateral chromatic aberration, color coma on the telephoto side, and astigmatism occur. Therefore, it is necessary to reduce the dispersion.

Further, as described above, the 31 lens corrects the zoom fluctuation of astigmatism, but there is a limit in the spherical system, and making it aspherical is effective in further improving the astigmatism on the wide angle side.

By the way, it is advantageous in terms of the catalog to express the total lens length of the zoom lens at the wide-angle end or the telephoto end, or at the intermediate zoom position when the first lens group reciprocates, such as the shortest total length at the time of storage. Let's do it. However, it has the drawback of being susceptible to shock. For example, I heard that when I hung a camera from my shoulder and boarded a sightseeing bus, I accidentally hit the tip of a lens against an iron pipe on a railing and broke it. In this way, as a structure that is resistant to inadvertent mistakes, a fixed lens barrel that is longest in zooming and stronger on the object side than the longest part that moves to the object side when the focus is extended may be provided. As described above, in order to ensure compactness under the disadvantageous condition of the total lens length, it is preferable that the above-mentioned refractive power is shared so that the total lens lengths at the wide-angle end and the telephoto end are substantially equal.

Next, numerical examples of the present invention will be shown. In the numerical example
Ri is the radius of curvature of the i-th lens surface in order from the object side, Di
Is the i-th lens thickness and air gap from the object side, Ni and ν
i is the refractive index and the Abbe number of the glass of the i-th lens in order from the object side.

The aspherical shape has an x-axis in the direction of the optical axis, an H-axis in the direction perpendicular to the optical axis, a positive traveling direction of light, and R is a paraxial radius of curvature, A, B,
When C, D, E, ..., A ', B', C ', D', ... are aspherical coefficients respectively It is expressed by

Numerical Example 1 Numerical Example 2 Numerical Example 3 Numerical Example 4 [Effects of the Invention] As described above, even if the zoom range of the standard zoom lens of the 35 to 70 mm class is expanded to the telephoto side by adopting the configuration of the present invention, the total lens length, especially at the longest zoom position It has the effect of shortening the overall lens length.

Furthermore, by effectively using the aspherical surface, the number of lenses and the lens barrel structure can be simplified, and the lens can be provided at low cost. At present, a technique for molding glass softened at a high temperature with an aspherical mold has been completed, and an aspherical lens is no longer an expensive element as before.

[Brief description of drawings]

FIG. 1 is a diagram showing a state of an aspherical surface according to the present invention. 2, FIG. 4, FIG. 6 and FIG. 8 are lens cross-sectional views of Numerical Embodiments 1, 2, 3, and 4 of the present invention, respectively. 3, 5 and 7
FIG. 9 and FIG. 9 are various aberration diagrams of Numerical Examples 1, 2, 3, and 4 of the present invention.

Claims (4)

(57) [Claims] 1. A first lens group having a negative refracting power and moving along the optical axis during zooming, and a second lens group having a positive refracting power and moving along the optical axis during zooming in order from the object side. The first lens group has, in order, a negative lens having a negative refractive power and a third lens group having a negative refractive power and fixed during zooming. The first lens group has a strong concave surface on the image side. And 12 lenses having a meniscus shape having an aspherical surface and having a convex surface directed toward the object side, and the second lens group sequentially has 21 lenses having positive refractive power, 22 lenses having positive refractive power, and biconcave surface 23 lenses and 24 biconvex lenses, and the third lens group is a meniscus-shaped 31 lens with the concave surface facing the image side.
A zoom lens having a lens and satisfying the following conditions. (1) −0.65 <f1 / f _T <−0.45 (2) 0.43 <r2 / r3 <0.63 (3) r9 / r10 <−15 (4) n5> 1.8 However, r2; on the image side of the 11th lens group. Radius of curvature, r3; object-side radius of curvature of the 12 lens groups, r9; object-side radius of curvature of the 23 lens groups, r10; image-side radius of curvature of the 23 lens groups, f1; of the first lens group Focal length, f _T ; Focal length at telephoto end, n5: Refractive index of Franhofer d-line of the 23 lenses. 2. The zoom lens according to claim 1, wherein the conditional expression satisfies ν ₃ > 50 when the Abbe number of the 31 lenses is ν ₃ . 3. The zoom lens according to claim 1, wherein the 31 lens is an aspherical lens. 4. The zoom lens according to claim 1, wherein the total lens lengths at the wide-angle end and the telephoto end are substantially the same.