JP2551026B2 - Zoom lens - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a zoom lens, and more particularly to a zoom lens applicable to a video camera, a small camera such as an electronic still camera, or the like.

TECHNICAL BACKGROUND OF THE INVENTION AND PRIOR ART In recent years, packaging of electronic parts has progressed and the integration rate has increased, so that the volume and weight of a lens in a main body of a video camera or the like have become relatively large. In terms of cost, the lens system has become a bottleneck in reducing overall cost. In the case of current video cameras and the like, small size, light weight, and low cost are absolute requirements, and in order to achieve this, it is important to make the optical system small and inexpensive. As a result of pursuing them, there are many cases where they gave up mounting a zoom lens and adopted a single focus lens. However, although it is possible to realize small size, light weight, and low cost, the single focus will drastically reduce the attractiveness of the product. Of course, it is possible to use a converter or attachment to make the lens telephoto or wide-angle, but it is necessary to carry it separately from the camera, and when considering the camera and these attachments together, it is compact, lightweight, and low-profile. It's hard to say that the cost has been realized. In recent years, there is a compact camera or the like in which a converter is incorporated in the camera body and the focal length is switched by a simple operation. However, considering this as a whole, it cannot be said that the size, weight, and cost are small, and it is difficult to achieve a change rate of the focal length of about 2 times or less. Also, in movie shooting, it is continuously switched during shooting. Since this is impossible, this too lacks commercial appeal.

So I can think of a zoom lens,
Since many of the conventional ones are aimed at high zoom ratios, they are large and costly. In addition, there are those aimed at small size, light weight, and low cost with low magnification, such as those disclosed in Japanese Patent Laid-Open No. 58-143311, but correction of various aberrations, especially chromatic aberration, is insufficient and it cannot be put to practical use. .

OBJECT OF THE INVENTION The present invention has a low zoom ratio, but has a very short overall length, a small number of components, and a large aperture capable of realizing small size, light weight, and low cost, and particularly when used in a wide angle of view range required in recent years. The objective is to provide a zoom lens with high performance.

SUMMARY OF THE INVENTION To achieve the above object, a zoom lens according to the present invention has, in order from the object side, a first group having a positive refractive power, a second group having a negative refractive power, and a positive refractive power. The third lens unit comprises a total of three lens units, and the second lens unit and the third lens unit move during zooming, in which case the second lens unit plays a role of varying the magnification, and the third lens unit holds the image plane position at a constant second position. It works to form an image of the divergent light flux after the group exit. By using the three-group configuration in this way, it becomes easy to realize compactness and deletion of the number of sheets. More specifically, the first group consists of a negative meniscus lens convex to the object side and a biconvex lens having a strong surface facing the object side, and the second group is a negative lens having a strong surface facing the image side. , A negative lens unit and a positive lens are cemented together to form a total of three negative lens units. With such a configuration, sufficiently good performance can be obtained even in a wide field angle range, and the aberration variation during zooming can be made sufficiently small. Further, the front lens diameter can be made sufficiently small, which is suitable for weight reduction. However, for that purpose, it is desirable to satisfy the following conditions.

0 <R _1P / | R _2R | <0.5 (however, R _2R <0) 0.27 < _I / | _II | <0.40 (however _II <0) 0.85 < _III f _M <1.7 Here, R indicates the radius of curvature, the first subscript thereof corresponds to the lens number counted from the object side, the second subscript indicates P when it is the object-side surface, and R when it is the image-side surface. _I , _II ,
_III indicates the refracting power of the first, second, and third groups (the reciprocal of the focal length). Also, f _T and f _W are the combined focal lengths of the entire system at the tele and wide ends, respectively.

The condition corresponds to a condition that gives the ratio of the object-side refracting power and the image-side refracting power when the entire first group is regarded as one positive lens, and greatly affects the aberration of the peripheral light. Especially at the tele end, for light outside the chief ray of the ambient light,
If the lower limit is exceeded, the upper frame will be generated, and if the upper limit is exceeded, the lower frame will be generated, and sufficient performance cannot be guaranteed.

The condition indicates the refractive power ratio of the first group and the second group,
If the value goes below the lower limit, it is advantageous for compactness, but the high-order aberrations that occur in the second lens group, which is strongly negative, are large, and the performance is greatly degraded. Also, if the upper limit is exceeded, the peripheral high volume will become insufficient, so it is necessary to make the front lens large in order to secure this, and it becomes impossible to satisfy the small size and light weight, and the Petzval sum becomes large and the field curvature becomes large. Getting worse.

The condition shows how much refracting power is given to the third lens unit, and when the value goes below the lower limit, the amount of movement during zooming increases, the dead space increases, and the size increases. On the contrary, if the upper limit is exceeded, sufficient back focus cannot be obtained.

By satisfying the conditions described above, it is possible to realize a compact and lightweight zoom unit that has a small aberration variation during zooming and has little residual aberration even when used in a wide image area. Further, by arranging the third unit following these zoom units according to one of the following two methods, it is possible to obtain an inexpensive zoom lens which has a sufficiently short overall length, a large aperture, and good aberrations. That is, as the first method, the third lens group is a positive lens having a strong refracting surface directed toward the object side, a biconcave lens, a positive lens having a strong refracting power directed toward the image side, and a strong refracting surface directed toward the object side. It consists of four positive lenses. With the positive, negative, positive and positive configuration, it is possible to secure a compact but sufficiently long back focus and to easily correct various aberrations.

Furthermore, by imposing the following conditions on the third group, very good performance is obtained.

1.0 <A ₁ / A ₂ 0.4 <| R _7P | / R _7R <1.2 (where R _7P <0) where A ₁ and A ₂ are between the sixth lens and the seventh lens, respectively.
It is the axial air gap between the seventh lens and the eighth lens.

These two expressions describe the arrangement and shape of the seventh lens, which is the only negative lens in the third group.

Distortion aberration and chromatic aberration of magnification largely change depending on the arrangement of the seventh lens. If the lower limit of the expression is exceeded, the distortion will be large and negative, and chromatic aberration of magnification will also become large and correction will not be possible.

The condition indicates the power distribution on the front and rear surfaces of the seventh lens of the negative lens. Since the seventh lens is the only negative lens in the third group, it is an important lens that plays a role in correcting various aberrations that occur in the other three positive lenses. Below the lower limit of this equation, If the power is excessively concentrated on the side surface of the object, a large amount of distortion will be generated and the curvature of field will be deteriorated. On the contrary, if you exceed the upper limit and concentrate the power on the image side,
The sagittal flare also causes excessive generation of high-order aberrations, resulting in performance deterioration, and it becomes impossible to secure a sufficient back focus.

Apart from this, the third group is constructed as follows (second method)
You can still achieve your goals. That is, the third lens group is composed of a positive lens with a strong refracting surface facing the image side, a positive lens with a strong refracting surface facing the object side, a biconcave lens with a strong refracting surface facing the image side, and a biconvex lens. And satisfy the following conditions.

_{0.3 <f 7 / f 6 <} 0.6 0.05 <R 8R / | R 8P | <0.5 ( where, R _8P <0) where, f _6, f ₇ sixth respectively, the focal length of the seventh lens.

The condition shows the distribution of the refractive powers of the two positive lenses preceding in the third lens unit. When the lower limit is exceeded and the refractive power of the sixth lens is weakened, the positive refractive powers of the seventh lens and the ninth lens are reduced. It becomes necessary to strengthen it more than necessary, and the spherical aberration deteriorates. On the other hand, if the value exceeds the upper limit, the light rays converge rapidly, so that a sufficient back focus cannot be obtained, and it cannot be put to practical use.

The condition relates to the refractive power balance between the front and rear surfaces of the biconcave eighth lens. If the refractive power of the rear surface is increased beyond the lower limit, higher-order coma aberration will occur, and if the upper limit is exceeded, the front surface will also be affected. If a large amount of refracting power is given, spherical aberration will be overcorrected and distortion will also be negatively generated.

If either of the two methods described above is selected to form the third group, the total number of lenses is as small as 9 lenses, but the size is large, the performance is high, the size is small, the weight is low, and the cost is low. It is possible to realize a zoom lens that satisfies

When the optical finder or the beam splitter for autofocus is inserted, it is preferable to insert it in the rear part of the second group.

Embodiments of the Present Invention Embodiments of a small-diameter, lightweight, low-cost, large-diameter, high-performance zoom lens according to the present invention will be described below.

However, in each example, r _{1 to} r ₂₀ are radii of curvature, and d _{1 to} d
₁₉ shows an axial _{distance, N 1 ~N 19, ν 1} ~ν 19 shows a refractive index at the d-line, the Abbe number. In each of the embodiments, a flat plate corresponding to a low pass filter or a face plate is inserted at the end.

<Example 1> <Example 2> <Example 3> Next, FIG. 1 shows a schematic configuration at the telephoto end of Embodiments 1 to 2, wherein R _{1P to} R _{7R and} A ₁ and A ₂ described above in the section of the outline of the present invention and the present invention are described. R ₁ to r ₂₀ , d _{1 to} d _{19 and the} like described in the section of the embodiment are also described. (3) shown in front of the third group (III) represents a diaphragm, and the flat plate (4) arranged behind the third group (III) is a flat plate corresponding to a low-pass filter or a face plate. Is.

FIG. 2 shows a schematic configuration of a lens corresponding to Example 3. FIGS. 3 to 5 are aberration diagrams of Examples 1 to 3 above, in which (a) shows the telephoto end, (b) shows the intermediate focal length, and (c) shows the various aberrations at the wide end. . Also, the solid line (d) represents the aberration for the d line, and the dotted line (SC) represents the sine condition. The dotted line (DM) and solid line (DS) represent astigmatism on the meridional surface and the sagittal surface, respectively.

[Brief description of drawings]

1 and 2 are lens configuration diagrams of the embodiments of the present invention, and FIGS. 3, 4, and 5 are aberration diagrams corresponding to the respective embodiments.

Claims (3)

(57) [Claims] 1. A first group having positive refractive power in order from the object side,
Second group with negative refractive power, third group with positive refractive power 3
Groups, the second and third groups move during zooming,
At this time, the second group plays a role of varying the magnification, and the third group is a zoom lens configured to image the divergent light flux after the second group exits while keeping the image plane position constant, It consists of a negative meniscus lens convex to the object side and a biconvex lens with a strong surface facing the object side. The second lens group consists of a negative lens with a strong surface facing the image side, and a negative lens and a positive lens. A zoom lens comprising a total of three negative lens groups in total and satisfying the following conditions. 0 <R _1P / | R _2R | <0.5 (however, R _2R <0) 0.27 < _I / | _II | <0.40 (however _II <0) 0.85 < _III f _M <1.7 Here, R indicates the radius of curvature, the first subscript indicates the lens number counted from the object side, the second subscript indicates P when the surface is on the object side, and R when the surface is on the image side. _I , _II , and _III represent the refractive powers of the first, second, and third groups. Also, f _T and f _W are the combined focal lengths of the entire system at the tele and wide ends, respectively. 2. The third lens group, in order from the object side, a positive lens with a strong refracting surface facing the object side, a biconcave lens, a positive lens with a strong refracting surface facing the image side, a strong refracting surface facing the object side. The zoom lens according to claim 1, wherein the zoom lens comprises four positive lenses and satisfies the following condition. 1.0 <A ₁ / A ₂ 0.4 <| R _7P | / R _7R <1.2 (where R _7P <0) where A ₁ and A ₂ are the axial air gaps before and after the only negative lens in the third lens group. , Between the sixth and seventh lenses is A ₁ , and between the seventh and eighth lenses is A ₂ . 3. A third lens group, in order from the object side, a positive lens whose strong refracting surface faces the image side, a positive lens whose strong refractive surface faces the object side, and a biconcave lens whose strong refractive surface faces the image side. And a biconvex lens, and the following condition is satisfied, The zoom lens according to claim 1. _{0.3 <f 7 / f 6 <} 0.6 0.05 <R 8R / | R 8P | <0.5 ( where R _8P <0) where f _6, f ₇ is the focal length of the sixth lens and the seventh lens respectively.