JP2556046B2 - Compact zoom lens - Google Patents

Description

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

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. Among them, those disclosed in JP-A-62-261712, JP-A-62-21113 and the like can be mentioned as those in which the zoom ratio is reduced to about 3 times to achieve compactness and cost reduction. However, although the former is an 11-sheet configuration and the number of sheets has been reduced, the F-number is dark at 2.8, and the latter is a bright F-number at 1.3, but it is a 12-sheet configuration, both of which are still large and heavy, and cost reduction and compactness. It is hard to say that this has been achieved sufficiently. Further, there is an example disclosed in Japanese Patent Laid-Open No. 58-143311 as an example in which the zoom ratio is reduced to about 2 times and the cost and size are extremely reduced. However, this is insufficient in correction of chromatic aberration, It cannot be put to practical use in terms of performance. Also, from the viewpoint of the chromatic aberration, it is impossible to increase the zoom ratio beyond that. On the other hand, it is intended for lenses for silver salt films such as single-lens reflex cameras, and there is Japanese Examined Patent Publication No. 45-27849. However, this is also very useful in fields where bright F-numbers are required, such as video cameras. Cannot be diverted.

OBJECT OF THE INVENTION The present invention realizes a zoom lens having a zoom ratio of about 2 to 3 times, a large aperture, a very short overall length and a small weight with a small number of constituent elements, and further has high performance in the entire zoom range. It is an object of the present invention to provide an obtained compact zoom lens.

SUMMARY OF THE INVENTION In order to achieve the above object, a zoom lens according to the present invention comprises, in order from the object side, a first group having a negative refractive power, a second group having a positive refractive power, and a second group having a positive refractive power. It consists of 3 groups in total, and during zooming, the 1st group and the 2nd group move, in which case the 1st group keeps the image plane position constant, the 2nd group plays a role of varying the magnification, and the 3rd group. Is fixed and controls image formation.

In this way, by adopting a negative and positive configuration, the weight of the front lens, which is the biggest drawback of a zoom lens with a positive refractive power (1/2 to 2 of the total lens weight in a zoom lens for a video camera)
Occupy about / 3) can be extremely light. That is, when securing a sufficient amount of ambient light, the ambient light that is incident at a strong tilt angle
The diameter of the front lens can be significantly reduced because it quickly loosens due to the negative refracting power of the group. In addition to the significant reduction in weight due to the smaller diameter, it also gives a large margin to the design of the main camera. However, conversely, the drawbacks of zoom lenses with negative refractive power are that they are not suitable for large zoom ratios and that the amount of movement of the second lens group increases a little, but if the zoom ratio is about 3 there is no problem.

In such a configuration, if the refractive powers of the first and second movable groups, which are movable groups, are set within a range that satisfies the following conditions, the above-described lightweight zoom lens can be obtained.

0.48 <| _I | / _II <0.83 (where _I <0) 0.20 <| _I | f _w <0.34 where _I and _II are the refractive powers of the first and second lens units, f _w
Is the combined focal length of the entire system at the wide end.

The formula shows the refractive power ratio of both moving groups, and the moving locus of both groups, especially the first group, changes depending on this ratio. If the upper limit is approached, the first lens unit will be located at the tele end and closer to the object side at the wide end, and the amount of peripheral light at the tele end will be reduced. And the peripheral light amount at the wide end is reduced.
It is necessary to increase the diameter of the group, which deviates from the purpose of compactification.

The formula below shows the appropriate range of the refractive power of the first lens group. If the lower limit is not reached and the lens is weakened, the amount of movement of both lens groups during zooming will increase and the front lens diameter will also increase, resulting in a compact size. Against. On the contrary, if the refractive power is increased beyond the upper limit, it is advantageous for compactness, but when thickening and correcting aberrations, the amount of various aberrations increases, and it becomes impossible to realize an inexpensive configuration.

By providing such a refracting power and further satisfying the following two expressions, even if the first lens unit is composed of one cemented lens including one negative lens and one positive lens, a zoom lens with good peripheral aberration can be obtained. can get.

-0.7 <R _2R / R _1P <0.1 Δν> 18 where R is the radius of curvature, the first subscript of which is the lens number counted from the object side, and the second subscript is P for the object-side surface and the image side. In the case of the plane, R is to be shown. Further, Δν is a difference in Abbe number between the negative lens and the positive lens of the first group.

The condition shows the distribution of the refractive power to both surfaces of the cemented lens, but it is preferable that the image-side surface R _2R has a large refractive power in terms of aberration correction. If the object side surface R _1P is also given a strong negative refractive power beyond the lower limit of the equation, negative distortion and coma will occur greatly, and it will be impossible to correct them after the second lens group. Absent. On the other hand, if the concentration exceeds the upper limit and concentrates on the image side surface, high-order aberrations are largely generated, and spherical aberration, coma aberration, field curvature, etc., particularly at the telephoto end, are deteriorated.

The condition corresponds to the difference in dispersion between the two lenses so that the first group alone can perform sufficient color correction. If this condition is not satisfied, chromatic aberration variation due to zooming is very large. That is, since the color correction is insufficient or the paraxial color correction condition is satisfied, a very strong cemented surface is required, and therefore high-order chromatic aberration occurs.

A simple and compact zoom lens can be provided by satisfying the above-mentioned conditions, but by further constructing the second group with the following two positive lens components,
Even with a large aperture, it is easy to realize a zoom unit with small aberration variation. That is, it is preferable that one of the positive lens components is a positive lens component in which a negative lens and a positive lens are cemented together, and the other is a positive single lens, which satisfies the following conditions.

0.1 <F _P / F _N <0.7 ν _N <30 where F _P and F _N are the focal lengths of the two positive lens components of the second lens group, the single lens is F _P , and the cemented lens is F _N. Is. ν _N is the Abbe number of the negative lens in the cemented lens.

The negative refractive power preceding type has a higher height at which the axial zonal light flux passes through the second group than the positive refractive power preceding type, so it is decomposed into two positive refractive powers in order to satisfactorily correct spherical aberration. Preferably. Furthermore, in order to carry out color correction by the second group alone, one of them should be a cemented negative lens with high dispersion. At this time, it is desirable to allocate a large amount of the refractive power to the single lens side of the two positive lens components, and if the cemented lens has a strong positive refractive power exceeding the upper limit of the expression, the color correction will be facilitated. In addition, the curvature of the bonding surface becomes very strong, which causes a large amount of high-order aberrations.
On the other hand, if the lower limit is not reached and the refracting power is concentrated on the single lens side, spherical aberration will be greatly negative and it will be difficult to increase the aperture.

The condition is that the second group alone can perform sufficient color correction.
This indicates that a sufficiently high-dispersion glass must be used for the negative lens, and if this is not satisfied, the cemented surface must have a strong curvature more than necessary, and high-order chromatic aberration occurs.

With the above, in particular, the specific configurations of the zoom units of the first and second groups are completed, and a compact zoom unit with a small aberration variation can be obtained. Further, as a relay system following this, the third unit is as follows. With the configuration, a large-diameter and compact zoom lens can be easily realized.

That is, the third group is composed of a positive meniscus lens and a negative lens in order from the object side and has a negative refractive power as a whole;
It is composed of one or two positive lenses and is composed of two groups of a positive rear group as a whole, and the following conditions are satisfied.

0.1 <| _A | f _w <0.7 (where _A <0) 0.6 <r _A / f _w <1.3 where _A is the composite refractive power of the front group of the third group, and r _A is the most object side of the third group. Is the radius of curvature of the lens surface.

Since the light flux emitted from the zoom unit is convergent light, a sufficient back focus cannot be ensured unless the third lens unit is constructed of a negative and positive telephoto type. On the contrary, in order to be able to satisfactorily correct spherical aberration even with a large aperture, it is desirable that the front surface of the third lens unit be convex. By constructing the front group with a positive meniscus lens and a lens with a strong negative refractive power, and having a negative refractive power as a whole, not only good spherical aberration and sufficient back focus can be obtained, but also a large positive refractive power. The advantage is that the Petzval sum, which tends to shift to, can be reduced. If the lower limit of the condition is not reached, insufficient back focus will be a problem, and if the upper limit is exceeded, spherical aberration will be significantly deteriorated and it will be difficult to increase the aperture.

If the lower limit of the equation is not reached, the spherical aberration will be easily corrected, but the back focus will be insufficient. On the other hand, if the upper limit is exceeded, spherical aberration and coma of peripheral light will deteriorate.

If the above-mentioned conditions are satisfied, a zoom lens with a large aperture and high performance can be obtained, which is extremely lightweight, compact, and has a simple structure as compared with conventional ones.

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 embodiment, r _{1 to} r ₂₁ are radius of curvature, d ₁
˜d ₂₀ indicates the distance between the upper surfaces of the shafts, and N ₁ ˜N ₂₀ and ν ₁ ˜ν ₂₀ are d.
The refractive index and Abbe number for a line are shown. In each of the embodiments, a flat plate corresponding to a low pass filter or a face plate is inserted at the end. Further, there is also one in which a flat plate corresponding to an optical finder or a beam splitter for autofocus is inserted between the second group and the third group.

Next, FIGS. 1 to 10 show a schematic structure at the telephoto end of the first to tenth embodiments, of which the first group which is a moving group.
For the group (I) and the second group (II), the movement from the tele end (T) to the wide end (W) is indicated by the arrow line (1) in FIG.
It is schematically shown by (2). (3) shown in front of the third group (III) represents a diaphragm, and
The flat plate (4) arranged behind the group (III) is a flat plate corresponding to a low-pass filter or a face plate. Further, the flat plate arranged between the second group (II) and the third group (III) in FIGS. 6 to 10 is a flat plate as a beam splitter for autofocus.

FIGS. 11 to 20 are aberration diagrams corresponding to Examples 1 to 10, in which (a) is a telephoto end, (b) is an intermediate focal length, and FIG.
(C) represents various aberrations at the wide end. Further, the solid line (d) represents the aberration with respect to the d line, and the dotted line (SC) represents the sine condition. Further, a dotted line (DM) and a solid line (DS) represent astigmatism on the meridional surface and the sagittal surface, respectively.

[Brief description of drawings]

1, 2, 3, 4, 5, 5, 6, 7, 8, 9 and 10 are lenses corresponding to the respective embodiments of the present invention. It is a block diagram, FIG. 11, FIG. 12, 13
FIG. 14, FIG. 15, FIG. 15, FIG. 16, FIG. 17, FIG. 17, FIG. 18, FIG. 19 and FIG. 20 are aberration diagrams thereof.

Claims (3)

(57) [Claims] 1. A first lens element having a negative refractive power in order from the object side.
In the zoom lens in which the first lens group and the second lens group move on the optical axis during zooming, a total of three lens groups, a second lens group having a positive refractive power and a third lens group having a positive refractive power, are included. The first group is a zoom lens characterized by comprising one negative lens component obtained by cementing a negative lens and a positive lens, and satisfying the following conditions. 0.48 <| _I | / _II <0.83 (where _I <0) 0.20 <| _I | f _w <0.34 −0.7 <R _2R / R _1P <0.1 Δν> 18 where _I and _II are the first group and the second group. Shows the refractive power of the group,
f _w denotes the composite focal length at the wide end. R indicates the radius of curvature, the first subscript of which is the lens number counted from the object side,
The second subscript indicates P for the object side surface and R for the image side surface. Δν is the difference between the Abbe number of the negative lens and the Abbe number of the positive lens of the first group. 2. The second group is composed of two positive lens components, one of which is a positive lens component obtained by bonding a negative lens and a positive lens, and the other is a single lens, which satisfies the following conditions. The zoom lens according to claim 1, wherein the zoom lens is a zoom lens. 0.1 <F _P / F _N <0.7 ν _N <30 where F _P and F _N are the focal lengths of the two positive lens components of the second lens group, the positive single lens is F _P , and the cemented lens is This is indicated by F _N. Further, ν _N is the Abbe number of the negative lens in the cemented lens of the second group. 3. The third lens unit comprises, in order from the object side, a front group consisting of a positive meniscus lens and a negative lens and having a negative refracting power as a whole, and one or two positive lenses. The zoom lens according to claim 2, characterized in that the zoom lens is composed of two rear groups having power and satisfies the following condition. 0.1 <| _A | f _w <0.7 (where _A <0) 0.6 <r _A / f _w <1.3 where _A is the composite refractive power of the front lens group of the third lens group, and r _A is the most object side of the third lens group. Is the radius of curvature of the lens surface.