JP2556048B2 - Large aperture ratio 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 and integration rate of electronic parts have been increased, so that the volume and weight of a lens in a main body of a video camera or the like are relatively increased. In terms of cost, the lens system has become a bottleneck in reducing overall cost. In the case of current video cameras, etc.
Light weight and low cost are absolute requirements, and it is important to make the optical system small and inexpensive to achieve it. 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 achieve a wider range of telephoto, 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 in size. 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, when considering this as a whole, it cannot be said that the size, weight, and cost are all 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-61-261712, JP-A-62-21113 and the like can be mentioned as those having a variable magnification reduced to about 3 times to achieve compactness and cost reduction. However, the former has an 11-card configuration and the number has been reduced, but the open F-number is 2.8.
It is dark, and the latter is bright with an open F-number of 1.3, but both are still large and heavy as they have 12 sheets, so it is hard to say that cost reduction and compactness have been sufficiently achieved. 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 because the correction of chromatic aberration is insufficient.
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 JP-B-45-27849, etc., but this is also a field in which the open F number is very dark and bright lenses such as video cameras are required. Cannot be diverted to.

OBJECT OF THE INVENTION The present invention has a variable magnification of about 3 and an open F-number
Despite having a large diameter of about 1.0, we realized a zoom lens with a very short overall length and a small weight with a small number of components,
It is another object of the present invention to provide a large-aperture-ratio compact zoom lens that achieves high performance in the entire zoom range.

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 zooms 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.

With such a zoom configuration, the F number is 1.
In order to guide a light flux of about 0 to the third lens group while suppressing the occurrence of various aberrations, and further satisfy the cost reduction and the compactness, the first and second lens groups which are the variable power parts are set as follows. Configure.

That is, the first group is composed of two negative lens components, one of which is a cemented lens in which a positive lens and a negative lens are cemented together, and the second group is composed of two positive lens components, one of which is a negative lens. And a positive lens are bonded together.
By doing so, sufficient color correction can be performed in each group and various aberrations can be corrected, but sufficient compactness cannot be achieved unless the following conditions regarding the refractive power of both groups are satisfied.

0.6 <| _I | / _II <0.85 (where _I <0) 0.2 <| _I | f _W <0.4 where _I and _II are the refracting powers of the first and second lens groups, and f _W is the synthetic focus at the wide end. Indicates the distance.

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. Especially since it is a large aperture ratio, it is important to secure the peripheral light amount, and if it approaches the upper limit, the first group will be located at the tele end and closer to the object side at the wide end, and the peripheral light amount at the tele end will decrease. On the other hand, if it approaches the lower limit, it will be positioned closer to the image side than the wide end at the tele end, and the amount of peripheral light at the wide end will decrease. In both cases, the front lens diameter and the diameter of the second lens unit are increased to prevent this. The need arises, which deviates from the purpose of compactness.

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 the amount of various aberrations increases, and the performance cannot be guaranteed in a large aperture ratio lens with F of 1.0.

Furthermore, in order to make the fluctuations of various aberrations sufficiently small, it is desirable that the first lens group be arranged in the order of a negative lens having a strong refracting surface facing the image side from the object side and a cemented lens. It is desirable that the first group and the second group satisfy the following conditions.

(N _1P + N _2N ) / 2> 1.72 0.5 <f _2R / f _2P <1.1 where N _1P and N _2N are the positive lens in the cemented negative lens in the first group and the cemented positive lens in the second group, respectively. It is the refractive index of the negative lens in the lens. Also, f _2P and f _2R are ₂ in the second group.
F _2P on the object side and f _2R on the image side at each focal length of the two positive lens components
Indicated by

Each cemented lens in the first group and the second group is an important component from the viewpoint of color correction as well as from the viewpoint of correction of other various aberrations, and in particular, the positive lens in the first group and the second lens in the second group are The negative lens plays an important role, both of which are made of high-dispersion glass for color correction and good monochromatic aberration. Therefore, it is necessary that the negative lens be made of high refractive index glass so as to satisfy the formula.

Further, in the structure of the present invention, the Petzval sum tends to slightly shift to the positive side. Therefore, in order to suppress it, it is desirable that the refracting power of the object-side component in the second lens unit is not larger than that of the image-side component. . The formula shows the proper range, and if the upper limit is exceeded and the object-side component has a strong refractive power, the Petzval sum increases and the field curvature greatly occurs. Cannot be guaranteed. On the other hand, if the lower limit is exceeded and the refracting power is excessively concentrated on the image-side component, spherical aberration and coma aberration due to this increase, which also deteriorates the performance.

With the above, in particular, the specific configurations of the zoom units of the first and second groups are arranged, and a compact zoom unit with a small aberration variation can be obtained. Furthermore, as a relay system that follows 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 lens components, is composed of two groups of positive rear groups as a whole, and satisfies the following conditions.

_{0.2 <| A | f W <} 0.8 ( where _A <0) _{where, A} is the composite refractive power of the third front group.

Since the light flux emitted from the zoom unit is convergent light, a sufficient back focus cannot be secured unless the third lens unit is formed of the negative and positive reverse telephoto types. 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. The front lens group consists of a positive meniscus lens and a lens with a strong negative refractive power.By giving 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. There is also an advantage that the Petzval sum that 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, the spherical aberration will be significantly deteriorated and it will be difficult to increase the aperture.

Furthermore, it is desirable that the rear lens group of the third lens group be composed of two positive lenses so that the following two conditions are satisfied.

0.8 <r _A / f _W <1.5 0.35 <f _BR / f _BP <0.85 where r _A is the radius of curvature of the most object-side surface of the third group front group, and f _BP and f _BR are the third The focal lengths of the two positive lenses in the rear group are indicated by f _BP on the object side and f _BR on the image side.

The radius of curvature of the most object-side surface in the front group of the third lens group is a very important factor for correcting spherical aberration and ensuring the back focus, and if it is lower than the lower limit, the back focus tends to be insufficient, and conversely the upper limit is set. If it is made weaker than the above, spherical aberration and coma will be more likely to occur, and high performance cannot be guaranteed.

Further, the two positive lenses in the rear group play a role of favorably correcting off-axis aberrations in particular, but the refractive power of the object-side lens should not be made weaker than that of the image-side lens to the extent that the upper limit of the formula is not exceeded. Then, the back focus is likely to be insufficient. On the contrary, if the lower limit is exceeded and the refracting power is not concentrated on the image side lens, distortion and astigmatism will be excessively generated.

With the above-mentioned structure, the compact zoom lens with a large aperture ratio and a unique zoom ratio of about 3 times and an F-number of 1.0 is available at a low cost, even though it consists of only 10 elements in 8 groups. Can be achieved with.

Embodiments of the Invention Embodiments of a compact, lightweight, low cost, large aperture ratio compact zoom lens according to the present invention will be described below.

However, in each example, r _{1 to} r ₂₁ are radii of curvature, d _{1 to} d
₂₀ shows an axial _{distance, N 1 ~N 20, ν 1} ~ν 20 is a refractive index with respect to d-line, the Abbe number. In addition, f represents the focal length and F represents the F number.
The values at the tele end, middle end, and wide end are listed. Further, _{I 1} , _II , and _III represent the powers of the first, second, and third groups. In each of the embodiments, a flat plate corresponding to a low pass filter or a face plate is inserted at the end.

Power data _I -0.0292000 _II 0.0405000 _III 0.05330097 Variable distance d ₅ d ₁₀ Tele 1.900 26.325 Middle 15.773 8.641 Wide 31.861 1.500 Power data _I -0.0288000 _II 0.0405000 _III 0.0409349 Variable spacing d ₅ d ₁₀ Tele 1.500 23.714 Middle 16.865 7.748 Wide 34.684 1.300 Power data _I −0.0301000 _II 0.0400000 _III 0.0473587 Variable spacing d ₅ d ₁₀ Tele 1.200 29.517 Middle 13.801 9.560 Wide 28.416 1.500 Power data _I −0.0301000 _II 0.0400000 _III 0.0473666 Variable spacing d ₅ d ₁₀ Tele 1.100 29.863 Middle 13.592 9.731 Wide 28.080 1.601 Power data _I −0.0293000 _II 0.0397000 _III 0.0540194 Variable distance d ₅ d ₁₀ Tele 2.500 28.025 Middle 16.166 9.344 Wide 32.016 1.800 Power data _I −0.0290000 _II 0.0397000 _III 0.0578561 Variable spacing d ₅ d ₁₀ Tele 2.400 29.437 Middle 15.008 10.228 Wide 30.485 2.000 Power data _I −0.0296000 _II 0.0397000 _III 0.0584687 Variable spacing d ₅ d ₁₀ Tele 2.400 29.574 Middle 15.391 9.922 Wide 30.413 2.000 Next, FIGS. First, which is the mobile 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.

FIGS. 8 to 14 are aberration diagrams corresponding to Examples 1 to 7, 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, FIG. 2, FIG. 3, FIG. 4, FIG. 5, FIG. 6, and FIG. 7 are lens configuration diagrams corresponding to each embodiment of the present invention, and FIG. Figure 9, Figure 10, Figure 11, Figure 12,
13 and 14 are aberration charts thereof.

Claims (4)

(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 composed of two negative lens components, and one of the negative lens components is composed of a cemented lens in which a positive lens and a negative lens are cemented together, and the second group is composed of two positive lens components, one of the positive lens components. Is a large-aperture-ratio compact zoom lens that is composed of a cemented lens in which a negative lens and a positive lens are cemented together, and satisfies the following conditions. 0.6 <| _I | / _II <0.85 (where ₁ <0) 0.2 <| _I | f _w <0.4 where _I and _II are the refractive powers of the first and second groups,
f _w indicates the combined focal length at the wide end. 2. The first group is composed of a negative lens having a strong refracting surface directed toward the image side in order from the object side and a cemented negative lens, and the following conditions are satisfied: A large-aperture-ratio compact zoom lens according to item 1 of the range. (N _1P + N _2N ) / 2> 1.72 0.5 <f _2R / f _2P <1.1 where N _1P and N _2N are the positive lens in the negative lens and the positive lens in the second lens, respectively. It is the refractive index of the negative lens in the positive lens. Further, f _2R and f _2P are the focal lengths of the two positive lens components in the second lens unit,
f _2P and f _2R on the image side. 3. The third lens unit, which 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 lens components. A large-aperture-ratio compact zoom lens according to claim 2, characterized in that it is composed of a rear group having power and satisfies the following condition. _{0.2 <| A | f W <} 0.8 ( where _A <0) _{where, A} is the composite refractive power of the third front group. 4. The rear lens unit of the third lens unit is composed of two positive lenses, and is composed of ten lenses in eight groups as a whole, and the following conditions are satisfied: The large-aperture-ratio compact zoom lens described in item 3. 0.8 <r _A / f _W <1.5 0.35 <f _BR / f _BP <0.85 where r _A is the radius of curvature of the lens surface of the third group front group that is closest to the object side, and f _BP and f _BR are respectively The focal length of the two positive lenses in the rear group of the third group, f _BP for the object side lens and f _{BR for} the image side
Indicated by