JP2629288B2 - Zoom lens with long back focus - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a zoom lens, and particularly has a very long back focus applicable to an electronic still camera and the like, a high peripheral illuminance ratio, and a full screen. And a high-performance zoom lens.

TECHNICAL BACKGROUND OF THE INVENTION AND PRIOR ART In electronic still cameras, there are several types of imaging systems, finder systems, and the like. Here, the finder system is a single-lens reflex camera, which is the mainstream in silver halide film photographic cameras. It is assumed that the method is used.
This single-lens reflex camera has many advantages, such as no parallax, the image captured by the viewfinder can be taken as it is, a powerful viewfinder screen can be obtained easily, and a bright viewfinder can be obtained. is there.

On the other hand, however, it is necessary to ensure a space between the lens and the image plane where the size of the reflection mirror without any vignetting enters, so that a very long back focus is required. Further, when compared with a single-lens reflex camera for a silver halide film, in an electronic still camera, it is necessary to insert several optical low-pass filters in front of an image sensor, so that a longer back focus is required. In consideration of the optical low-pass filter and the mirror for the viewfinder, as well as the extra space for retreating the optical low-pass filter, a back focus (in terms of air length) of about twice or more the diagonal length of the imaging surface is required. Table 1 shows these.

Now, to get such a long back focus,
It would be nice to simply shift the focal length range of the zoom lens to the long focal length, but that would limit the use of the telephoto zoom lens, so here the wide-angle focal length should have both angles of a standard lens. Is assumed.

As a zoom lens having such a long back focus, for example, JP-A-61-90119 and JP-A-61-156016
In the former, the focal length f =
13.2 to 38.4, 3x zoom with F _NO = 1.8, compatible with 2/3 inch and secures about 24mm (air length) back focus, the latter is f = 15.5 to 43.6, F _NO = 1.4 With a 3x zoom, both angles are slightly longer depending on the focal length, but it is compatible with 2/3 inches and has a back focus of about 22mm (air length). However, since the four-component zoom type is adopted, the former has a configuration of 14 images, and the latter has a configuration of 15 images. Also, its full length,
The diameter and weight are extremely large, and the functionality and portability of the main body camera are extremely impaired.

In electronic still cameras, imaging devices of about 2/3 to 1/2 inch are often used in recent years, so the diagonal length is about 1/4 to 1/5 for 35 mm film, so of course, The lenses must be compact to match.
Further, since it is expected that the cost of electronic components such as an image sensor and a deck will be further reduced in the future, it is naturally required to significantly reduce the cost of the lens. As described above, it is impossible to achieve compactness and cost reduction in each of the conventional examples.

Also, when trying to incorporate TTL-AF or TTL-AE to further enhance the function of the electronic still camera, not only the finder reflection mirror between the lens and the imaging surface,
Actually, it is necessary to secure a longer back focus than the degree shown in Table 1 because the mirrors for AF and AE must be provided.

Next, a possible example is a negative-positive two-component zoom lens for a 35 mm film camera. These are superior in terms of compactness and cost reduction as compared with the above-described four-component zoom type, and since the refractive power distribution is basically of a retrofocus type, the back focus can be easily lengthened. However, even with these zoom lenses, the back focus is completely insufficient with respect to Table 1. Also, in the ultra wide-angle zoom lens back focus becomes longer than the focal length range, F _NO is also dark objects are many, there is a performance also difficulties, number of constituent lenses is also large,
Too expensive to apply here.

Another requirement of the electronic still camera is high optical performance. It is well known that a still image generally requires better performance than a moving image, and this must be guaranteed for the entire screen from the center to the periphery of the screen. Therefore, higher performance is required than a general video camera zoom lens. Also, compared to a 35mm zoom lens, the smaller screen size requires more advanced aberration correction. Furthermore, for silver halide film,
An imaging element such as a CCD has a low sensitivity latitude, so that the influence of a decrease in the illuminance around the screen is conspicuous. Therefore, a high peripheral illuminance ratio is required. General zoom lenses are
In order to prevent the deterioration of the peripheral aberration, the peripheral flare light is removed by regulation by the effective diameter of the lens. Therefore, the peripheral illuminance ratio tends to be low, and this cannot be used for an electronic still camera.

Object of the present invention In view of the above points, the present invention has a very long back focus, is compact, and has a small number of lens components.
Aiming to provide a high-performance zoom lens that constitutes a zoom lens with a zoom ratio of about 2.5 times and a maximum aperture ratio of about F2.8 and that has a high degree of aberration correction over the entire screen and a sufficiently high peripheral illumination ratio. And

SUMMARY OF THE INVENTION The zoom lens according to the present invention imposes the following condition as a reference for determining the length of the back focus with respect to the focal length range.

BF> 1.2f _M Here, BF is the back focus (air length) in the minimum state during zooming, and f _T and f _W are the combined focal lengths of the entire system at the telephoto end and the wide end, respectively. In the above-described back focus criterion (see Table 1), since the factor of the focal length is not included, the minimum back focus is defined by an equation so as to be more realistic. If this expression is satisfied, an optical low-pass filter, a reflection mirror for viewfinder, and other AF and A
Even if a small mirror such as an E mirror enters, there is no mechanical interference. In the above-mentioned two conventional examples, the values on the right side of this equation are 27.0 and 31.2, respectively, and the back focus is much lower than both, and the back focus is not sufficiently long in both cases. You can see that.
Only the super wide-angle two-component zoom such as f = 23-35 / F3.5 disclosed in Japanese Patent Application Laid-Open No. 61-116315, which satisfies this equation, clearly shows how long the back focus is required.

Now, with a sufficiently long back focus,
To make it compact and simple, it consists of two groups: a front group with negative refractive power from the object side and a rear group with positive refractive power. It is desirable to configure by zooming. Further, regarding the refractive power distribution of the front and rear groups, it is necessary to satisfy the following expression.

0.7 <| _I | / _II <1.1 where _I <0, where _I and _II are the refractive power (reciprocal of the focal length) of the front group and the rear group, respectively.

When the value falls below the lower limit of this equation, the refractive power of the rear group becomes stronger with respect to the refractive power of the front group, in other words, the focal length of the rear group becomes shorter, which is necessary for securing a sufficient back focus. The target back focus cannot be achieved in a very disadvantageous direction. Conversely, if the upper limit is exceeded, although it is advantageous to secure the back focus, the power of the front group becomes too strong, or the distance between the front group and the rear group is large, and the higher Since the occurrence of aberration is large, the peripheral performance is particularly greatly deteriorated, and it is difficult to obtain the target high performance. In the latter case, wasteful space increases, and it is necessary to increase the diameter of the lens of the front group or the rear group in order to secure the amount of peripheral light. Decrease.

Now, assuming that the above conditions are satisfied,
Consider a specific configuration of the lens shape. Among them, the configuration of the rear group, which is very important for securing the back focus, will be described first. In the past, many configurations proposed as a rear group of a two-component zoom type have a complicated configuration of five or more, and it is difficult to achieve cost reduction with this configuration. Some have a four-sheet configuration, but for example, those disclosed in JP-A-59-142515 and JP-A-60-55311 have a positive, positive, negative, and positive configuration, and are basically Since the positive refractive power is concentrated on the object side of the rear group, it is disadvantageous for securing the back focus.

In order to make the back focus sufficiently long, it is desirable to reduce the positive refractive power on the object side of the rear group as much as possible. Therefore, in the present invention, the rear group is composed of two positive components,
The object-side component includes at least one positive lens and one negative lens, and the image-side component is arranged with a certain space therebetween, and includes at least one positive lens and one negative lens. The following conditions are imposed on the refractive power of the quantity component.

0.5 < _P / _R <0.93 where _P is the refractive power of the object-side component in the rear group, and _R is the refractive power of the image-side component. In this way, the divergent light beam emitted from the front group is refracted almost afocal or slightly convergent by the object side component with weaker positive refractive power, and the final image is formed by the rear group with stronger positive refractive power. Let it. With this configuration, a long back focus longer than the focal length of the entire rear unit or longer can be easily obtained. This is because, although the focal length of the image side component of the rear group is relatively long, it can be much longer than the focal length of the entire rear group.

By the way, if the refractive power is below the lower limit of the expression and the refracting power is concentrated too much on the image side, it is advantageous for securing the back focus, but since the aberration generated in the image side component becomes large, the image side component is reduced. A complicated configuration is required, which leads to an increase in cost. Conversely, if you exceed the upper limit and increase the refractive power on the object side,
A desired back focus cannot be obtained.

As described above, the rear group is composed of the object-side component and the image-side component,
By satisfying the condition of the expression, a sufficient back focus can be obtained, but a specific configuration of the rear unit will be described in consideration of aberration correction.

That is, the object-side components in order from the object side to the rear group and the positive lens L _4,
Composed of two negative lens L ₅ having a strong surface at the object side, it is to construct by two negative lens L ₆ and the positive lens L ₇ having a strong surface the image side component on the image side.

Aberration correction, why such an arrangement is desired, the spherical aberration on the axis in the negative lens L ₅ of the object-side components, chromatic aberration, coma aberration outside the axis is at a negative lens L ₆ of the image-side components, the shaft This is because external lateral chromatic aberration, curvature of field, and distortion can be satisfactorily and efficiently corrected. By adopting such a configuration, even if the off-axis light beam passes with a sufficient light beam width (to ensure the peripheral illuminance ratio),
There is almost no performance degradation. Further, it is preferable to satisfy the following conditions from the viewpoint of aberration correction.

1.0 <| _5P | / _II <1.6 0.5 _<6/5 <1.0 (where _{5, 6} <0) where, _5P is a negative lens which is the second lens in the rear group
The refractive power of the object side surface of the L _5, n _5, R _5P is the refractive index and the object-side curvature radius of the d-line of L _{5. 5, 6} are each refractive power of the negative lens L ₆ followed by the L _5.

Expression, spherical aberration, but prescribes a negative lens L ₅ having a shape at the center of the correction of such coma and weaken the refractive power of the object side surface than the lower, spherical aberration is insufficiently corrected, reverse When the upper limit is increased beyond the upper limit, not only the spherical aberration is overcorrected, but also coma aberration is increased, and higher-order aberrations are more likely to occur in the peripheral light beam.

The formula is the two negative lenses that are the center of the aberration correction in the rear group.
But shows the power balance of L ₅ and L _6, when to have a strong refractive power to L ₅ than the lower, peripheral curvature of field and distortion, and correction of lateral chromatic aberration becomes difficult, peripheral The performance is significantly deteriorated. Conversely, when exceeding an upper limit to have a strong refractive power in L _6, but preferred to improve the performance of the peripheral, spherical aberration tends to become over-corrected on axis, in particular degradation when the diameter of the axial chromatic aberration is raised It is large and the contrast tends to decrease on the axis.

Subsequently, the configuration of the front group will be described. The configuration of the front group mainly affects the securing of the back focus, the spherical aberration at the telephoto end, and the off-axis aberration at the wide end. In combination with the rear group described above, the front group is sequentially arranged from the object side to the negative meniscus lens L ₁ convex to the object side, the negative lens L ₂ having the strong surface facing the image side, and the positive lens having the strong surface facing the object side. it is desirable to configure three lenses L _3.

By arranging negative, positive, and negative, the principal point in the front group can be brought closer to the object side, so that the back focus can be easily lengthened. Further, by both negative lens and such a shape, the aberration occurrence amount by a negative lens as small as possible, which is a configuration that is corrected by strong positive lens L ₃ surface.
As a result, a front group having a sufficiently small amount of aberration from the on-axis to the off-axis is obtained, though it has a three-element configuration.

With the above-described configuration, a zoom lens having a very long back focus can be realized with a simple configuration of only seven lenses, and its compactness, high performance, and high peripheral illuminance ratio have been conventionally achieved. It has been achieved several levels higher than those proposed.

Embodiments of the Present Invention Hereinafter, embodiments of a high performance zoom lens having a long back focus according to the present invention will be described. However, in each embodiment, r _{1 to} r ₁₇ indicate the radius of curvature, d _{1 to} d ₁₆ indicate the distance between the shaft upper surfaces, and N 1
_{1 to} N ₁₆ and ν _{1 to} ν ₁₈ indicate the refractive index and Abbe number at d-line.

The aberration is corrected with a flat plate corresponding to a low-pass filter or the like inserted at the end.

<Example 1> <Example 2> Next, FIG. 1 shows a schematic configuration at the telephoto end of the first and second embodiments. Among the moving groups, the front group (I) and the rear group (II) are shown in FIG. The movement from T) to the wide end (W) is schematically shown by arrow lines (1) and (2). (3) shown in front of the rear group (II) represents a stop, and a flat plate (4) arranged behind the rear group (II) is a flat plate corresponding to a low-pass filter or a face plate. . Incidentally, (15) indicates an image plane.

2 and 3 are aberration diagrams corresponding to the first and second embodiments, respectively, wherein (a) shows the telephoto end, (b) shows the intermediate focal length, and (c) shows various aberrations at the wide end. . 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 the drawings]

FIG. 1 is a lens configuration diagram for each embodiment of the present invention, and FIGS. 2 and 3 are aberration diagrams thereof.

Claims (4)

(57) [Claims] 1. A zoom lens comprising, in order from the object side, a front group having a negative refractive power and a rear group having a positive refractive power, and changing the distance between both groups to change the magnification. A zoom lens having a long back focus, comprising two positive lens components each including two positive lenses including one positive lens and one negative lens, and satisfying the following conditions. _{0.7 <| I | / II <} 1.1 ( _{although, I <0) 0.5 <P} / R <0.93 where, _{I, II} is the refractive power of the front and rear groups respectively (reciprocal of focal length), _{P, R} indicates the refractive power of the object-side component and the image-side component of the two positive lens components in the rear group, respectively. 2. A method rear group in order from the object side, a negative lens facing the positive lens L ₄ and the strong surface at the object side component and an image side consisting of two negative lens L ₅ having a strong surface at the object side L ₆ and the positive lens long back focus zoom lens according to the first claim, characterized in that it has a total of four image-side component consisting of two L _7. 3. The zoom lens having a long back focus according to claim 2, wherein the following condition is satisfied. 1.0 <| _5P | / _II <1.6 0.5 _<6/5 <1.0 (where _{5, 6} <0) where, _5P is negative lens L ₅ which is the second lens in the rear group
N ₅ and R _5P are the d-line refractive index of L ₅ and the object-side radius of curvature. _{5, 6} are each refractive power of the negative lens L ₆ followed by the L _5. The method according to claim 4 front unit in order from the object side, a negative meniscus lens L ₁ convex to the object side, a positive lens L ₃ having a strong surface in the negative lens L ₂ and the object side having a strong surface on the image side 4. The zoom lens having a long back focus according to claim 3, wherein the zoom lens includes three lenses.