JPH0247616A - Zoom lens with long back focus - Google Patents

Abstract

PURPOSE:To obtain the high-performance zoom lens which has its aberrations compensated at high level and a sufficiently large marginal illuminance rate by composing a rear group of two positive lens elements each including one positive lens and one negative lens, and satisfying specific conditions. CONSTITUTION:The zoom lens consists of two groups, i.e. a front group with negative refracting power and the rear group with positive refracting power in order from the object side, and the gap between both groups is varied to vary the power. Then the inequalities I and II hold as to the refracting power distribution of the front and rear groups. In the inequality I, psi1 and psi2 are the refracting power values (reciprocal of focal length) of the front and rear groups and in the inequality II, psiP and psiR are the refracting power values of the object-side element and image-side element of the two positive lens elements in the rear group. Further, the rear group consists of the two lens elements each including one positive lens and one negative lens. Consequently, the zoom lens which has the long back focus, high marginal illuminance rate, and excellent performance over the entire image plane is obtained.

Description

[Detailed Description of the Invention] The present invention relates to a zoom lens, which has a very long back focus that can be applied to electronic still cameras, etc., has a high peripheral illuminance ratio, and This invention relates to a zoom lens that has good performance across the entire screen.

For electronic still cameras, the imaging method.

There are several types of one-sided finders, etc.
Here, the single-lens reflex viewfinder type is assumed to be used, which is the mainstream in silver halide film photography cameras. This eye-reflex system has many advantages, such as there is no variation, it is possible to capture the image as seen in the viewfinder, it is easy to modify the viewfinder, it is easy to create an impressive viewfinder screen, and it can provide a bright viewfinder. be.

However, on the other hand, it is necessary to secure space between the lens and the image plane for a large reflective mirror without vignetting, which requires an extremely long back focus. Furthermore, when compared with an eye reflex camera for silver halide film, an electronic still camera requires several optical low-pass filters to be installed in front of the image pickup device, and therefore requires a longer back focus force. Considering these optical low-pass filters, finder mirrors, and enough space to move them, a back focus (air equivalent) of approximately twice the diagonal length of the imaging surface is required. Table 1 shows these.

<Table 1> Now, in order to obtain a long back focus like this, you can simply shift the focal length range of the zoom lens to a long focal length, but that would be limited to telephoto zoom lenses, so we will discuss it here. Let's assume that the focal length at the wide end has an angle of view comparable to that of a standard lens.

Examples of zoom lenses with such a long back focus include JP-A-61-90119 and JP-A-61-90119.
156016, the former has a focal length f = 13.2 to 38.4. FNO=1.
It is a 3x zoom of the 8, and has a bank focus of about 24 mm (air length) with inch support, and the latter is f
=15.5~43.6. With the 3x zoom of F, O-1, and 4, the angle of view is a little longer than the focal point, and a back focus of about 22 meters (air length) is obtained, corresponding to a distance of 1 inch.

However, since these employ a four-component zoom type, the former has a 14-element structure, and the latter has a 15-element structure, resulting in an extremely large number of lenses, leading to an increase in cost.

Furthermore, the overall length and weight per diameter are also extremely large, severely impairing the functionality and portability of the camera body.

In recent years, electronic still cameras have often used image sensors of about 1 to η inches, so the diagonal length is about A to 3 compared to that for 35m film, so naturally the lens must be compact to match. There must be. Furthermore, it is expected that the cost of electronic components such as image pickup devices and decks will be drastically reduced in the future, so it is natural that the cost of lenses will also be required to be significantly reduced. In this way, it is impossible to achieve compactness and cost reduction in each of the above-mentioned conventional examples.

In addition, when trying to incorporate TTL-AF or TTL-AE in order to further improve the functionality of an electronic still camera, it is necessary to install not only a reflective mirror for the viewfinder but also mirrors for AF and AE between the lens and the imaging surface. In reality, it is necessary to ensure a longer back focus than that shown in Table 1.

Next, a possible example is a negative-positive two-component zoom lens for a thirty-five film camera.

These lenses are superior in terms of compactness and cost reduction compared to the four-component zoom type described above, and basically have a retrofocus type refractive power distribution, making it easy to lengthen the back focus.

However, even with these zoom lenses, the back focus is completely insufficient compared to Table 1. Additionally, ultra-wide-angle zoom lenses that have a long back focus relative to the focal length range often have a dark FNo, have performance problems, and require a large number of lenses, making them too expensive to be used here.

Another requirement for electronic still cameras is high optical performance. It is well known that still images generally require better performance than videos, and this must be guaranteed across the entire screen from the center to the periphery. is required. Furthermore, even compared to a 35m+s zoom lens, the smaller screen size requires more advanced aberration correction. Furthermore, compared to a silver halide film, an image sensor such as a COD has a lower latitude, so the influence of a decrease in illuminance around the screen becomes noticeable, so a high peripheral illuminance ratio is required.

In general zoom lenses, in order to prevent deterioration of peripheral aberrations, peripheral flare light is removed by regulating the effective diameter of the lens, so the peripheral illuminance ratio tends to be low, which is not suitable for electronic still cameras. I can not use it.

In view of the above points, the present invention has a very long back focus, is compact, has a small number of lens elements, has a variable power ratio of about 2.5x, and has a maximum aperture ratio of about F2.8. It is an object of the present invention to provide a high-performance zoom lens in which aberrations are highly corrected over the entire screen of the lens, and the peripheral illuminance ratio is sufficiently high.

Oh λkun Bhii! In the zoom lens according to the present invention, the following condition is imposed as a standard for determining the length of the back focus with respect to the focal length range.

■B F >1.2fM However, fH=Jft-
fw Here, BF is the back focus (air length) in the minimum state during zooming, and fT and fw are the combined focal lengths of the entire system at the tele end and wide end, respectively.

Since the above-mentioned back focus standard (see Table 1) does not include the focal length factor, the minimum bank focus is defined by formula (2) here to be more in line with reality. If this formula (■) is satisfied, even if a small mirror such as an optical low-pass filter, finder reflection mirror, or other AFAE mirror is inserted between the lens and the imaging surface, there will be no mechanical interference. In the conventional example, the values on the right side of this equation are 27.0.31°2
The back focus of both is much lower than this, and it can be seen that the back focus of both cannot be said to be sufficiently long. An example that satisfies this equation is f = 23 disclosed in Japanese Patent Application Laid-open No. 116315/1983.
It is only an ultra-wide-angle two-component zoom such as -35/F3.5, which clearly shows how long a back focus is required.

Now, in order to have a sufficiently long back focus and to have a compact and simple configuration, it is necessary to have two groups, a front group with negative refractive power and a rear group with positive refractive power from the object side. It is desirable to construct the lens with a two-format zoom that changes magnification by changing the distance between the groups, and furthermore, regarding the distribution of refractive power between the front and rear groups, it is necessary to satisfy the following formula: ■0.7<19'; l / steam < 1.1
however,? , <0 Here, 9'i 9y are the refractive powers (reciprocals of the focal length) of the front group and the rear group, respectively.

If the lower limit of formula 0 is exceeded, the refractive power of the rear group will be stronger than the refractive power of the front group, or in other words, the focal length of the rear group will become shorter, which means that sufficient back focus cannot be secured. The target back focus cannot be achieved in a very unfavorable direction. On the other hand, if the upper limit is exceeded, although it is advantageous for securing back focus, the power of the front group will become too strong, or the distance between the front and rear groups will be widened; in the former case, high-order Since a large amount of aberration occurs, the peripheral performance in particular deteriorates significantly, making it difficult to achieve the targeted high performance. In addition, in the latter case, not only is there an increase in wasted space, it is also necessary to make the lens diameter of the front group or rear group extra large in order to secure the amount of peripheral light, which also increases the tendency for flare to occur, resulting in poor peripheral performance. decreases.

Now, on the premise that the above-mentioned conditions are satisfied, we will consider specifically configuring the lens shape. Among these, we will first explain the configuration of the rear group, which is extremely important for ensuring back focus. In the past, many of the configurations that have been proposed for the rear group of 2-element zoom types have complex configurations of 5 or more elements, making it difficult to achieve cost reductions. However, for example, the ones disclosed in JP-A-59-142515 and JP-A-60-55311 have a positive, negative, and positive configuration.
Basically, positive refractive power is concentrated on the object side of the rear group, so
This is disadvantageous for securing back focus.

In order to make the bank focus sufficiently long, it is desirable to weaken 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, and the object side component is at least It includes one positive lens and one negative lens, and the image side component is placed with a certain amount of space between them, and it includes at least one positive lens and one negative lens, and the refractive power of both components meets the following conditions. shall be imposed.

■0.5<ψ, /ψ<0.93 Here, ψ is the refractive power of the object side component in the rear group, and ψ1 is the refractive power of the image side component. In this way, the object-side component with weak positive refractive power refracts the divergent light beam emitted from the front group almost afocal or slightly convergent, and the rear group with strong positive refractive power forms the final image. let With this configuration, a long bank focus that is equal to or longer than the focal length of the entire rear group can be easily obtained. This is because although the focal length of the image side component of the rear group is strong, it can be made much longer than the focal length of the entire rear group.

Now, if the lower limit of formula 0 is exceeded and the refractive power is concentrated too much on the image side, it is advantageous for securing back focus, but the aberration generated in the image side component becomes large, so the image side component It becomes necessary to configure the system in a complicated manner, leading to an increase in costs. On the other hand, if the refractive power on the object side is strengthened by exceeding the upper limit, the desired back focus cannot be obtained.

As described above, the rear group is composed of an object side component and an image side component, and 0
Sufficient back focus can be obtained by satisfying the conditions of the formula, but a specific configuration of the rear group will be described with further consideration to aberration correction.

That is, in order from the object side in the rear group, the object side component is a positive lens L4,
It consists of two lenses, a negative lens with a strong surface facing the object side, and a positive lens L, and a negative lens L with a strong surface facing the image side, and an image side component.

The reason why such an arrangement is desirable for correcting aberrations is that the negative lens L, which is the object side component, corrects on-axis spherical monochromatic aberration and off-axis comatic aberration, and the negative lens L6, which has the image side component, corrects the off-axis spherical aberration and monochromatic aberration. This is because chromatic aberration of magnification, curvature of field, and distortion can be corrected well and efficiently. By adopting such a configuration, there is almost no performance deterioration even if the off-axis beam passes through with a sufficient beam width (to ensure the peripheral illuminance ratio).Furthermore, the following conditions must be satisfied in order to correct aberrations. ,preferable.

■1.0< lψSr + / 9', <1.6 [However, f,, =; 2sp” O] ■0.5<ψ, /ψs
<1.0 (However, ψ5.9'&<0) Here, ψ
3. is the object side refractive power of the second negative lens in the rear group, and n Sr R5P is the d-line refractive index and object side curvature radius of n Sr R5P. ψ3. The ψ6 beam, followed by a negative lens, has the refractive power of each.

Formula (2) regulates the shape of the negative lens L, which is the center of correction for spherical aberration, coma, etc., but if the refractive power of the object side is weakened beyond the lower limit, spherical aberration will be insufficiently corrected, On the other hand, if it is strengthened beyond the upper limit, not only will spherical aberration be over-corrected (comatic aberration will occur significantly), but also high-order aberrations will likely occur in peripheral light beams.

Equation 0 shows the refractive balance of the two negative lenses, which are the center of aberration correction in the rear group. It becomes difficult to correct field curvature and distortion as well as lateral chromatic aberration, resulting in significant deterioration of peripheral performance. On the other hand, if the upper limit is exceeded and the lens has strong refractive power, it is good for improving peripheral performance, but axial spherical aberration tends to be overcorrected, and axial chromatic aberration in particular deteriorates as the aperture increases. If it is large, the contrast tends to decrease on the axis.

Next, the configuration of the front group will be explained. The configuration of the front group mainly affects securing back focus, spherical aberration at the telephoto end, and off-axis aberration at the wide end.

In combination with the rear group described above, the front group is made up of, in order from the object side, a negative meniscus lens LI convex toward the object side, a negative lens L construction with a strong surface facing the image side, and a positive lens L3 with a strong surface facing the object side. It is preferable that it consists of three sheets.

By arranging the lenses negative and negative, the principal point in the front group can be brought closer to the object side, making it easier to lengthen the back focus. Further, by forming the double-page lenses in such a shape, the amount of aberration generated by the negative lens is minimized (and this is corrected by the strong surface of the positive lens L).

As a result, although it has a three-element structure, it is possible to obtain a front group with sufficiently low aberration generation from on-axis to off-axis.

By adopting the configuration described above, a zoom lens with an extremely long back focus can be realized with a simple configuration of only 7 elements, and it is also compact and high-performance.
The high ambient illumination ratio has been achieved at a level that is several steps higher than those that have been proposed in the past.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An example of a high-performance zoom lens with a long back focus based on the present invention will be shown below. However, in each example, r I−r I? is the radius of curvature, dl~(tea indicates the axial spacing, N1 to N14. C1 to C5. indicate the refractive index and Abbe number at the d-line.

A flat plate equivalent to a low-pass filter or the like is inserted at the end of the lens to correct aberrations.

<Example 1> f = 10.3-14.5-24.7 B F
=25.0FNo=2.82~3.22~4.24 Power data ψ -0,0460000 ψ, 0.0475072 times fl corner tele middle wide d.

2,300 15.332 28.201 <Example 2> f=10.3~14.5~24.7 F8°=2.82~3.25~4.281s 13.953 4.077 0.010 B F = 23.0 ■ Passing distance - 0,0460000 ψ, 0.0521464 Variable plate width ψI at Tele Middle 2,000 13,873 12,713 3,715 Wide 25.596 0.0 IO Next FIG. 1 shows the schematic configuration of the first and second embodiments at the telephoto end, of which the front group (1) and the rear group (
For the old version, see Figure 1 from the tele end (T) to the wide end (W).
The movement to is schematically shown by arrow lines (1) and (2). (3) shown in front of the rear group (ff) represents an aperture, and the flat plate (4) placed at the rear of the rear group (ff) is a flat plate that corresponds to a low-pass filter or face plate. . Note that (15) indicates the image plane.

Figures 2 and 3 are aberration diagrams corresponding to Examples 1 and 2, where (a) shows various aberrations at 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, and the dotted line (open) and solid line (O
5) represents astigmatism on the meridional and sagittal surfaces, respectively.

[Brief explanation of the drawing]

Claims (1)

[Claims] (1) A zoom lens that consists of, in order from the object side, a front group with negative refractive power and a rear group with positive refractive power, and whose magnification is varied by changing the distance between the two groups, A zoom lens with a long back focus, characterized in that the rear group is composed of two positive lens components each including one positive lens and one negative lens, and satisfies the following conditions. 0.7<|ψ_ I |/ψ_II<1.1 (however, ψ_
I <0) 0.5<ψ_P/ψ_R<0.93 Here, (ψ_ I and ψ_II represent the refractive power (reciprocal of the focal length) of the front group and the rear group, respectively, and ψ_P and ψ_R are the refractive powers of the rear group, respectively. This shows the refractive power of the object side component and image side component of the two positive lens components. (2) The rear group is made up of two lenses in order from the object side: a positive lens L_4 and a negative lens L_5 with a strong surface facing the object side. Negative lens L_6 and positive lens L_ with object side component and strong surface facing the image side
2. A zoom lens with a long back focus according to claim 1, characterized in that the zoom lens has a total of four lenses, an image side component consisting of two lenses No. 7. (3) A zoom lens with a long back focus according to claim 2, which satisfies the following conditions. 1.0<|ψ_5_P|/ψ_II<1.6 [However, ψ_5_P=(n_5-1)/R_5_P<
0]0.5<ψ_6/ψ_5<1.0 (However, ψ_5, ψ_6<0) Here, ψ_5_P is the object side refractive power of the negative lens L_5, which is the second lens in the rear group, and n_5 , R_5_
P is the refractive index of the d-line of L_5 and the radius of curvature on the object side. ψ＿
5 and ψ_6 are the refractive powers of L_5 and the negative lens L_6 following it. (4) In the front group, starting from the object side, there is a negative meniscus lens L_1 that is convex toward the object side, and a negative lens L_2 that has a strong surface facing the image side.
4. The zoom lens with a long back focus according to claim 3, characterized in that it is composed of three lenses: a positive lens L_3 having a strong surface facing toward the object side.