JP2000171709A - Wide-angle conversion lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-performance wide-angle conversion lens equipped with a zoom lens whose zoom ratio is about 8 to 10, suitable for a high- performance electronic still camera using an imaging device whose number of pixels is >=1,000,000 and hardly having deterioration such as the curvature of field and distortion aberration at a wide-angle end. SOLUTION: This almost afocal system wide-angle conversion lens 1 is attached to the object side of a photographing lens so as to shorten the focal distance of an entire lens system, and it is constituted of a 1st lens L1 being a concave lens whose concave surface having strong curvature faces to an image surface side, a 2nd lens L2 being a convex lens, and a combined lens consisting of a 3rd lens L3 being a biconcave lens and a 4th lens L4 being a biconvex lens in order from the object side. Angular magnification is set to about 0.75 in the lens, which satisfies respective conditions 0.3<|h2/r2|<0.8, 0.15<|h4/r4|<0.45, 0,015<n4-n3<0.2, 0.25<|h6/r6|<0.85 and ν3<ν4 when it is assumed that (hi) is the effective radius of a surface i-th from the object side, (ri) is the radius of curvature of the surface i-th from the object side, (ni) is the refractive index of an i-th lens on a d-line and ∼i is the Abbe number of the i-th lens.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photographing lens mainly for a video camera and an electronic still camera, and more particularly to a photographing lens having a zoom ratio exceeding 3 times and having an angle of view at a wide-angle end of about 55 to 60 degrees, and 100. The present invention relates to a high-performance wide-angle conversion lens that is compatible with a high-performance lens corresponding to an image sensor having 10,000 or more pixels.

[0002]

2. Description of the Related Art Recently, particularly in electronic still cameras, the number of pixels exceeding 1 million pixels has been used as an image pickup device, and the size of an image pickup screen has increased with the increase in the number of pixels. In order to increase the number of pixels and reduce the size, many imaging lenses for electronic still cameras with a high number of pixels have a zoom ratio of about three times.

However, a video camera for photographing moving images generally uses a photographing lens having a zoom ratio of 10 times or more.

[0004] Accordingly, in electronic still cameras as well, there is a strong demand for a higher zoom ratio, and small, high-performance, high-magnification zoom lenses have come into practical use.

Such a compact, high-performance, high-magnification zoom lens for an electronic still camera, like a zoom lens of the same type for a video camera, requires a wide-angle end to achieve both miniaturization and high-magnification. When the angle of view is about 55 ° to 60 °, a well-balanced design is possible.

However, as the zoom ratio is increased,
When the angle of view is narrow, there is a problem that the image quality is likely to be degraded due to camera shake.In addition, for still images where shooting methods such as panning cannot be used unlike moving images, there is a demand to increase the angle of view at the wide-angle end. Is also strong.

Accordingly, there has been an increasing demand for a high-performance wide-angle conversion lens capable of shifting the focal length range to the wide-angle side without deteriorating the image quality of the high-performance zoom lens.

A wide-angle conversion lens used in a conventional consumer video camera is, as shown in an example a of a wide-angle conversion lens shown in FIGS.
Generally, three lenses L1, L2, and L3 having negative, negative, and positive refractive powers, respectively, are arranged in order from the object side.

The wide-angle conversion lens a
Have been designed mainly in pursuit of miniaturization and low cost, rather than imaging performance.

[0010] The wide-angle conversion lens a is
As shown in FIG. 7, when the zoom lens 2 is attached to and used as an example 2 of the zoom lens, the meridional field curvature at the wide-angle end of the zoom lens 2 is reduced.
Compared to a single state, the lens barrel falls to the under side, and barrel-shaped distortion increases extremely.

In addition, since chromatic aberration of magnification is generated from the conversion lens a and there is no design freedom for correcting the chromatic aberration, color bleeding becomes remarkable in a peripheral portion of the screen.

FIGS. 5 and 6 show the zoom lens 2 respectively.
9 shows the image plane astigmatism, distortion, and meridional lateral aberration at the single wide-angle end and the telephoto end respectively. FIGS. 8 and 9 show the wide-angle end and the wide-angle end when the wide-angle conversion lens a is attached to the zoom lens 2, respectively. The graph shows astigmatism, distortion, and meridional lateral aberration at the telephoto end.

In the astigmatism diagram, the sagittal field curvature is shown by a solid line, and the meridional field curvature is shown by a broken line. In the meridional transverse aberration diagram, the solid line is the d line and the broken line is g.
Lines and alternate long and short dash lines indicate those on the C line.

That is, FIGS. 5 and 6 and FIGS. 8 and 9
As is apparent from the comparison with the above, when the wide-angle conversion lens a is attached to the zoom lens 2, the astigmatism is good on the sagittal image plane, but falls on the under side on the meridional image plane, and This is a main cause of deteriorating the imaging performance in the section.

Further, since the distortion greatly occurs on the minus side from the wide-angle conversion lens a, it is added to the distortion at the wide-angle end of the zoom lens 2, and the barrel-shaped distortion becomes extremely noticeable.

In order to correct the distortion in the meridional image plane on the wide-angle conversion lens a side, it is effective to use a concave lens on the object side where the height of the principal ray is high, and to make the divergence effect stronger as the peripheral light flux becomes stronger. But
If the refractive power of the concave lens on the object side is increased in order to perform the above correction, the distortion aberration curve will be further bent to the minus side, and simultaneously improving the curvature and the distortion on the meridional image plane will be similar to the wide-angle conversion lens a. 3 having negative, negative, and positive refractive power in order from the object side.
This was not possible with a single lens configuration.

The evaluation of a still image is the most strict image quality evaluation that requires a printed image to be observed by hand.
In addition, when a still image is noticeably degraded in image quality and distorted in a peripheral portion of an image as compared with a moving image, this deteriorates the impression of a printed image.

The conventional wide-angle conversion lens a satisfies such a requirement for a still image.
It was extremely difficult.

Therefore, in a still camera pursuing high image quality, the curvature and distortion on the meridional image plane are simultaneously improved, and only the angle of view is changed by using a conversion lens. Must be impossible.

[0020]

SUMMARY OF THE INVENTION In view of the above-mentioned problems, the present invention provides a high-performance image pickup device having a zoom lens having a zoom ratio of about 8 to 10 times and using an image pickup element having more than one million pixels. It is an object of the present invention to provide a high-performance wide-angle conversion lens which has almost no deterioration of field curvature and distortion at the wide-angle end, which is optimal for an electronic still camera.

[0021]

In order to solve the above-mentioned problems, a wide-angle conversion lens according to the present invention is mounted on an object side of a photographing lens to shorten the focal length of the entire lens system. In the lens,
In order from the object side, it is composed of a first lens of a concave lens having a concave surface with a strong curvature directed to the image surface side, a second lens of a convex lens, and a cemented lens of a third lens of a biconcave lens and a fourth lens of a biconvex lens. , The angle magnification is about 0.75, hi is the effective radius of the i-th surface from the object side, ri is the radius of curvature of the i-th surface from the object side, and ni is the i-th lens (i-th lens from the object side). Lens) refractive index at d-line,
If νi is the Abbe number of the i-th lens (i-th lens from the object side), 0.3 <| h2 / r2 | <0.8, 0.
15 <| h4 / r4 | <0.45, 0.015 <n4-
n3 <0.2, 0.25 <| h6 / r6 | <0.85,
This is to satisfy each condition of ν3 <ν4.

Therefore, it is possible to provide a wide-angle conversion lens that can shift the focal length range to the wide-angle side without substantially deteriorating the curvature of field, distortion, and chromatic aberration at the wide-angle end of the main zoom lens. It becomes possible.

[0023]

Embodiments of the wide-angle conversion lens of the present invention will be described below with reference to the accompanying drawings.

In the following description, "si" is the i-th lens surface (i-th surface) from the object side, "ri" is the radius of curvature of the i-th surface si, and "di" is the i-th surface si and the i-th surface. i + 1 surface s
The surface interval on the optical axis between the (i + 1) th surface and “ni” is the d-line (wavelength 587.m) of the i-th lens (i-th lens) from the object side.
The refractive index at “6 nm),“ νi ”, is the Abbe number of the i-th lens.

The wide-angle conversion lens 1 is a nearly afocal wide-angle conversion lens which is mounted on the object side of the zoom lens 2 which is a photographing lens to reduce the focal length of the entire lens system.

The wide-angle conversion lens 1 includes, in order from the object side, a first lens L1 of a concave lens having a concave surface with a strong curvature facing the image plane side, a second lens L2 of a convex lens,
It is composed of a cemented lens of a third bi-concave lens L3 and a bi-convex fourth lens L4.

Further, the wide-angle conversion lens 1 has an angular magnification of about 0.75 (0.7209 times in the wide-angle conversion lens 1) and h.
i is the effective radius of the i-th surface from the object side, ri is the radius of curvature of the i-th surface from the object side, ni is the refractive index at the d-line of the i-th lens (i-th lens from the object side), and νi is the i
Assuming the Abbe number of the lens (i-th lens from the object side), 0.3 <| h2 / r2 | <0.8 (conditional expression 1) 0.15 <| h4 / r4 | <0.45 (conditional expression) 2) 0.015 <n4-n3 <0.2 (conditional expression 3) 0.25 <| h6 / r6 | <0.85 (conditional expression 4) ν3 <ν4 (conditional expression 5) It is like that.

Hereinafter, the above-mentioned conditional expressions will be described.

Conditional expression 1 is for correcting the curvature of the meridional image plane at the wide-angle end toward the over side.

That is, by making the absolute value of h2 / r2 larger than the lower limit of 0.3, the principal ray passes through the peripheral portion of the first lens L1 where the curvature is large, and the principal ray passes through the second surface. The incidence angle formed with the normal of s2 suddenly increases in the peripheral portion, so that the meridional field curvature at the wide-angle end can be lowered to the over side.

However, since the improvement of the meridional field curvature at the wide-angle end simultaneously promotes the negative distortion generated from the first lens L1, the absolute value of h2 / r2 is 0, which is the upper limit. It is necessary not to exceed .8. When the absolute value of h2 / r2 exceeds the upper limit, the first lens L1 is moved by the second lens L2.
, It becomes difficult to correct the distortion on the negative side, which is generated from.

Conditional expression 2 is for correcting the negative side distortion generated from the first lens L1 as a side effect of the correction of the meridional field curvature by the conditional expression 1.

That is, the second lens L2 is disposed adjacent to the first lens L1, and the absolute value of the curvature r4 of the fourth surface s4 is set to be larger than the lower limit value 0.15. The deflection angle of the principal ray is increased to correct the distortion.

When the absolute value of the curvature r4 of the fourth surface s4 exceeds the upper limit value of 0.45, the balance between the correction of the meridional field curvature in the first lens is lost, and the curvature of field and distortion are reduced. At the same time, the object of the present invention cannot be achieved.

If the Abbe number of the second lens L2 is set to 40 or less in addition to the condition defined by the conditional expression 2, the chromatic aberration of magnification can be corrected at the same time.

Conditional expressions 3 to 5 mainly define conditions for correcting spherical aberration and axial chromatic aberration at the telephoto end.

That is, the junction surface s6 between the third lens L3 and the fourth lens L4 has a positive refractive power, and the value of n4-n3 and h
The absolute value of 6 / r6 is 0.01, which is the lower limit.
By making them larger than 5 and 0.25, spherical aberration at the telephoto end is prevented from falling to the under side.

In order to correct the chromatic aberration of magnification at the wide-angle end, the Abbe number of the first lens L1 and the second lens L2 satisfies ν2 <ν1, and this is insufficient for axial chromatic aberration at the telephoto end. Becomes Therefore, by adding the condition defined by Conditional Expression 5 to the condition defined by Conditional Expression 3 and Conditional Expression 4, the luminous flux is spread and the height of the principal ray is reduced. Can be corrected.

Note that n4 defined by conditional expressions 3 and 4
If the value of −n3 and the absolute value of h6 / r6 exceed the upper limits of 0.2 and 0.85, respectively, it becomes difficult to correct spherical aberration and axial chromatic aberration at the telephoto end in a well-balanced manner. .

Hereinafter, the wide-angle conversion lens 1 of the present invention will be described.
Here is a numerical example.

[0041]

[Table 1]

The focal length of the zoom lens 2 is as follows: the numerical value of the zoom lens 2 as the main lens to which the wide-angle conversion lens 1 is attached and the numerical value of the composite optical system of the wide-angle conversion lens 1 and the zoom lens 2. f
= 4.31 (wide-angle end) and f = 41.82 (telephoto end),
The F value of the zoom lens 2 is F1.84 (wide-angle end) and F
2.22 (telephoto end), the distance from the seventh lens surface s7 to the entrance pupil of the zoom lens 2 is 20.57 (wide-angle end) and 1
45.8 (telephoto end), angle of view of zoom lens 2 is 57.6
° (wide-angle end) and 6.03 ° (telephoto end), and the angle of view in the combined optical system is 76.0 ° (wide-angle end) and 8.36 °
(Telephoto end).

Table 3 shows each value related to the conditional expressions 1 to 4.

[0044]

[Table 2]

FIGS. 3 and 4 show astigmatism, distortion, and meridional lateral aberration at the wide-angle end and the telephoto end of the combined optical system of the wide-angle conversion lens 1 and the zoom lens 2, respectively.

FIGS. 5 and 6 show astigmatism, distortion and meridional lateral aberration of the zoom lens 2 alone at the wide-angle end and the telephoto end.

In the astigmatism diagram, the solid line shows the value on the sagittal image plane, and the broken line shows the value on the meridional image plane. In the meridional transverse aberration diagram, the solid line shows the d-line,
The dashed line indicates the value on the g line, and the dashed line indicates the value on the C line.

As described above, in the wide-angle conversion lens 1 of the present invention, in comparison with the conventional wide-angle conversion lens a shown in FIGS. 8 and 9, the disadvantages of the conventional example are remarkably eliminated. Is obvious, and
Also in comparison with various aberrations when only the zoom lens 2 shown in FIGS. 5 and 6 is used, the original purpose of the wide-angle conversion lens 1 is a wide angle of view, without substantially impairing the performance of the zoom lens 2. It is clear that the transformation has been achieved.

It should be noted that the specific shapes and structures of the respective parts shown in the above-described embodiments are merely examples for embodying the present invention, and the technical features of the present invention will be described below. The scope should not be construed as limiting.

[0050]

As is apparent from the above description, the wide-angle conversion lens of the present invention is a substantially afocal wide-angle conversion lens which is mounted on the object side of a photographing lens to shorten the focal length of the entire lens system. In order from the object side, the first lens is a concave lens having a concave surface with a strong curvature directed to the image surface side, the second lens is a convex lens, and the cemented lens is a third lens of a biconcave lens and a fourth lens of a biconvex lens. In addition to the angular magnification of about 0.75, hi is the effective radius of the i-th surface from the object side, ri is the radius of curvature of the i-th surface from the object side, and ni is the i-th lens (i-th lens from the object side). N) refractive index at d-line, νi
Is the Abbe number of the i-th lens (i-th lens from the object side), 0.3 <| h2 / r2 | <0.8, 0.15
<| H4 / r4 | <0.45, 0.015 <n4-n3
<0.2, 0.25 <| h6 / r6 | <0.85, ν3
Since each condition of <ν4 is satisfied, the focal length range can be shifted to the wide-angle side without substantially deteriorating the curvature of field, distortion, and chromatic aberration at the wide-angle end of the zoom lens that is the main lens. .

According to the second aspect of the present invention, if ν2 is the Abbe number of the second lens, ν2 <40.
Is satisfied, the chromatic aberration of magnification can be simultaneously corrected by the second lens in addition to the correction of the distortion.

[Brief description of the drawings]

FIG. 1 shows an embodiment of the wide-angle conversion lens of the present invention together with FIG. 2 to FIG. 4, and FIG. 1 schematically shows the lens configuration of the wide-angle conversion lens.

FIG. 2 is a view schematically showing a state in which the wide-angle conversion lens of the present invention is mounted on an example of a zoom lens.

FIG. 3 shows astigmatism at the wide-angle end in the state shown in FIG. 2;
It is a figure which shows a distortion aberration and a meridional lateral aberration.

FIG. 4 shows astigmatism at the telephoto end in the state shown in FIG. 2;
It is a figure which shows a distortion aberration and a meridional lateral aberration.

5 is a diagram showing astigmatism, distortion, and meridional lateral aberration at the wide-angle end in the zoom lens alone shown in FIG. 2;

6 is a diagram showing astigmatism, distortion, and meridional lateral aberration at the telephoto end of the zoom lens shown in FIG. 2 alone;

FIG. 7 shows an example of a conventional wide-angle conversion lens together with FIGS. 8 and 9 and shows a case. FIG. 7 shows an example of a conventional wide-angle conversion lens mounted on a zoom lens shown in FIG. FIG.

FIG. 8 shows astigmatism at the wide-angle end in the state shown in FIG. 7;
It is a figure which shows a distortion aberration and a meridional lateral aberration.

9 shows astigmatism at the wide-angle end in the state shown in FIG. 7;
It is a figure which shows a distortion aberration and a meridional lateral aberration.

[Explanation of symbols]

1: wide-angle conversion lens, 2: photographing lens, L1
... first lens, L2 ... second lens, L3 ... third lens,
L4: 4th lens

Claims (2)

[Claims] 1. A substantially afocal wide-angle conversion lens which is mounted on the object side of a taking lens to shorten the focal length of the entire lens system, wherein a concave surface having a strong curvature is directed toward an image surface in order from the object side. The first lens of the concave lens, the second lens of the convex lens, the cemented lens of the third lens of the biconcave lens and the fourth lens of the biconvex lens, the angular magnification is about 0.75, and the following A wide-angle conversion lens characterized by satisfying each condition. 0.3 <| h2 / r2 | <0.8 0.15 <| h4 / r4 | <0.45 0.015 <n4-n3 <0.2 0.25 <| h6 / r6 | <0.85 ν3 <ν4 where hi: effective radius of the i-th surface from the object side, ri: radius of curvature of the i-th surface from the object side, ni: refraction at the d-line of the ith lens (the ith lens from the object side) Rate, νi: Abbe number of the i-th lens (i-th lens from the object side). 2. The wide-angle conversion lens according to claim 1, wherein the following condition is satisfied. ν2 <40, where ν2 is the Abbe number of the second lens.