JPH07209580A - Wide converter lens - Google Patents

Abstract

(57) [Summary] [Purpose] To obtain a wide converter lens that is mounted in front of the taking lens and displaces the focal length of the entire system to the shorter side. [Constitution] In a wide converter lens which is mounted in front of a photographing lens and displaces the focal length of the entire system to the shorter side, the wide converter lens is composed of a first lens having a negative refractive power and a positive refractive power in order from the object side. Has a second lens,
An afocal system is formed as a whole, and the focal length fi of the i-th lens, the radius of curvature Ri of the i-th lens surface, and the i-th lens
The refractive index Ni of the lens material with respect to the e-line, the Abbe number νi with respect to the d-line, and the effective diameter ai of the i-th lens surface are properly set.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wide converter lens mounted on the front side (object side) of a photographing lens (also referred to as a master lens) for displacing the focal length of the entire photographing system to a shorter one, and particularly to a photographic camera. Suitable for use in shooting lenses such as zoom lenses with a wide field angle of 30 degrees or more used in video cameras and video cameras and single focal length lenses, the entire lens system is compact and has high optical performance. It relates to a wide converter lens having.

[0002]

2. Description of the Related Art Conventionally, various front wide converter lenses have been proposed which are mounted in front of a taking lens and change the focal length of the entire taking system to a shorter one while keeping the focal plane of the entire system at a fixed position. Has been done.

For example, in Japanese Laid-Open Patent Publication No. 59-204817, an afocal system is constructed by five lenses as a whole, which is composed of a first group having a negative refractive power and a second group having a positive refractive power in order from the object side. We have proposed a wide converter lens with a focal magnification of about 0.7.

Further, in Japanese Patent Laid-Open No. 63-253319, the number is 3
It proposes a wide converter lens for finder system or small cameras, which is composed of 4 lenses and has an afocal magnification of about 0.6 times.

In general, a front type wide converter lens mounted in front of a taking lens has an advantage that the F number and focal plane of the entire system can be kept unchanged even if the wide converter lens is mounted.

[0006]

[Problems to be Solved by the Invention] Recent video cameras
With the downsizing and weight reduction of photographing lenses (master lenses) in still cameras and the like, there is also a demand for small and lightweight wide converter lenses to be mounted on them.

However, in the case where the taking lens to which the wide converter lens is attached has a wide angle of view, for example, in the case of a zoom lens having a taking angle of view of 60 degrees or more at the wide-angle end, the object of the wide converter lens is secured in order to secure off-axis light flux. The effective diameter of the first lens group on the side becomes large. Therefore, it is necessary to correct the aberration by increasing the number of lenses in the first and subsequent lens groups, and as a result, the size of the wide converter lens as a whole tends to increase.

For example, in Japanese Examined Patent Publication (Kokoku) No. 4-34722, a wide converter lens for mounting on a zoom lens (master lens) having a zoom ratio of 6 and a shooting angle of view of 53.2 degrees at the wide-angle end is composed of five lenses. ing.

According to the present invention, by appropriately setting the lens configuration, the lens can be mounted on the object side of a wide-angle photographing lens and the total focal length of the two lenses can be displaced to the shorter side. A wide converter lens that has an afocal magnification of about 0.8 times, can maintain excellent optical performance of the entire system when mounted, and is compact and lightweight for the entire lens system. With the goal.

[0010]

The wide converter lens of the present invention is mounted on the front side of a photographing lens and displaces the focal length of the entire system to the shorter side. The wide converter lens is arranged in order from the object side. It has a first lens having a negative refractive power and a second lens having a positive refractive power, and constitutes an afocal system as a whole, the focal length of the i-th lens is fi, and the radius of curvature of the i-th lens surface is R
i, the refractive index of the i-th lens material with respect to the e-line is Ni, d
When the Abbe number with respect to the line is νi and the effective diameter of the i-th lens surface is ai

[0011]

[Equation 2] It is characterized by satisfying the following condition.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1, 2 and 3 are lens cross-sectional views of Examples 1, 2, and 3 when the wide converter lens of the present invention is mounted in front of a taking lens (master lens).

In the figure, WC is a wide converter lens, which is composed of an afocal system and has a negative refracting power first lens C1 having a negative refracting surface which is stronger toward the image side than the object side and both lens surfaces. The second lens C2 having a convex positive refractive power
Is made up of. Reference numeral M denotes a photographing lens (master lens), and in the figure, a case where a zoom lens is used is shown.

F is a focus unit, V1 is a variator,
V2 is a compensator, and R is a relay lens. S
P is a diaphragm, G is a color separation prism, a filter, etc.,
It is shown as a glass block in the figure.

In the present invention, in order to reduce the weight of the wide converter lens as a whole, the first lens C1 having a focal length f1 having a negative refractive power and the second lens having a θ point distance f2 having a positive refractive power are sequentially arranged from the object side. The double-lens configuration of the lens C2 constitutes an afocal system having a magnification of about 0.8.

Since the wide converter lens of the present invention is an afocal system, the object-side focal point of the first lens C1 and the object-side focal point of the second lens C1 coincide with each other, and the principal point interval is e, then e-f1 = It satisfies f2. The afocal magnification is f1 / f2.

In general, since the taking lens itself takes an image, the taking lens alone is excellently corrected for aberrations.
Therefore, in order to obtain a good optical performance as a whole when the wide converter lens is attached, it is necessary to achieve good aberration correction by the wide converter lens alone.

In this embodiment, when the wide converter lens WC is attached to the zoom lens M, the off-axis light beam is incident on the first lens at the highest level on the optical axis when the zoom lens is zoomed to the wide-angle end. Correction of external aberration becomes important. Further, since the axial light flux is highest incident on the optical axis of the second lens when the zoom lens is zoomed to the telephoto end, correction of spherical aberration becomes important at this time.

Therefore, in the present embodiment, the first lens C1 is a negative lens having a concave surface on both sides with the negative refracting surface having a strong negative refracting surface facing the object side, and the second lens C2 is a positive lens having both convex lens surfaces. Constituting a lens, the conditional expression (1) to
Each element is set so as to satisfy (8).

As a result, the amount of aberration generated by the wide converter lens itself is reduced, the optical performance of the entire photographing system when mounted on the master lens is kept good over the entire zoom range, and a compact and lightweight wide converter lens is achieved.

Next, the technical meanings of the above conditional expressions will be described.

Conditional expression (1) defines the afocal magnification as a wide converter lens with respect to the ratio of the focal lengths of the first lens and the second lens. If the lower limit of conditional expression (1) is exceeded, the effect of widening the wide converter lens, which is the object of the present invention, cannot be sufficiently obtained. On the other hand, when the value exceeds the upper limit, the afocal ratio becomes too large, which makes it difficult to correct aberrations well.

Conditional expression (2) defines the ratio between the focal length of the second lens and the effective diameter of the final lens surface. Second
When the master lens to be mounted is a zoom lens, the effective diameter of the final lens surface of the lens is approximately equal to the effective diameter of the axial light flux at the telephoto end. Conditional expression (2) represents the aperture ratio of the second lens and represents the difficulty of aberration correction at the telephoto end.

In the conditional expression (2), when the lower limit value is exceeded, the power (refractive power) of the second lens increases, and from the relationship of the conditional expression (1), the power of the first lens also increases and aberration correction is performed. It will be difficult. Conversely, if the upper limit is exceeded,
This is not good because the power of the second lens becomes too small and the entire lens system becomes large.

Conditional expression (3) defines the shape factor of the first lens having a negative refractive power. When the master lens is a zoom lens, in order to reduce the distortion on the wide-angle side, it is preferable that the master lens has a meniscus shape that is concentric with respect to the principal ray, that is, the shape factor is small.

However, in this lens shape, the power of the lens surface on the image side of the first lens becomes strong, so that spherical aberration extremely occurs on the telephoto side, which makes correction difficult. In order to correct the spherical aberration on the telephoto side, it is preferable that both the object-side lens surface and the image-side lens surface have a negative power, that is, the shape factor is close to zero. However, this shape increases barrel distortion on the wide-angle side.

Conditional expression (3) shows the lens shape of the first lens capable of appropriately correcting the distortion on the wide angle side and the spherical aberration on the telephoto side. When the lower limit is exceeded, the spherical aberration on the telephoto side becomes large, On the other hand, if the upper limit is exceeded, barrel distortion on the wide-angle side increases, which is not good.

Conditional expression (4) defines the shape factor of the second lens having a positive refractive power. When the master lens is a zoom lens, the second lens corrects the distortion generated by the first lens that precedes on the wide-angle side by the refracting action of the lens surface on the object side, and on the telephoto side, the distortion of the first lens that precedes is corrected. Over spherical aberration that occurs on the concave lens surface on the strong image side is corrected.

In order to effectively perform these corrections under the condition regarding the lens shape of the first lens of the conditional expression (3), both the object-side lens surface and the image-side lens surface are positive. It is necessary that the lens has a power of 1 and the curvature of the lens surface on the object side is tighter than that of the lens surface on the image side.

Conditional expression (4) expresses this condition by a shape factor. If the upper limit of conditional expression (4) is exceeded, distortion correction by the preceding first lens on the wide-angle side becomes insufficient. On the other hand, if the value goes below the lower limit, the spherical aberration of the overshooting first lens on the telephoto side is insufficiently corrected, which is not preferable.

Conditional expression (5) relates to the ratio of the radius of curvature of the image side lens surface of the first lens to the radius of curvature of the object side lens surface of the second lens, and mainly in the image side lens surface of the first lens. In order to correct the spherical aberration on the telephoto side in particular, when the master lens is a zoom lens, the spherical aberration on the overside that is mainly generated is generated on the lens surface on the object side of the second lens and the underside is generated. It is a thing. Conditional expression (5)
If the value exceeds the upper limit of, the over-correction of spherical aberration by the first lens on the telephoto side will be overcorrected, and if it exceeds the lower limit, the correction will be insufficient.

Conditional expression (6) relates to the ratio of the radius of curvature of the object-side lens surface of the first lens to the radius of curvature of the image-side lens surface of the second lens, and mainly in the object-side lens surface of the first lens. This is for causing the generated under distortion to cause over distortion on the image side lens surface of the second lens, and particularly for correcting the distortion on the wide angle side when the master lens is a zoom lens.

If the upper limit of conditional expression (6) is exceeded, distortion on the wide-angle side will be corrected well, but it will be difficult to correct spherical aberration on the telephoto side. Conversely, if the lower limit is exceeded, distortion on the wide-angle side will be corrected. Is not good because it is under-corrected.

Conditional expressions (7) and (8) relate to the refractive indexes of the materials of the first lens and the second lens, respectively. Conditional expression (7) is for selecting a material having a refractive index as high as possible in order to configure the first lens group with one lens and to make the curvature gentle. When the lower limit of conditional expression (7) is exceeded and the refractive index becomes low, the curvature of the lens surface of the first lens becomes strong, and it becomes difficult to correct distortion on the wide-angle side.

Conditional expression (8) is for reducing the refractive index of the material of the second lens to cancel the negative Petzval sum generated in the first lens, and to strengthen the curvature to some extent to correct distortion and spherical aberration. belongs to. If the conditional expression (8) is not satisfied, it becomes difficult to correct distortion and spherical aberration well.

The conditional expressions (9) and (10) relate to the Abbe numbers of the materials of the first lens and the second lens. Conditional expression (9) is for increasing the Abbe number as much as possible in order to suppress the negative axial chromatic aberration which occurs because the first lens has a strong negative power. If the Abbe number of the material of the first lens deviates from the conditional expression (9), it becomes difficult to satisfactorily correct the axial chromatic aberration.

Conditional expression (10) mainly relates to the Abbe number of the second lens for correcting the axial chromatic aberration generated in the first lens. If the lower limit of conditional expression (10) is exceeded, axial chromatic aberration will be undercorrected, and conversely, if it exceeds the upper limit, overcorrection will result.

Next, numerical examples of the wide converter lens of the present invention and a photographing lens (master lens) to which the wide converter lens is attached will be shown.

In the numerical examples, Ri is the radius of curvature of the i-th lens surface in order from the object side, and Di is the i-th lens surface from the object side.
The th lens thickness and the air gap, Ni and νi are the refractive index and the Abbe number of the glass of the ith lens in order from the object side.

Table 1 shows the relationship between the above-mentioned conditional expressions and various numerical values in the numerical examples.

Numerical Example 1 is an example in which glass having a high refractive index is used as the material of the first lens. The curvature of each lens surface can be made relatively gentle, and the distortion at the wide-angle end is corrected to be small.

In Numerical Examples 2 and 3, glass having a relatively low refractive index of the material of the second lens, but a large Abbe number is selected for chromatic aberration correction. The master lens is a high magnification, wide angle zoom lens with a zoom ratio of 13 times, a half angle of view of 31.5 degrees at the wide angle end, an F number of 1.6 at the wide angle end, and an F number of 1.9 at the telephoto end. .

[Wide Converter Lens] <Numerical Example 1> R 1 = -961.351 D 1 = 2.80 N 1 = 1.73852 ν 1 = 49.8 a 1 = 112.803 R 2 = 157.859 D 2 = 27.03 a 2 = 103.645 R 3 = 185.016 D 3 = 19.03 N 2 = 1.51678 ν 2 = 54.7 a 3 = 87.366 R 4 = -270.185 a 4 = 90.679 <Numerical Example 2> R 1 = -602.952 D 1 = 2.80 N 1 = 1.68108 ν 1 = 50.7 a 1 = 112.523 R 2 = 91.253 D 2 = 34.94 a 2 = 97.160 R 3 = 141.197 D 3 = 13.22 N 2 = 1.54212 ν 2 = 59.5 a 3 = 83.427 R 4 = -200.510 a 4 = 81.967 <Numerical example 3> R 1 = -602.486 D 1 = 2.80 N 1 = 1.68082 ν 1 = 55.3 a 1 = 112.005 R 2 = 90.157 D 2 = 34.43 a 2 = 96.687 R 3 = 138.465 D 3 = 13.16 N 2 = 1.52736 ν 2 = 64.6 a 3 = 83.500 R 4 = -188.075 a 4 = 82.337 (Master lens) <Numerical example> R 1 = 705.470 D 1 = 2.50 N 1 = 1.81264 ν 1 = 25.4 R 2 = 90.710 D 2 = 5.80 R 3 = 528.960 D 3 = 6.20 N 2 = 1.49845 ν 2 = 81.6 R 4 = -189.480 D 4 = 0.17 R 5 = 103.400 D 5 = 12.24 N 3 = 1.43496 ν 3 = 95.1 R 6 = -136.990 D 6 = 0.17 R 7 = 52.570 D 7 = 10.34 N 4 = 1.69979 ν 4 = 55.5 R 8 = 219.360 D 8 = Variable R 9 = 71.980 D 9 = 1.00 N 5 = 1.88814 ν 5 = 40.8 R10 = 17.870 D10 = 3.75 R11 = -110.470 D11 = 0.80 N 6 = 1.80811 ν 6 = 46.6 R12 = 78.620 D12 = 3.80 R13 = -15.560 D13 = 1.00 N 7 = 1.77621 ν 7 = 49.6 R14 = 121.380 D14 = 3.52 N 8 = 1.93306 ν 8 = 21.3 R15 = -31.110 D15 = Variable R16 = -24.590 D16 = 0.90 N 9 = 1.77621 ν 9 = 49.6 R17 = -36.260 D17 = 4.30 N10 = 1.81265 ν10 = 25.4 R18 = -269.500 D18 = Variable R19 = (Aperture) D19 = 2.84 R20 = -176.460 D20 = 3.96 N11 = 1.76168 ν11 = 27.5 R21 = 40.580 D21 = 0.17 R22 = 90.990 D22 = 10.86 N12 = 1.48915 ν12 = 70.2 R23 = -23.600 D23 = 1.30 N13 = 1.83932 ν13 = 37.2 R24 = -54.300 D24 = 21.69 R25 = -1049.550 D25 = 6.25 N14 = 1.60548 ν14 = 60.7 R26 = -47.850 D26 = 0.17 R27 = 95.900 D27 = 1.65 N15 = 1.81265 ν15 = 25.4 R28 = 31.110 D28 = 9.07 N16 = 1.48915 ν16 = 70.2 R29 = -220.140 D29 = 0.17 R30 = 41.330 D30 = 5.74 N17 = 1.48915 ν17 = 70.2 R31 = 208.110 D31 = 8.27 R32 = ∞ D32 = 65.00 N18 = 1.51825 ν18 = 64.2 R33 = ∞ Value when a wide converter lens is attached, wide converter lens Distance between the master lens is 10.

F _NO 1.6-1.9

[0045]

[Table 1]

[0046]

As described above, according to the present invention, by properly setting the lens configuration, the lens is mounted on the object side of a wide-angle shooting lens and the focal length of the entire system is displaced to the shorter side. It is composed of 2 lenses as a whole, has an afocal magnification of about 0.8 times, and can maintain good optical performance of the entire system when mounted, and the size and weight of the entire lens system can be reduced. A wide converter lens designed for

[Brief description of drawings]

FIG. 1 is a lens cross-sectional view when a wide converter lens according to Numerical Example 1 of the present invention is mounted on a master lens.

FIG. 2 is a lens cross-sectional view when a wide converter lens according to Numerical Example 2 of the present invention is mounted on a master lens.

FIG. 3 is a lens cross-sectional view when a wide converter lens according to Numerical Example 3 of the present invention is attached to a master lens.

FIG. 4 is an aberration diagram at the wide-angle end when the wide converter lens of Numerical Embodiment 1 of the present invention is attached to a master lens.

FIG. 5 is an aberration diagram at the telephoto end when the wide converter lens of Numerical Example 1 of the present invention is attached to a master lens.

FIG. 6 is an aberration diagram at the wide-angle end when a wide converter lens according to Numerical Example 2 of the present invention is attached to a master lens.

FIG. 7 is an aberration diagram at a telephoto end when a wide converter lens according to Numerical Example 2 of the present invention is attached to a master lens.

FIG. 8 is an aberration diagram at the wide-angle end when the wide converter lens of Numerical Example 3 of the present invention is attached to a master lens.

FIG. 9 is an aberration diagram at a telephoto end when a wide converter lens according to Numerical Example 3 of the present invention is attached to a master lens.

[Explanation of symbols]

 WC wide converter lens M master lens C1 first lens C2 second lens F focus group V variator C compensator R relay lens group g g line ee line ΔS sagittal image surface ΔM meridional image surface

Claims (1)

[Claims] 1. A wide converter lens mounted in front of a taking lens for displacing the focal length of the entire system to the shorter side, wherein the wide converter lens has a negative refractive power and a positive refractive power in order from the object side. Has a second lens having a power and constitutes an afocal system as a whole, and the focal length of the i-th lens is fi and the radius of curvature of the i-th lens surface is R.
i, the refractive index of the i-th lens material with respect to the e-line is Ni, d
When the Abbe number with respect to the line is νi and the effective diameter of the i-th lens surface is ai, Wide converter lens characterized by satisfying the following conditions.