JPH06250080A - Inverted telescope type photographic lens - Google Patents

Abstract

PURPOSE:To simply compose a lens and to make the lens high performance inspite of the small size of the lens and the small aperture of an attachment such as a filter mounted on the object side. CONSTITUTION:The concave surface in contact with air on the image side(diaphragm side) of a negative lens located closest to the object side is made aspherical surface whose curvature monotonously decreases with the increase of a distance from an optical axis among negative lenses included in a lens group G1 on the object side and having a negative power. Concretely, the reference curved surface of a vertex is fallen within the range of a conical constant kapparepresented by-1<kappa<0.8, by designating the focal length of a whole system as (f), the focal length of a negative lens having an aspherical surface as-f1, the effective diameter on the tangent plane at the vertex of the surface closet to the object side as phi and an half viewing angle as theta, the condition: 0.7f<f1<2 fphi<3.3.f.tan<2>theta is satisfied.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inverse telephoto type super wide-angle lens having a back focus larger than the focal length of the entire system.

[0002]

2. Description of the Related Art In a reverse telephoto wide-angle lens, a negative lens on the object side (front) is necessary to obtain a back focus larger than the focal length, but at the same time, on the image side (rear).
Has the effect of reducing the angle of view with respect to the positive lens group and correcting the curvature of the image surface and directing the Petzval sum in the negative direction. However, in a lens system in which the ratio of the back focus to the focal length is not so large, the action of these negative lenses can be a factor for improving the imaging performance by correcting the aberration of the entire lens system.

However, in a lens having an angle of view of 80 ° or 100 ° which requires a relatively long back focus, or a lens system which requires a back focus of more than that, negative distortion is generated. It is well known that the Petzval sum becomes too negative and that adverse effects such as disturbance of coma aberration are prominent. In order to minimize the negative distortion generated from the negative meniscus lens, and also from the viewpoint of keeping the image surface good and suppressing the spherical aberration of the pupil,
It is desirable to have a meniscus shape with a convex surface facing the object side so that the chief ray passes through these negative lenses in the vicinity of the minimum deviation angle. However, at this time, negative comatic aberration occurs due to the concave surface of the negative lens. In particular, a large negative coma aberration occurs with respect to the light outside the principal ray, and the correction thereof is a heavy burden on the lens design.

In many of these cases, aberrational problems other than distortion can be reduced by enlarging the lens system, especially the front lens system. This is to enlarge the lens proportionally to reduce the absolute value of the Petzval sum in that portion and to reduce the occurrence of coma aberration by using only the light nearer to the principal ray. However, this method cannot correct distortion in principle. This is because the distortion is an amount indicated by a ratio, and if the angle of view is the same, the amount of distortion does not change if the relative optical path of the principal ray does not change even if the lens system is expanded proportionally. Even if it is effective for correction of coma aberration and positive curvature of the image plane, it is not effective for correction of distortion. This is easy to understand if one imagines a wide-angle conversion system. This is because the magnification and the distortion do not change even if the conversion system is enlarged, but the spherical aberration and the coma are improved.

Next, the shape and attachment size of the lens closest to the object will be described. The size of the attachment (aperture size) such as a filter attached to the object side of the lens system determines the practical size of the lens in addition to the effective diameter of each surface of the lens. Is. As described above, it is desirable that the bending of the negative lens is performed so that the chief ray passes with the minimum deviation angle, so that the degree of bending increases as the angle of view increases. In other words, the top surface becomes convex with respect to the object, and the "depth" of the surface increases. This tendency is further promoted by reducing the power of the negative lens in order to prevent the occurrence of large distortion. Since the attachment size is not smaller than the effective diameter of the tangent plane of the apex of the leading lens surface, the larger the "depth" of the surface, the larger the difference between the effective diameter of the lens and the attachment size. In particular, when the leading lens is a negative lens, the radius of curvature becomes smaller than the effective diameter due to strong bending, so that the attachment size becomes considerably larger than the effective diameter of the lens. This tendency increases rapidly with an increase in the angle of view, and is a theoretical drawback to some extent.

[0006]

To avoid this drawback, the power of the leading negative lens may be increased to increase the radius of curvature of the leading surface. The result is as described above. Inconvenience such as frame distortion occurs. In addition, there are many cases in which a positive lens is placed near the negative lens to correct distortion, but this tends to further increase the volume of the lens system. A system in which the convex surface of the front negative meniscus lens is an aspherical surface is also known for the purpose of correcting distortion. This is an effective means, but the fact is that it has only an auxiliary meaning of a system composed only of spherical surfaces, and the drawback that the attachment size is considerably larger than the effective diameter of the lens remains. However, even if such a method is adopted, the angle of view is 10
Designing a lens system at 0 ° or more is not easy and the configuration is complicated, and the volume of the lens system tends to be large both in the optical axis direction and in the aperture direction.

An object of the present invention is to solve these problems and to provide a compact and high performance super wide-angle large-aperture-ratio photographic lens in which an attachment such as a filter mounted on the object side has a small aperture. .

[0008]

The phenomenon as described above is caused by the fact that the curvature of the spherical surface is constant. Therefore, if the concave surface of the negative lens is a curved surface whose curvature monotonously decreases as it moves away from the optical axis, the occurrence of such distortion and coma will be reduced. In the present invention, among the negative lenses included in the lens group having the negative power arranged in front to obtain a back focus larger than the focal length,
These problems are solved by making the concave surface in contact with the air on the image side (diaphragm side) of the negative lens closest to the object side an aspherical surface whose curvature monotonously decreases as the distance from the optical axis increases.

Specifically, it is arranged on the object side of the diaphragm.
First lens unit G having negative refractive power₁And the first lens group G₁When
The second lens group G having a positive refractive power and arranged between the diaphragm and the diaphragm.
₂And a third lens of positive refracting power arranged on the image side of the diaphragm.
Group G₃And has a back distance larger than the focal length of the entire system.
In the reverse telephoto wide-angle lens having a focus,
1 lens group G₁Has a negative lens with a convex surface facing the object side.
However, the concave surface of the negative lens in contact with the air on the image side is
Is a rotationally symmetric aspherical surface, and x is the optical axis from the vertex of the curved surface.
Direction distance, y is the distance from the optical axis, C is the apex of the curve
Curvature, C _Four, C₆... is a constant, the negative
The aspherical shape in the meridional section of the concave surface of the lens is x = Cy²/ {1+ (1-κC²y²)^1/2} + C_Foury^Four+ C
₆y⁶+ ... And in the range represented by -1 <κ <0.8, the curvature becomes farther away from the optical axis.
The shape is monotonically decreasing.

[0010]

In the construction of the present invention as described above, the curved surface shape of the concave surface on the image side of the negative lens closest to the object in the first lens group G ₁ having negative refractive power will be described first. Since the lens is rotationally symmetric with respect to the optical axis, the curve of the meridional section may be described. In general, a quadratic curve has two focal points, and the coincidence of these is a circle. The two focal points are ellipses on the same side of the curve, one is a parabola at infinity, and one is a hyperbola on the opposite side of the curve.

Of these, the ones whose curvature decreases monotonously with the distance from the optical axis are three except for the circle, but in the case of an ellipse, the major axis must coincide with the optical axis. .
Further, the hyperbola has a straight line asymptote and has little change in curvature at a place sufficiently distant from the optical axis. Speaking extremely, since it is a concave conical shape, the deviation angles with respect to principal rays having different inclinations are approximately the same, which is not a preferable phenomenon for aberration correction. For this reason, the hyperboloid is not suitable as an aspherical surface to be introduced into the surface far from the diaphragm of the wide-angle lens having a large angle of view, that is, the surface closest to the object side. However, this does not apply if the region is not very close to the asymptote.

Therefore, a curved surface suitable for the image-side (diaphragm-side) concave surface of the negative lens closest to the object side of the wide-angle lens is an ellipse or a parabola whose major axis coincides with the optical axis, and is relatively curved along the hyperbolic axis. It will be a near limited area. In the meridional plane, x = Cy ² / {1+ (1-κC ² y ² ) ^1/2 } + C ₄ y ⁴ + C is generally adopted in optical design as an expression expressing the shape of an aspherical surface.
The expression is ₆ y ⁶ +.

Here, x is the distance in the optical axis direction, y is the distance from the optical axis, C is the curvature at the apex of the curve, the first term on the right side represents a quadratic curve, and κ determines its shape. Is the amount.
It is a hyperbola if κ is negative, a parabola if zero, an ellipse between zero and 1, and a circle if κ = 1. In some cases, a squared term is added to the right side, but since it overlaps with the first term and a curve can be completely represented even if omitted, the above equation is used.

In such an aspherical expression, κ
Has the possibility of the region −∞ <κ <1. However, when κ is close to 1, it becomes close to a circle, and the effect as an aspherical surface for correcting distortion is small. In addition, since it becomes a shape close to a cone in the vicinity of −∞, it is not suitable as a lens. Therefore, the present invention has found that κ in the range of −1 <κ <0.8 (1) is effective for reducing the aperture of the lens while correcting distortion. .

When the value goes below the lower limit of this range, there are many portions that can be regarded as the hyperbolic straight line portion within the effective diameter of the lens, so that the lens surface has a shape close to a cone, and as a lens, the difference in curvature between the central portion and the peripheral portion. Is too large, and the difference between the middle part and the peripheral part is small, and as a result, the curvature of the image plane and the bending of the distortion aberration become large, and the correction thereof becomes difficult. If the upper limit is exceeded and the value is close to 1, the lens surface is close to a circle as described above, and the effect on aberration correction is small. In this aspherical shape, it is practical that the negative lens has a meniscus shape with its convex surface facing the object side.

When the terms after the second term are present, the shape of the curved surface changes and disappears as a quadric surface, but it is included between the two curves determined by the value of κ above, and the curvature is from the optical axis. It is also possible to add a higher-order term under the condition of monotonically increasing with respect to the distance and to modify the curve as necessary. Of course, higher order terms may be added to the object side surface of the lens having this aspherical surface.

In the conventional reverse telephoto wide-angle lens system whose angle of view exceeds 75 °, the first lens group G _{1 having} negative refractive power is used.
In order to correct the negative distortion aberration in (2), most of them need to have a positive lens near the lens mainly for the purpose of correcting the distortion aberration. That is, the first lens group G
Distortion aberrations are corrected by arranging positive and negative lenses in combination in ₁ to compose a lens group having negative power. However, this is essentially uneconomical because the necessary negative power is canceled by the positive power, and as a result, a lens having a larger aperture is required.

In the above-described negative lens having an aspherical concave surface according to the present invention, the power of the negative lens does not need to be canceled by the positive lens in order to reduce the occurrence of negative distortion. That is, it is possible for a single negative lens to bear the action of a combination of positive and negative lenses in a conventional wide-angle lens centering on a spherical system. In addition, since negative coma aberration from the front (object side) negative lens is much smaller than that of the spherical system, the absolute value of the power is made larger without detrimentally affecting the aberration. It is possible to keep the aperture of the lens system small,
A large back focus can be obtained. This also makes it possible to increase the radius of curvature of the convex surface of the negative meniscus lens, so that the size of the attachment such as a filter mounted on the object side can be made smaller.

However, on the other hand, there is a drawback that the Petzval sum becomes more negative. Therefore, in the present invention, the focal length -f ₁ of the negative meniscus lens is 0. It is desirable that 7 <f ₁ / f <2 (2).

If the upper limit is exceeded, aberration correction will be more advantageous, but the negative refracting power of the first lens group G ₁ will be weakened, and the required effective diameter will be large, and therefore a large attachment size will be required. When the value goes below the lower limit, the negative refractive power of the first lens group G ₁ becomes too strong, and the Petzval sum becomes too large to be negatively corrected, which makes it difficult to satisfactorily correct field curvature and astigmatism. . And
If the upper limit of the condition (1) is set to 1.28, it is advantageous to maintain the aperture of the first lens group G _{1 in} a smaller size even in a lens having a wider angle of view.

In order to obtain a small lens having an effective diameter, when the effective diameter at the tangent plane of the apex of the surface closest to the object is φ and the half angle of view is θ, φ <3.3 · f · tan ² It is desirable to have a configuration that satisfies the condition of θ (3). Next, the rear lens group including the diaphragm will be described. As described above, the rear lens group has the second lens group G ₂ having a positive refracting power and the third lens group G ₃ having a positive refracting power with the diaphragm interposed therebetween. The second lens group G ₂ on the object side of the aperture has a positive refractive power,
On-axis color correction tends to be insufficient. This is advantageous in correcting the lateral chromatic aberration that tends to occur in the first lens group G ₁ having the negative refractive power located closest to the object side. Moreover, in a conventional wide-angle lens composed of only spherical surfaces, the negative refracting power lens group on the object side tends to generate a positive field curvature, which means that the coma of the light ray incident on the outside of the principal ray is negative. There is also a cause for becoming. Therefore, it is the second lens group G ₂ that has a great effect of correcting this coma aberration in the positive direction and correcting the image surface in the negative direction. In a conventional lens system, measures are taken such that the front negative lens is made large, the power of the positive lens is made large, and the convex surface is strongly bent toward the object side.

By using the above concave aspherical surface according to the present invention, the occurrence of negative coma in the first lens group G ₁ having a negative refractive power is reduced, and conversely, positive coma is generated. Also increases. Therefore, the second lens group G _{2 in} front of the diaphragm produces negative coma, and the first lens group G ₁
In many cases, it will be necessary to adopt a structure for correcting the positive coma aberration in. This can be realized within a range of techniques that can be easily applied by a conventionally known person skilled in the art, such as a bonding surface having a negative refractive power, as will be seen in Examples of the present invention described later. In addition, this method acts so that the Petzval sum moves toward the positive direction like a bonded surface with negative refracting power, and cancels the action of moving the Petzval sum of the front group having negative refracting power toward negative. , Often has the effect of keeping good overall aberrations at the same time. It is also effective in correcting spherical aberration, which tends to be negative compared to the case of using only a spherical system.

Next, the third lens group G ₃ located behind the diaphragm will be described. The lens group located behind the diaphragm needs to include at least two lenses, one for each of positive and negative, and at least two, in order to correct spherical aberration, chromatic aberration, and coma of light incident on the inside of the principal ray. Of course, the brighter the whole system of lenses is, the more lenses this group needs. In addition, it is desirable that a concave surface be present on the image side in contact with air in order to correct the property that the ray incident on the inside of the chief ray tends to be inward coma and to make the image plane flat. It is desirable that the air lens formed by the surface and the surface following it be biconvex.

In the structure of the present invention as described above, in order to maintain the brightness of F number of 2.0 or less, it is preferable to provide an aspherical surface in the third lens group G ₃ on the image side of the diaphragm. In order to correct the spherical aberration that depends on the aperture, it is effective to move the third lens group G _{3 to} the most object side, that is, the position closer to the diaphragm. In addition, when it also serves to correct off-axis aberrations, it is effective to provide an aspherical surface slightly on the image side of the diaphragm. In this case, the aspherical surface is effective as a reference curved surface at the apex.

[0025]

Embodiments of the present invention will now be described in detail with reference to the drawings. The following examples all have a back focus larger than the focal length. In the following numerical examples, the i-plane (i = 1.
2.3, ...) is the radius of curvature r _i , the i-th surface and the (i +
1) the surface distance on the optical axis from the surface is d _i , and the i-th surface and the (i +
1) The refractive index of the medium between the surface and the d-line is n _di , i-th
The Abbe number of the medium between the surface and the (i + 1) th surface is represented as ν _di (however, the refractive index of air and the Abbe number are blank).
In this case, the surface distance d _i of the last surface is the back focus, and the effective diameter of the lens on the first surface is the maximum effective diameter of the entire system.

In the first embodiment according to the present invention, as shown in the lens configuration diagram of FIG. 1, the first lens group G _{1 having} negative refracting power, the second lens group G _{2 having} positive refracting power, and the diaphragm S are sequentially arranged from the object side. , And a third lens group G _{3 having} a positive refractive power. The first lens group G ₁ is composed of a negative meniscus lens L ₁ having a convex surface facing the object side, and the second lens group G ₂ having a positive refractive power is a combination of a negative meniscus lens having a convex surface facing the object side and a positive lens. It is composed of a cemented positive lens L ₂ and a weak refractive power L ₃ . The third lens group G ₃ having a positive refractive power has a negative meniscus lens L ₄ having a convex surface directed toward the object side and a positive lens L ₅ having a positive meniscus lens having a convex surface directed toward the image side and a negative meniscus lens cemented together. , A positive meniscus lens L ₆ having a convex surface facing the image side, and a positive lens L ₇ having a surface having a stronger curvature facing the image side.

The values relating to the lens specifications and the aspherical surface of the first embodiment are as follows.

[0028]

(Table 1) (first embodiment) Focal length f = 100.000 F-number 1.8 Angle of view 2θ = 84.0 ° ir _i d _i n _di ν _di 1 166.398 10.000 1.6968 55.6 2 50.119 35.000 3 226.840 12.500 1.62041 60.1 4 75.799 70.833 1.74950 35.2 5 −798.914 3.000 6 607.693 7.500 1.74950 35.2 7 77.846 25.000 1.71300 53.9 8 − 1430.331 29.167 9 317.550 8.333 1.75692 31.6 10 164.697 10.417 11 − 1178.677 25.833 1.64006 60.0 12 − 64.710 7.500 1.75520 27.6 13 −129.205 0.833 14.10.392.316 12. −158.645 0.833 16 48325.827 12.500 1.69680 55.6 17 −370.156 158.342 (Value relating to aspherical surface) 2nd surface κ ＝ 0.527 C ₄ ＝ 0.47689 × 10 ^-7 C ₈ = 0 C ₆ = 0 C ₁₀ = 0 13th surface κ ＝ 1.000 C ₄ = 0.21013 × 10 ⁻⁷ C ₈ = 0 C ₆ = −0.26774 × 10 ⁻¹¹ C ₁₀ = 0 f ₁ /f=−1.067 φ = 172.5 = 2.13f · tan ² θ Various aberrations of the first embodiment The figure is shown in FIG. From this aberration diagram, it has a brightness of F number 1.8 at an angle of view of 84 °, and has an extremely excellent imaging performance in spite of the configuration in which the object side lens L ₁ has the smallest aperture. It is clear.

In the second embodiment of the present invention, as shown in the lens construction diagram of FIG.
A lens group G ₁ , a second lens group G _{2 having} a positive refractive power, a diaphragm S,
It is composed of a third lens group G _{3 having a} positive refractive power. First
The lens group G ₁ includes a negative meniscus lens L ₁ having a convex surface directed toward the object side, and the second lens group G ₂ having a positive refractive power has a negative meniscus lens having a convex surface directed toward the object side, a biconvex positive lens, and an image side. It is composed of a cemented positive lens L ₂ which is formed by laminating three negative meniscus lenses having a convex surface facing to and a positive lens L ₃ whose surface has a stronger curvature. The third lens group G ₃ having a positive refractive power is a positive lens L _{4 which} is a cemented double positive lens and a negative double lens, a positive meniscus lens L ₅ having a convex surface facing the image side, and a lens having a stronger curvature on the image side. It is composed of a positive lens L ₆ with its surface facing and a biconvex positive lens L ₇ .

The values relating to the lens specifications and the aspherical surface of the second embodiment are as follows.

[0031]

[Table 2] (Second Embodiment) Focal length f = 100.000 F number 2.0 Angle of view 2θ = 84.0 ° ir _i d _i n _di ν _di 1 214.393 6.161 1.63246 63.8 2 49.452 31.626 3 324.095 9.858 1.80411 46.5 4 65.635 62.020 1.66755 42.0 5 −10 2.654 8.215 1.58913 61.1 6 −774.419 0.411 7 131.479 36.555 1.56883 56.0 8 − 5039.657 15.813 9 253.995 32.037 1.67003 47.1 10 −176.610 10.268 1.78470 26.1 11 133.906 10.884 12 −361.549 12.322 1.60311 60.6 13 − 96.530 0.821 148913 6319 11.15 -227.130 0.411 16 636.648 8.625 1.56384 60.7 17 -817.920 158.852 (value related to the aspherical surfaces) the second surface _{κ = 0.430 C 4 = 0.99719 ×} 10 -7 C 8 = 0 C 6 = 0 C 10 = 0 f 1 / f = - 1.016 φ = 180 = 2.22f · tan ² θ FIG. 4 shows various aberration diagrams of the second example. From this aberration diagram, it has a brightness of F number 2.0 at an angle of view of 84 ° and has an extremely excellent imaging performance in spite of the configuration in which the aperture of the object side lens L ₁ is the smallest. It is clear.

Also in the third embodiment according to the present invention, as shown in the lens construction diagram of FIG.
A lens group G ₁ , a second lens group G _{2 having} a positive refractive power, a diaphragm S,
Although it is composed of the third lens group G _{3 having a} positive refractive power, it is an ultra wide-angle lens whose field angle reaches 100 °. The first lens group G ₁ is composed of a negative meniscus lens L ₁ having a convex surface facing the object side, and the second lens group G ₂ having a positive refractive power is a negative meniscus lens having a convex surface facing the object side and a biconvex positive lens. and a positive lens L ₃ toward the surface with a stronger curvature by cemented positive lens L ₂ and the object side cemented consisting together of. Also,
The third lens group G ₃ having a positive refractive power cemented with a negative meniscus lens having a convex surface directed toward the same image side and a positive meniscus lens having a convex surface directed toward the image side mating positive lens L _4, pasting of a biconvex lens and a biconcave lens It is composed of a compound lens L ₅ , a positive lens L ₆ with a surface having a stronger curvature facing the image side, and a biconvex positive lens L ₇ .

The values of the lens specifications and the aspherical surface of the third embodiment are as follows.

[0034]

[Table 3] (Third Example) Focal length f = 100.000 F number 2.8 Angle of view 2θ = 100.0 ° i r _i d _i n _di ν _di 1 453.027 9.783 1.63246 63.8 2 59.783 46.196 3 351.331 54.348 1.80411 46.5 4 68.542 43.478 1.59507 35.5 5 − 1142.440 0.543 6 137.125 65.217 1.60717 40.3 7 ∞ 11.960 8 −434.445 21.739 1.58913 61.1 9 − 55.435 8.152 1.80218 44.7 10 −107.065 13.315 11 763.587 22.283 1.54739 53.5 12 −114.098 6.522 1.80384 33.9 13 258.109 9.783 16 1.6282 204179. −100.419 0.543 16 2717.390 15.217 1.62041 60.1 17 −203.516 205.125 (Value related to aspherical surface) 2nd surface κ = 0.000 C ₄ = 0.15574 × 10 ^-6 C ₈ = 0 C ₆ = 0 C ₁₀ = 0 f ₁ / f =- 1.089 φ = 245 = 1.73f · tan ² θ FIG. 6 shows various aberration diagrams of the third example. From this aberration diagram, it is possible to obtain an extremely excellent imaging performance despite the fact that the object-side lens L ₁ has the smallest aperture while having an F-number of 3.5 with a super-wide angle of view of 110 °. It is clear that

Also in the fourth embodiment, as shown in the lens configuration diagram of FIG. 6, the first lens group G _{1 having} a negative refractive power, the second lens group G _{2 having} a positive refractive power, the diaphragm S, and the positive lens element are arranged in this order from the object side. It is composed of the third lens group G _{3 having a} refractive power, but the angle of view is 100
It is an ultra-wide-angle lens that reaches °. The first lens group G ₁ is composed of a negative meniscus lens L ₁ having a convex surface facing the object side, and the second lens group G ₂ having a positive refractive power is a negative meniscus lens having a convex surface facing the object side and a biconvex positive lens. 3 with negative lens
It is composed of a cemented positive lens L ₂ formed by laminating two sheets and a biconvex positive lens L ₃ . Further, the third lens group G ₃ having a positive refractive power cemented positive lens L ₄ of a negative meniscus lens having a convex surface directed toward the positive lens and the image side, laminating lens L ₅ between the positive lens and the negative lens, the image side Is composed of a positive lens L ₆ and a biconvex positive lens L ₇ with a surface having a stronger curvature.

The values relating to the lens specifications and the aspherical surface of the fourth embodiment are as follows.

[0037]

(Table 4) (Fourth Example) Focal length f = 100.000 F number 3.5 Angle of view 2θ = 110.0 ° ir _i d _i n _di ν _di 1 320.261 9.804 1.77279 49.5 2 73.529 63.331 3 281.045 25.393 1.80411 46.5 4 84.479 103.555 1.60342 38.0 5 − 92.924 13.072 1.69680 55.6 6 − 2241.584 0.654 7 220.832 36.601 1.60342 38.0 8 − 466.738 13.072 9 1289.284 27.451 1.60342 38.0 10 − 67.218 15.686 1.77279 49.5 11 −280.933 13.072 12 − 1293.096 19.551 1.57501 41.4 13 − 71.895 14.804 1.93 15 -236.968 20.915 1.58913 61.1 16 - 90.196 0.654 17 1669.346 23.529 1.56384 60.7 18 -162.792 248.439 (value related to the aspherical surfaces) the second surface _{κ = 0.312 C 4 = 0.34757 ×} 10 -8 C 8 = 0 C 6 = 0 C 10 = 0 f ₁ /f=−1.257 φ = 295 = 1.45 f · tan ² θ FIG. 8 shows various aberration diagrams of the fourth embodiment. From this aberration diagram, it is possible to obtain an extremely excellent imaging performance, despite having a configuration in which the aperture of the object-side lens L ₁ is the smallest while having an F-number of 3.5 and a super wide angle of view of 110 °. It is clear that

[0038]

As described above, according to the present invention,
As seen in the examples, it is possible to obtain a wide-angle photographic lens having a brightness reaching an F number of 1.8 at an ultra-wide angle of view of 84 ° to 110 °, a small size, and good correction of aberrations. Then, a simple lens configuration is possible without the need for a positive lens that has been arranged mainly for the purpose of correcting distortion in the negative lens group on the object side,
Together with the downsizing of the front lens aperture, the volume of the lens is further reduced, which contributes to the weight reduction of the lens. Moreover, in the aspherical surface according to the present invention, the coma aberration can be corrected well, so that there is also an effect that it is possible to sufficiently secure the light quantity in the peripheral portion of the screen, which is often insufficient in the wide-angle lens.

[Brief description of drawings]

FIG. 1 is a lens configuration diagram of a first embodiment of the present invention.

FIG. 2 is a diagram of various types of aberration of the first embodiment of the present invention.

FIG. 3 is a lens configuration diagram of a second embodiment of the present invention.

FIG. 4 is a diagram of various types of aberration of the second embodiment of the present invention.

FIG. 5 is a lens configuration diagram of a third embodiment of the present invention.

FIG. 6 is a diagram of various types of aberration of the third embodiment of the present invention.

FIG. 7 is a lens configuration diagram of a fourth embodiment of the present invention.

FIG. 8 is a diagram of various types of aberration of the fourth embodiment of the present invention.

[Explanation of symbols]

G ₁ ... First lens group having negative refracting power G ₂ ... Second lens group having positive refracting power G ₃ ... Third lens group having positive refracting power S ...

Claims (3)

[Claims] 1. A first lens group G ₁ having a negative refractive power arranged closer to the object side than a stop, and a second lens group having a positive refractive power arranged between the first lens group G ₁ and the diaphragm. In the reverse telephoto wide-angle lens having G ₂ and a third lens unit G ₃ having a positive refractive power arranged on the image side of the diaphragm and having a back focus larger than the focal length of the entire system, 1 lens group G
₁ has a negative lens with a convex surface facing the object side, and the concave surface of the negative lens in contact with air on the image side is an aspherical surface whose curvature decreases monotonically as it moves away from the optical axis and which is rotationally symmetric with respect to the optical axis. Yes, x is the distance from the vertex of the curved surface in the optical axis direction, y is the distance from the optical axis, C is the curvature at the vertex of the curve, and C ₄ , C ₆
Is a constant, the aspherical shape of the concave surface of the negative lens in the meridional section is: x = Cy ² / {1+ (1-κC ² y ² ) ^1/2 } + C ₄ y ⁴ + C
An inverse telephoto wide-angle lens characterized by having a range of ₆ y ⁶ + ... And −1 <κ <0.8. 2. The focal length of the entire system of the reverse telephoto lens is f,
The negative lens having the aspherical surface has a focal length of −f _1, which satisfies the condition of 0.7f <f ₁ <2f φ <3.3 · f · tan ² θ. Reverse telephoto wide-angle lens. Wherein an effective diameter of at tangent plane at the apex of the most object side surface of the negative lens having the aspherical phi, when the half field angle _{θ, 0 · 7f <f 1} <2f φ <3. The inverse telephoto wide-angle lens according to claim 2, wherein the condition of 3 · f · tan ² θ is satisfied.