JPH0593852A - Distortion correction filter - Google Patents

Abstract

PURPOSE:To provide the filter which corrects the remaining distortion of a photographic lens. CONSTITUTION:This filter is a flat plate type filter F which is fitted to the photographic lens Ob and used, its refractive index varies with the radial distance from the optical axis, and ¦N1.t.f¦<0.05 holds. Here, N1 is a coefficient showing the refractive index distribution of a radial type heterogeneous lens and the coefficient is obtained by expanding the refractive index into n(y)=N0+ N1y<2>+N2y<4>+..., where (y) the distance in the direction perpendicular to the optical axis, n(y) the refractive index at a position of a radius (y), (t) the thickness of the filter, and (f) the focal length of the photographic lens Ob fitted with the filter F.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a distortion correction filter, and more particularly to a distortion correction filter using a radial type inhomogeneous lens whose refractive index changes with distance from the optical axis in the radial direction.

[0002]

2. Description of the Related Art In a photographing lens such as a camera, it is desirable that the distortion (distortion aberration) be favorably corrected in order to photograph a subject as faithfully as possible. It is not preferable to shoot with a photographic lens having a large distortion, even if a rectangular object is shot, because it is reflected in a barrel shape or a spool shape.

However, at the wide-angle end of a wide-angle lens or zoom lens, a large negative (barrel-shaped) distortion often occurs due to the characteristics of the lens system. this is,
This is because, in these wide-angle lens systems, in general, in order to secure the back focus of the lens system, the lens system has a so-called retrofocus type power arrangement of negative and positive in order from the object side. At the telephoto end of the zoom lens, on the contrary, positive (pincushion) distortion tends to occur.

In the actual use of these taking lenses, at present, distortion is allowed to some extent.

[0005]

The present invention has been made in view of such circumstances, and an object thereof is to provide a filter for correcting the distortion remaining in the taking lens as described above. That is.

[0006]

In the present invention, in order to solve the above-mentioned problems, a distortion correction filter having the following constitution is attached in front of the taking lens and used.

That is, a flat plate-shaped filter used by being attached to a photographing lens or the like is characterized in that its refractive index changes with the distance from the optical axis in the radial direction and satisfies the following conditions. It is a distortion correction filter.

(1) | N ₁ · t · f | <0.05 where N ₁ is a coefficient representing the refractive index distribution of the radial type inhomogeneous lens, and the distance in the direction orthogonal to the optical axis is y. , When the refractive index at the position of radius y is expressed as n (y),
This refractive index is n (y) = N ₀ + N ₁ y ² + N ₂ y ⁴ +.
... is a coefficient when it is expanded, t is the thickness of the filter, and f is the focal length of the photographic lens or the like to which the filter is attached.

[0009]

The reason and operation of adopting the structure of the present invention will be described below. In the present invention, it is considered that the distortion remaining in the taking lens is corrected by attaching the correction filter in front of the taking lens.

That is, it is possible to correct the distortion generated in the taking lens by attaching another lens system in front of the taking lens. However, when a lens system having a refractive power is attached, there are problems that the focal length of the entire photographing lens system changes, the angle of view changes, and the image forming position changes. Further, when the afocal system is attached, the image forming position does not change, but in general, the focal length of the entire system changes and the angle of view also changes.
If an afocal optical system with an afocal magnification of 1 is attached, only the aberration can be controlled, but with a normal afocal optical system, the entire lens system becomes large.

Therefore, according to the present invention, in order to realize an afocal optical system having an afocal magnification of 1 with a simple structure, the refractive index is increased inside the flat glass plate as the distance from the optical axis increases in the radial direction of the lens. We considered a radial type inhomogeneous lens system in which the value changes.

The refractive index distribution of the radial type inhomogeneous lens is
The distance in the direction orthogonal to the optical axis is y, the refractive index at the radius y is n (y), the refractive index on the optical axis (base refractive index) is N ₀ ,
When the distribution coefficient is N ₁ , N _2, ..., It is expressed by the following equation (a).

(A) n (y) = N ₀ + N ₁ y ² + N ₂ y ⁴ + ... The medium of this radial type inhomogeneous lens is roughly the refractive power φ _m given by the following equation (b). have.

(B) φ _m ≈−2 · N ₁ · t Here, N ₁ is the distribution coefficient of the equation (a), and t is the thickness of the lens.

As described above, the radial type inhomogeneous medium is
Generally, it has refractive power, but if the refractive index distribution used for the filter is appropriately selected, only the aberration can be controlled without the refractive power of the filter itself. With this configuration, it is possible to correct only the distortion without changing the angle of view or changing the image forming position even if the filter is attached. Therefore, in the present invention, the following condition (1) is set by considering a filter including one flat plate radial type inhomogeneous lens.

(1) | N ₁ · t · f | <0.05 where f is the focal length of the taking lens to which the filter is attached.

The condition (1) is a condition for preventing the medium of the radial type inhomogeneous lens from having too much refractive power. When this condition is satisfied, the medium does not have much refractive power, and the refractive power of the entire system of the flat plate radial type inhomogeneous lens becomes almost zero. If the range of this condition (1) is exceeded, the medium has a large refractive power, and when mounted in front of the taking lens, the angle of view and the image forming position change greatly, which is not preferable.

In addition, the paper "PJ Sands, JO"
pt. Soc. Am. 60 , 1436 (1971) ”, it is shown that when N ₀ and N ₁ are fixed, the third-order distortion generated in the radial type heterogeneous medium is determined by N ₂ . Therefore, considering the correction of the distortion using the coefficient N ₂ , the following condition (2) is set.

(2) N ₂ <0 (when correcting negative distortion) N ₂ > 0 (when correcting positive distortion) This condition (2) is a condition for favorably correcting distortion. .. If the range of the condition (2) is exceeded, the distortion will worsen, which is not preferable.

The principle of correcting the distortion when the above conditions (1) and (2) are satisfied will be described by taking the case of correcting the negative distortion as an example. Figure 1
As shown in (b), consider a case where the distortion correction filter F of the present invention is inserted on the object side of the taking lens Ob. It is assumed that the filter F has a refractive index distribution as shown in FIG. 1A that satisfies the conditions (1) and (2). Since the coefficient N ₁ is almost 0 and the coefficient N ₂ is negative,
As shown in FIG. 1A, the refractive index distribution is flat near the center and gradually lowers toward the periphery.
An off-axis chief ray without the filter F is shown by a dotted line in FIG. 1B, and at this time, it is assumed that a large negative distortion is generated as shown by a dotted line in FIG. .. When the filter F is inserted in such a state, the off-axis chief ray is bent inward as shown by the solid line in FIG. 1B due to the refractive index distribution. This is because the light ray has a property of bending from a lower refractive index to a higher refractive index. As a result, the distortion is corrected as shown by the solid line in FIG. Also, since the center of the refractive index distribution is flat,
The paraxial amount of the taking lens does not change.

The condition (2) is a condition assuming that the taking lens and the diaphragm are on the image side of the filter.
If there is a diaphragm on the object side of the filter, the conditions are exactly opposite.

Further, in order to satisfactorily correct only the distortion without affecting aberrations other than the distortion as much as possible, the height of the off-axis chief ray is higher than that of the axial marginal ray of the taking lens. It is desirable to arrange the filter of the present invention in The object side of the taking lens often has such a condition.

Further, in the present invention, a filter for correcting distortion is considered, but conversely, the concept of the present invention can be applied to a filter for photographing for worsening distortion and aiming at a special effect.

[0024]

Embodiments of the distortion correction filter of the present invention will now be described with reference to the drawings. First, before describing the embodiments, an example of a taking lens to which the distortion correction filter of the present invention is attached will be described.

Example of taking lens f = 8.26 to 23.22 F _NO = 2.80 to 2.80 ω = 28.3 ° to 9.9 ° Maximum image height = 4 r ₁ = 24.7617 d ₁ = 1.2000 e ₁ = 15.000 n _d1 = 1.84666 ν _d1 = 23.78 r ₂ = 17.1917 d ₂ = 0.5000 e ₂ = 13.450 r ₃ = 17.9223 d ₃ = 8.0000 e ₃ = 13.450 nd ₂ = 1.77250 ν _d2 = 49.66 r ₄ = 292.8233 d ₄ = (variable) e ₄ = 13.450 r ₅ = 50.4082 d ₅ = 0.9000 e ₅ = 7.000 n _d3 = 1.69680 ν _d3 = 55.52 r ₆ = 8.4202 d ₆ = 3.4000 e ₆ = 5.400 r ₇ = -16.7250 d ₇ = 0.9000 e ₇ = 5.400 n _d4 = 1.69680 ν _d4 = 55.52 r ₈ = 35.7904 d ₈ = (variable) e ₈ = 5.000 r ₉ = 21.8570 d ₉ = 2.0000 e ₉ = 5.000 n _d5 = 1.84666 ν _d5 = 23.78 r ₁₀ = -326.2179 d ₁₀ = (variable) e ₁₀ = 5.000 r ₁₁ = ∞ (aperture) d ₁₁ = 1.5000 e ₁₁ = 2.507 r ₁₂ = 9.3737 d ₁₂ = 2.5000 e ₁₂ = 3.000 n _d6 = 1.74950 ν _d6 = 35.27 r ₁₃ = -14.4963 d ₁₃ = 1.1838 e ₁₃ = 3.000 r ₁₄ =- 7.3176 d ₁₄ = 1.0000 e ₁₄ = 3.000 n _d7 = 1.84666 ν _d7 = 23.78 r ₁₅ = 10.8802 d ₁₅ = 1.8442 e ₁₅ = 3.000 r ₁₆ = -14.1044 d ₁₆ = 2.3000 e ₁₆ = 3.700 n _d8 = 1.60311 ν _d8 = 60.70 r ₁₇ = -7.2266 d ₁₇ = 0.1500 e ₁₇ = 4.300 r ₁₈ = 23.7943 d ₁₈ = 3.8000 e ₁₈ = 5.300 n _d9 = 1.60311 ν _d9 = 60.70 r ₁₉ = -12.2012 d ₁₉ = 2.0000 e ₁₉ = 5.500 r ₂₀ = ∞ d ₂₀ = 15.0000 e ₂₀ = 6.000 n _d10 = 1.51633 ν _d10 = 64.15 r ₂₁ = ∞ e ₂₁ = 6.000 Zoom interval

In the above, f is the focal length of the entire system, F _NO
Is the F number, ω is the half angle of view, r ₁ , r ₂ ... Is the radius of curvature of each lens surface, d ₁ , d ₂ ... Is the distance between the lens surfaces,
e _1, e ₂ ... are effective radius of each lens surface, n _d1, n _d2 ... d-line refractive index of each lens, ν _d1, ν _d2 ... is the Abbe number of each lens.

2 (a) and 2 (b) show the lens cross sections at the wide-angle end and the telephoto end of the taking zoom lens M, respectively (however, as will be described later, the sectional view is shown with the distortion correction filter of the present invention attached). It is shown.). Further, aberration diagrams at the wide-angle end and the telephoto end with respect to an object point at infinity are shown in FIGS. 3 (a) and 3 (b), respectively. From these aberration diagrams, it is understood that in the taking lens M, a large amount of distortion is produced in the negative direction at the wide-angle end and in the positive direction at the telephoto end.

Next, an embodiment of the distortion correction filter of the present invention will be shown.

Example 1 Surface number Curvature radius Radius spacing Effective radius Medium 1 ∞ 5.0 20.0 Radial inhomogeneity 1 2 ∞ 5.0 (d ₀ ) 20.0 Distribution coefficient of radial inhomogeneity 1 Wavelength (nm) N ₀ N ₁ N ₂ N ₃ or more 587.56 (d line) 1.70154 0 -2.000 × 10 ^-7 0 656.28 (C line) 1.69651 0 -2.000 × 10 ^-7 0 486.13 (F line) 1.71352 0 -2.000 × 10 ^-7 0 435.84 (g line) 1.72333 0 -2.000 × 10 ^-7 0.

Example 2 Surface number Radius of curvature Surface spacing Effective radius Medium 1 ∞ 5.0 20.0 Radial inhomogeneity 2 2 ∞ 5.0 (d ₀ ) 20.0 Radial inhomogeneity 2 Distribution coefficient Wavelength (nm) N ₀ N ₁ N ₂ N ₃ or more 587.56 (d line) 1.70154 0 1.000 × 10 ^-7 0 656.28 (C line) 1.69651 0 1.000 × 10 ^-7 0 486.13 (F line) 1.71352 0 1.000 × 10 ^-7 0 435.84 (g line) 1.72333 0 1.000 × 10 ^-7 0.

The first embodiment described above is a filter for correcting negative (barrel-shaped) distortion, and N ₁ = 0, N
_{When 2} <0, the refractive index distribution is high in the central part and gradually decreases toward the periphery. However,
Since the coefficient N ₁ is 0, the filter itself has no refractive power. FIG. 2A shows an arrangement in which the filter F ₁ of the first embodiment is attached to the object side d _{0 in} the state where the taking lens M is at the wide-angle end. An aberration diagram for an object point at infinity at that time is shown in FIG. Comparing FIG. 4A and FIG. 3A, it can be seen that the distortion that has largely appeared in the negative direction is significantly improved and other aberrations are hardly changed. Therefore, if this filter F ₁ is attached in front of the taking lens M as required, an image in which distortion is favorably corrected can be obtained.

The second embodiment is a filter for correcting positive (pincushion type) distortion, and N ₁ = 0,
When N ₂ > 0, the refractive index distribution is low at the center and gradually increases toward the periphery. However, since the coefficient N ₁ is 0, the filter itself has no refractive power. FIG. 2B shows the filter F _{2 of the} second embodiment when the taking lens M is at the wide-angle end.
Is attached to the position on the object side d ₀ . Fig. 4 (b) shows the aberration diagram for the object point at infinity at that time.
Shown in. Comparing FIG. 4B and FIG. 3B, it can be seen that the distortion that largely appears in the positive direction is significantly improved and other aberrations are hardly changed. Therefore, by attaching this filter F ₂ in front of the taking lens M as required, an image with favorable distortion can be obtained.

Although some embodiments of the present invention have been described above, the present invention is not limited to these embodiments and various modifications can be made.

[0034]

As described above, according to the present invention,
It is possible to realize a distortion correction filter that can effectively correct distortion that remains in the taking lens with a simple configuration.

[Brief description of drawings]

FIG. 1 is a diagram for explaining the principle of distortion correction of the present invention.

FIG. 2 is a diagram showing a lens cross section at a wide-angle end and a telephoto end of an example of a photographing zoom lens in which a distortion correction filter according to an exemplary embodiment of the present invention is attached.

FIG. 3 is an aberration diagram at a wide-angle end and a telephoto end of the photographing zoom lens itself in FIG.

FIG. 4 is an aberration diagram at a wide-angle end and a telephoto end of the taking zoom lens in FIG. 2 after the distortion correction filter according to the example of the present invention is attached.

[Explanation of symbols]

Ob, M ... Shooting lens F, F ₁ , F ₂ ... Distortion correction filter

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[Procedure amendment]

[Submission date] May 8, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0032

[Correction method] Change

[Correction content]

The second embodiment is a filter for correcting positive (pincushion type) distortion, and N ₁ = 0,
When N ₂ > 0, the refractive index distribution is low at the center and gradually increases toward the periphery. However, since the coefficient N ₁ is 0, the filter itself has no refractive power. FIG. 2B shows the filter F _{2 of the} second embodiment when the taking lens M is at the telephoto end.
Is attached to the position on the object side d ₀ . Fig. 4 (b) shows the aberration diagram for the object point at infinity at that time.
Shown in. Comparing FIG. 4B and FIG. 3B, it can be seen that the distortion that largely appears in the positive direction is significantly improved and other aberrations are hardly changed. Therefore, by attaching this filter F ₂ in front of the taking lens M as required, an image with favorable distortion can be obtained.

Claims (1)

[Claims] 1. A flat plate-shaped filter used by being attached to a photographing lens or the like, characterized in that its refractive index changes as it moves away from the optical axis in the radial direction, and that the following condition is satisfied. Distortion correction filter: (1) | N ₁ · t · f | <0.05 where N ₁ is a coefficient representing the refractive index distribution of the radial type inhomogeneous lens, and the distance in the direction orthogonal to the optical axis is y. And the refractive index at the position of radius y is expressed as n (y),
This refractive index is n (y) = N ₀ + N ₁ y ² + N ₂ y ⁴ +.
... is a coefficient when it is expanded, t is the thickness of the filter, and f is the focal length of the photographic lens or the like to which the filter is attached.