JPH05127081A - Endoscope objective - Google Patents

Abstract

PURPOSE:To decrease the number of lenses and to excellently compensate aberrations by bringing a cemented lens into contact with an image pickup element and satisfying specific conditions. CONSTITUTION:This objective system consists of a 1st lens L1 with negative power, a 2nd lens L2 which has positive power and also has a convex surface on the image side, a brightness stop S, a filter group F which is arranged right behind the brightness stop, and a cemented lens which consists of a 3rd biconvex lens L3 and a 4th plano-convex lens L4 and is brought into contact with the image pickup element in order from the object side; and the conditions shown by inequalities I are satisfied. Here, Im is the maximum image height of the lens system, fR the composite focal length of the 3rd and 4th lenses, f1 the focal length of the 1st lens, and f2 the focal length of the 2nd lens.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an endoscope objective lens.

[0002]

2. Description of the Related Art As an endoscope, a fiberscope using an image guide fiber is widely used.
However, recently, videoscopes using various solid-state image pickup devices have been widely used instead of image guides.

Image pickup methods of the videoscope mainly include a frame sequential method and a dot sequential method. Here, the dot-sequential method is a solid-state image sensor in which a color coding filter (usually called a color mosaic filter) in which minute color filters are integrated in a mosaic pattern corresponding to each pixel of the solid-state image sensor is used. It is provided on the incident side of to obtain a color signal.

When a mosaic type image pickup device is used, each color filter is arranged on each picture element of the solid-state image pickup device. However, if the distance between the filter and the light-receiving surface of the light-receiving element of the solid-state image sensor is large, a light ray having a large incident angle with respect to the solid-state image sensor will pass through the color filter and then enter the pixel that should be originally incident. However, the image obtained by being incident on the picture element adjacent to it is uneven in color (called color shading).
May occur. This cannot prevent color shading in a lens system of a type that obliquely enters the image sensor, such as the one described in Japanese Patent Laid-Open No. 62-173415 shown in FIG.

Further, as a means for solving the above-mentioned drawbacks, there is an optical system described in JP-A-2-74912.

This optical system is as shown in FIG. 20, and in order from the object side, a concave lens having a strong negative power, a convex lens, an aperture stop, an infrared cut filter, a cemented lens, a field lens, and an optical element closest to the image side. A low pass filter is arranged.

In this optical system, a convex lens is arranged in front of the aperture stop in order to correct coma and astigmatism generated by the concave lens having a strong negative power. The convex lens also corrects the chromatic aberration of magnification generated by the concave lens.

However, this objective lens has a large number of constituent lenses.

Further, if the number of lenses is reduced and a relatively simple structure is attempted, the power of each lens generally increases, and various aberrations increase accordingly. In order to correct this aberration, the total length of the lens system becomes long and the compactness required by the endoscope cannot be maintained. If the cemented lens in the rear group is a single lens, it becomes impossible to sufficiently correct lateral chromatic aberration.

Further, an optical low-pass filter such as a crystal filter arranged immediately before the solid-state image pickup device becomes very expensive because the solid-state image pickup device becomes smaller as the endoscope becomes thinner and becomes very expensive.

[0011]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an endoscope objective lens which is a telecentric system, has a wide angle, a short overall length, a small outer diameter, a small number of lenses and a well-corrected aberration. It is in.

[0012]

The objective lens of the present invention comprises:
As shown in FIG. 9, in order from the object side, a first lens L ₁ having negative power, a second lens L ₂ having positive power and having a convex surface on the image side, an aperture stop S, and an aperture stop. A lens system in which a cemented lens of a filter group F disposed immediately after, a third lens L _{3 that} is a biconvex lens, and a fourth lens L ₄ that is a plano-concave lens, and the cemented lens is in close contact with an image sensor Then, the following conditions are satisfied. (1) 0.18 <I _m / f _R <0.642 (2) 0.1 <| f ₁ / f ₂ | <2 where I _m is the maximum image height of the lens system and f _R is the third lens The composite focal length of the fourth lens, f ₁ is the focal length of the first lens, and f ₂ is the focal length of the second lens.

Further, in the present invention, at least one surface having an optical low-pass effect is provided near the aperture stop.

The present invention integrates the functions of a cemented lens and a field lens, which are arranged behind the aperture stop in the lens system described in Japanese Patent Laid-Open No. 2-74912, without deteriorating the aberration correction capability. Therefore, the birefringent plate arranged immediately before the image pickup element for integration is eliminated.

In the objective lens of the present invention, as described above, the second lens L ₂ having a positive power is arranged between the first lens L ₁ having a negative refractive power and the aperture stop S, and the second lens L ₂ is provided for an image. By directing the convex surface, a coma aberration on the positive side opposite to the coma aberration on the negative side generated on the concave surface of the first lens L ₁ is generated on the rear surface of the second lens L ₂ , so that the coma aberration on the front side of the aperture stop is generated. Can be corrected well. Here, as for the sign of coma aberration, the direction of forming an image on the optical axis side of the principal ray on the glass image plane is minus, and the opposite direction is plus.

Next, regarding the astigmatism, the first lens L ₁
The positive astigmatism generated on the concave surface of the second lens L ₂
Negative astigmatism is generated on the rear surface to correct the aberration. In this way, it is possible to eliminate coma and astigmatism occurring in the front group before the stop in the front group.

On the rear side of the aperture stop, it has a function as a field lens and is arranged so that the chief ray is incident almost vertically on the image pickup device, for example, CCD.

At this time, the third lens L ₃ and the diaphragm S are opened to some extent so that the power of the third lens L ₃ is not too strong, and the third lens L ₃ is generated on the object side surface. If the astigmatism is not kept small, the cemented surface cannot correct it. Further, the astigmatism generated at the front group in the insufficient correction, the third lens when so as to balance with the surface on the object side of L _3, an aperture stop S and the aberration correction on between the third lens L ₃ Is also advantageous, you can take space to put the filter.

As described above, it is possible to satisfactorily correct other than chromatic aberration.

The chromatic aberration can be independently corrected before and after the aperture stop, and since the second lens is arranged on the object side of the aperture stop, it is canceled by the first lens and the second lens. You can make it fit. In this way, it is possible to increase the power of both the first lens and the second lens, so that the size can be reduced.

Even at the rear side of the aperture stop, the negative chromatic aberration generated by the convex power of the third lens can be sufficiently corrected by the cemented surface of the third lens and the fourth lens.

In recent years, CCDs have also increased in number of pixels, and since there are no objects having a high spatial frequency in the human body (stomach or intestine), moire due to interference between the objects and the CCD is unlikely to occur. Therefore, the low-pass filter made of crystal or the like, which is conventionally arranged at this position for removing moire, can be omitted, and a field lens including a cemented lens can be obtained.

The optical system having such a structure requires conditions for disposing a filter in the rear group.

The condition (1) is such that the angle of incidence of the incident light on the filter is limited when the filter is arranged.

In the case of a videoscope using an infrared cut filter, a YAG cut filter, etc., it is desirable to arrange the above filter immediately after the aperture stop so as to satisfy the condition (1) as described above.

As shown in FIG. 9, an optical system in which the chief ray incident on the image plane is vertical is expressed as follows, where the angle with respect to the optical axis of the off-axis chief ray passing through the aperture stop is θ '. You can sin θ '= I / f _{R where} I is the image height.

Usually, the interference type infrared cut filter is
When the incident angle of the light beam becomes large, the transmittance in the infrared region sharply increases and it becomes impossible to block the light in the infrared region.

Further, in the absorption type filter, when the incident angle of a light beam becomes large, a difference occurs in the glass path due to a difference in image height.
This may cause uneven color on the screen.

From the above, both the interference type filter and the absorption type filter need to have a small incident angle to them. The angle is about 40 ° or less, and if the upper limit of the condition (1) is satisfied, color unevenness can be removed. On the contrary, when the value exceeds the upper limit of the condition (1), color unevenness occurs, which is not preferable.

On the other hand, if the value goes below the lower limit, the total length of the rear group becomes long and the optical system becomes large, which is not preferable. The condition (2) is for favorably correcting the aberration in the front group.

If the lower limit of this condition (2) is exceeded, the power of the first lens is too strong for the power of the second lens, and the first lens
Negative coma and positive astigmatism produced by the lens cannot be corrected by the second lens.

On the contrary, if the upper limit of the condition (2) is exceeded, the correction by the second lens becomes excessive, which is not preferable.

In order to correct the chromatic aberration in the front group to some extent while satisfying the above condition (2), it is desirable to satisfy the following condition (3). (3) | ν ₁ −ν ₂ | <15 where ν ₁ and ν ₂ are the Abbe numbers of the first lens and the second lens, respectively. If this condition (3) is not satisfied, it becomes impossible to correct chromatic aberration in the front group, and it becomes impossible to correct lateral chromatic aberration and axial chromatic aberration at the same time even when trying to correct in the rear group. Next, in order to secure a space for placing the filter immediately after the aperture stop, it is preferable to satisfy the following condition (4). _{(4) 0.5f <f R <} 5f objective lens of the present invention, a telecentric system is that since as can be seen from Figure _{3 f R ≒ D s (D} s is the object side of the third lens from the aperture stop Air equivalent length to the top of the surface)
The relationship is established.

D _s is a space for arranging the filter immediately after the diaphragm, and condition (4) is a condition for arranging the filter having a certain characteristic.

If the lower limit of the condition (4) is exceeded, the filter cannot be arranged. If the upper limit is exceeded, the overall length of the lens system will be too long and the size will be too large to be practical. If you try to force it to be shorter by the front group, the power of the rear group will become too weak and it will not be a telecentric system.

In order to suppress the occurrence of chromatic aberration in the rear group to some extent, it is desirable to satisfy the following condition (5). (5) | ν ₃ −ν ₄ | ≧ 20 where ν ₃ and ν ₄ are the Abbe numbers of the third lens and the fourth lens, respectively.

To independently suppress the chromatic aberration of magnification in the rear group, it is necessary to satisfy the above condition (5).

Further, since this objective lens has a fixed focus, the F number cannot be increased so much because the depth of field is gained. Therefore, the thickness of the light beam incident on the CCD is limited, and when dust adheres to the surface close to the CCD, the shadow thereof is easily reflected on the CCD.

In order to solve this drawback, in the present invention, the field lens, that is, the cemented lens of the third lens L ₃ and the fourth lens L ₄ in FIG. 3 is constructed to help prevent dust adhesion. That is, the surface on the image side composed of the third lens L ₃ and the fourth lens L ₄ is made to be a flat surface and directly adhered or adhered to the CCD cover glass or the CCD so that the following condition (6) is satisfied. There is. (6) 0.5f <D _R <5f where D _R is the air-equivalent length from the object side surface of the third lens L ₃ to the image forming position.

In this lens system, dust is most likely to adhere to the surface of the third lens L _{3 on} the object side. If the distance from this surface is large, the influence of dust is eliminated.
That is, it is desirable that D _R is equal to or more than the lower limit value of the condition (6), and if it exceeds the lower limit of this condition, the shadow of dust is likely to appear. On the other hand, if the upper limit of the condition (6) is exceeded and D _R becomes large, the principal ray enters vertically, and the outer diameter of the field lens becomes large, which is not preferable in practice.

[0041]

EXAMPLES Next, examples of the endoscope objective lens of the present invention will be shown. Example 1 f = 1.000, F / 5.481, IH = 0.8021, object distance = -9.7222 r ₁ = ∞ d ₁ = 0.2222 n ₁ = 1.88300 ν ₁ = 40.78 r ₂ = 0.5376 d ₂ = 0.3472 r ₃ = 1.3829 d _{3 = 0.9208 n 2 = 1.64769 ν} 2 = 33.80 r 4 = -0.9650 d 4 = 0.1042 r 5 = ∞ ( _{stop) d 5 = 0.0208 r 6 =} ∞ d 6 = 0.2778 n 3 = 1.52287 ν 3 = 59.89 r 7 = ∞ d ₇ = 0.0208 r ₈ = ∞ d ₈ = 1.0417 n ₄ = 1.52000 ν ₄ = 74.00 r ₉ = ∞ d ₉ = 0.7549 r ₁₀ = 1.6092 d ₁₀ = 1.0488 n ₅ = 1.72916 ν ₅ = 54.68 r ₁₁ = -1.8194 _{d 11 = 0.8532 n 6 = 1.84666} ν 6 = 23.78 r 12 = ∞ d 12 = 0.2778 n 7 = 1.52287 ν 7 = 59.89 r 13 = ∞ I m / f R = 0.326, | f 1 / f 2 | = 0.587, | Ν ₁ −ν ₂ | = 6.98, f _R /f=2.461, | ν ₃ −ν ₄ | = 30.9, D _R /f=1.29 Example 2 f = 1.000, F / 5.636, IH = 0.7804, physical point Distance = -9.4595 r ₁ = ∞ d ₁ = 0.1741 n ₁ = 1.51633 ν ₁ = 64.15 r ₂ = 0.4113 d ₂ = 0.6757 r ₃ = -30.5166 d ₃ = 0.3971 n ₂ = 1.61405 ν ₂ = 54.95 r ₄ = -0.7508 d ₄ = 0.1014 r ₅ = ∞ (diaphragm) d ₅ = 0.0203 r ₆ = _{∞ d 6 = 0.2703 n 3 =} 1.52287 ν 3 = 59.89 r 7 = ∞ d 7 = 0.0203 r 8 = ∞ d 8 = 1.0135 n 4 = 1.52000 ν 4 = 74.00 r 9 = ∞ d 9 = 0.9285 r 10 = 1.4373 d _{10 = 1.3514 n 5 = 1.65844 ν} 5 = 50.86 r 11 = -1.4377 d 11 = 0.5100 n 6 = 1.84666 ν 6 = 23.78 r 12 = ∞ d 12 = 0.2703 n 7 = 1.52287 ν 7 = 59.89 r 13 = ∞ I m _{/ f R = 0.293, | f} 1 / f 2 | = 0.639, | ν 1 -ν 2 | = 9.2, f R /f=2.659, | ν 3 -ν 4 | = 27.08, D R /f=1.31 embodiment example 3 f = 1.000, F / 5.297 , IH = 0.8128, object distance _{= -9.8522 r 1 = ∞ d 1} = 0.2252 n 1 = 1.88300 ν 1 = 40.78 r 2 = 0.5321 d 2 = 0.3519 r 3 = 1.2799 d 3 _{= 0.8707 n 2 = 1.64769 ν 2} = 33.80 r 4 = -0.9606 d 4 = 0.10 56 r ₅ = ∞ (aperture) d ₅ = 0.0211 r ₆ = ∞ d ₆ = 0.2815 n ₃ = 1.52287 v ₃ = 59.89 r ₇ = ∞ d ₇ = 0.0211 r ₈ = ∞ d ₈ = 1.0556 n ₄ = 1.52000 v ₄ = 74.00 r ₉ = ∞ d ₉ = 0.7210 r ₁₀ = 1.6137 d ₁₀ = 1.0724 n ₅ = 1.72916 ν ₅ = 54.68 r ₁₁ = -2.0150 d ₁₁ = 0.6162 n ₆ = 1.84666 ν ₆ = 23.78 r ₁₂ = ∞ d ₁₂ = 0.2815 n ₇ = 1.52287 ν ₇ = 59.89 r ₁₃ = ∞ I _m / f _R = 0.333, | f ₁ / f ₂ | = 0.603, | ν ₁ −ν ₂ | = 6.98, f _R /f=2.44, | ν ₃₋ ν ₄ | = 30.9, D _R /f=1.22 Example 4 f = 1.000, F / 5.340, IH = 0.8327, object point distance = -10.0937 r ₁ = ∞ d ₁ = 0.2307 n ₁ = 1.88300 ν ₁ = 40.78 r ₂ = 0.7089 d ₂ = 0.6870 r ₃ = 1.6655 d ₃ = 1.0718 n ₂ = 1.74950 ν ₂ = 35.27 r ₄ = -1.5461 d ₄ = 0.1081 r ₅ = ∞ (aperture) d ₅ = 0.0216 r ₆ = ∞ d ₆ = 0.2884 n ₃ = 1.52287 ν ₃ = 59.89 r ₇ = ∞ d ₇ = 0.0216 r ₈ = ∞ _{d 8 = 1.0815 n 4 = 1.52000} ν 4 = 74.00 r 9 = ∞ d 9 = 0.5078 r 10 = 1.3148 d 10 = 1.0941 n 5 = 1.51112 ν 5 = 60.48 r 11 = -2.1699 d 11 = 0.4596 n 6 = 1.84666 ν _{6 = 23.78 r 12 = ∞ d} 12 = 0.2884 n 7 = 1.52287 ν 7 = 59.89 r 13 = ∞ I m / f R = 0.231, | f 1 / f 2 | = 0.643, | ν 1 -ν 2 | = 5.51 , F _R /f=3.602, | ν ₃ −ν ₄ | = 36.7, D _R /f=1.21, where r ₁ , r ₂ , ...
₁ , d ₂ , ... Is the thickness of each lens and the lens interval, n
₁ , n ₂ , ... Are the refractive indices of the respective lenses, ν ₁ , ν ₂ ,.
Is the Abbe number of each lens.

These Examples 1 to 4 are shown in FIGS. 1 to 4, respectively, and the two parallel plane plates immediately after the aperture stop remove the influence of YAG laser or the like on the CCD from the object side.
A G laser cut filter or the like, and then an infrared cut filter.

In these examples, by satisfying the condition (1), the incident angle of the chief ray on the filter group at the maximum image height is 25 °. The surface of the second lens on the brightness side is convex toward the image side in order to correct coma and astigmatism generated in the first lens.

In Examples 1 to 4 described above, if the object has a portion having a high spatial frequency or a small white bright spot due to the omission of the crystal filter, moire may occur at that portion. is there.

In order to prevent this, it is effective to adopt the following structure.

When observing the inside of a human body using an endoscope, a phenomenon called moire caused by interference between a subject and a solid-state image pickup device such as a CCD is unlikely to occur. However, even if the object to be observed has a white small bright spot even in the human body, when a CCD image is taken, a color moire called suspicious white is generated around the white bright spot.

The endoscope is used not only for medical purposes but also for industrial purposes, for observing inside pipes of water pipes and inside engines.
It may be inspected. In this case, many objects having a high spatial frequency are used as subjects, and moire is likely to occur, which causes a problem in actual use.

The objective lens of the present invention is adapted to the above situation as described below.

Generally, there is an image pickup device having a discrete sampling structure and a Nyquist limit due to sampling of the image pickup device, and an image pickup optical system for forming an object plane on a predetermined image forming plane is used. It suffices to dispose at least one surface having an optical low-pass effect capable of providing uniform blurring in the vicinity of the aperture stop of the imaging optical system.

Here, it is desirable that the surface having the low-pass effect satisfies the following condition (7). (7) S / 2 ≦ f _R sin {| θ _m | (n ₁ −n ₀ / n ₀ )} ≦ 3S where | θ _m | <90 ° Here, S is for obtaining the luminance signal of the solid-state image sensor. The minimum sampling interval, θ _m, is the maximum value of the angular difference between the normal of the surface having the low-pass effect and the normal of the reference surface of that surface, n ₀ , n _1.
Are the refractive indices of the medium on the object side and the image side of the surface having the optical low-pass effect, respectively.

In order to give such an optical low-pass effect, for example, there is a method of giving a high-order aberration having a uniform directivity to the image plane. FIG. 10 shows the configuration of an optical system adopting the above method. Since blurring due to high-order aberrations is given only to the horizontal scanning direction of the CCD, it is almost aligned with the horizontal scanning direction immediately after the aperture stop. In this direction, a filter having a substantially wave-shaped surface of one cycle or more is arranged. This filter has a cylindrical shape in a plane substantially vertical to the horizontal scanning direction of the CCD.

The principle of this moire removing means will be described below.

FIG. 11 shows the rear group behind the aperture stop. The focal length of the rear lens group 2 is f _R, and the filter 1 having the surface R ₁ whose angle of the normal to the optical axis is θ is arranged.

Here, if the spherical aberration of the rear lens group 1 is favorably corrected, the axial rays converge on the image plane. However, this axial ray is refracted by the surface R ₁ of the filter and has an angle of Δ _i with respect to the optical axis.

At this time, the following relationship is established. B = f _R sin Δ _i (a) Also, Δ _i ≈θ (n ₁ −n ₀ / n ₀ ).

Here, n ₀ and n ₁ are the refractive indexes of the medium on the object side and the image side of the surface R ₁ , respectively. Therefore, B is given by the following equation. B = f _R sin {θ (n ₁ −n ₀ / n ₀ )} (b) FIG. 12 shows another surface having an optical low-pass effect, which is a more complicated shape and has a waviness.

In FIG. 12, the optical axis is the z axis and the CCD is
When the scanning direction is the x-axis and the vertical direction is the y-axis, a surface having a waviness as shown in FIG. 12 only in the x-direction can be expressed by the following equation (c). z = f (x) (c) In this case, the above value will be referred to as a deviation amount from the reference surface.

[0058] At this time, the change delta _i of the ray angle due to inclination of the surface is as following formula (d). Δ _i = {df (x)} / dx
(D) The following equation is obtained from the equation (a). B = f _R sin {df (x) / dx (n ₁ -n ₀ / n ₀ )}
(E) Further, FIGS. 13 and 14 show how higher-order aberrations are generated by the surfaces shown in FIGS. 11 and 12, respectively.

As already expressed by the formula, the relation between the surface shape and B is as shown in formula (a) and formula (e).

FIG. 13A shows the deviation of the surface shape from the reference surface. Here, the reference surface can be considered as a surface in which the spherical aberration of the optical system is sufficiently corrected, and is not limited to a flat surface.

FIG. 13B shows the value of B obtained from the equation (c).

Using the surface of this shape, FIG.
The spot is separated into two points as shown in. However, ΔB
Is the absolute value of the maximum value of B. Therefore, the response (MTF) is close to a cosine curve as shown in FIG.

FIG. 14A shows a surface in which the waviness of one cycle or more is provided with respect to the reference surface within the range of the aperture of the aperture stop. From f)
The shape is as shown in FIG. When a surface having such a shape is used, the spot becomes closer to a rectangular shape as shown in FIG. 16A, and therefore the MTF becomes as shown in FIG. 16B, which is lower than that in FIG. 15B. Then, the MTF is low, but at high frequencies, the MTF is well suppressed.

Next, the one shown in FIG. 17A in which the above-mentioned surface shapes having the optical low-pass effect are combined is also conceivable.

With this surface shape, an MTF as shown in FIG. 18 can be obtained.

The above description has described one direction of one plane. However, it is also possible to create such an optical low-pass effect in other directions, for example in other planes in the vertical direction. In this case, the low-pass effect may be simultaneously produced in two or more directions on the same surface, or may be rotationally symmetrical with respect to the optical axis.

When an effect is simultaneously produced in two directions on one surface, the surface is represented by the following formula (f). Z = f (x) × f (y) (f) The rotationally symmetric surface is expressed by the following equation (g). Z = f (r) (g) Here, how to take the reference plane will be described.

In the present invention, the effect of the optical low-pass filter is imparted by deforming one surface of the objective lens whose aberration is favorably corrected.
Therefore, the reference surface is a surface on which various aberrations are favorably corrected when the deformation is removed. More specifically,
Since the surface having the effect of the optical low-pass filter of the present invention is often provided near the pupil of the objective lens, the surface that minimizes spherical aberration is the reference surface.

In the present invention, the shapes of the reference surface are flat, spherical,
If the spheroid is a spheroid or a hyperboloid, and the reference surface is a spherical surface, the one having a radius of curvature that minimizes spherical aberration is considered as the reference surface. When the reference surface is a spherical surface, the amount of deviation from the reference surface is the so-called aspherical surface amount itself.

As can be seen from FIGS. 18, 19 and 21, the value of ΔB determines the spot diameter. In this equation (c) and equation (f), if the difference between the angle between the normal line of the surface having the optical low-pass effect and the normal line of the reference plane is θ and its maximum value is θ _m , the value of ΔB is Can be expressed as ΔB = f _R sin {| θ _m | (n ₁ -n ₀ / n ₀ )} where | θ _m | <90 ° The current mainstream method for simultaneous solid-state imaging is the volume 37, 10 of the Institute of Television Engineers (1983). (Year) pages 89-96, "Single-panel colorization system for filled storage mode CCD".

In the image pickup method described in the above-mentioned document, a color filter array having a color array as shown in FIG. 21 is arranged in front of a solid-state image pickup element, and when reading out an image signal, the color filter array is adjacent in the vertical direction (vertical direction in the drawing). The signals of the two pixels to be mixed are mixed. That is, the first field (A field)
Then, as shown in the right side of the figure, vertical two pixels are mixed to perform horizontal scanning. In the second field (B field), horizontal scanning is performed pixel by pixel in the vertical direction as shown in the left side of the figure. To do. At this time, the luminance signals of the n and n + 1 lines in the A field are Y _An and Y _{An + 1} , and the luminance signals of the n and n + 1 lines in the B field are Y an and Y _{An + 1} .
_{Let Bn} and Y _{Bn + 1} be the charge of each pixel due to the light after passing through each filter, Mg (magenta), Ye (yellow), G
Assuming (green) and Cy (cyan), the luminance signal is obtained as follows.

Y _An = (Ye + Mg) + (Cy + G) = Y _{An + 1} = (Ye + G) + (Cy + Mg) = 2R + 3G + 2B Y _Bn = (Mg + Ye) + (G + Cy) = Y _{Bn + 1} = ( G + Ye) + (Mg + Cy) = 2R + 3G + 2B When such a solid-state image sensor is used, the horizontal low-pass filter cutoff frequency ν _c generally satisfies the following relationship. ν _n ≤ν _c ≤ν _s (h) where ν _s is the sampling frequency of the luminance signal of this solid-state image sensor, and ν _n is the Nyquist limit frequency.

Further, from FIGS. 15, 16 and 18, the cutoff frequency due to the spot shape can be moved within the following range by combining (B) of FIG. 1 / 4ΔB ≦ ν _c ≦ 3/4 ΔB (i) Letting S be the minimum sampling interval for obtaining the luminance signal of the solid-state imaging device, the following relationship is established. ν _s = 1 / s = 2ν _n (j) First, when ν _c = ν _n , the relationship between ν _n and ΔB is as follows from equation (h) and equation (i). 1 / 4ΔB ≦ ν _n ≦ 3/4 ΔB (k) Next, when v _c = ν _s , the following is obtained. 1 / 4ΔB ≦ ν _s ≦ 3 / 4ΔB (m) The following relationships are obtained from the formula (k) and the formula (j) and the formulas (m) and (j). 1 / 2S ≦ ΔB ≦ 3/2 S S ≦ ΔB ≦ 3S or S / 2 ≦ ΔB ≦ 3S Therefore, the following relationship is obtained. S / 2 ≦ f _R sin {| θ _m | (n ₁ −n ₀ / n ₀ )} ≦ 3S, where | θ _m | <90 °.

This is the above-mentioned condition (7). That is, by satisfying this condition, it is possible to effectively remove the moire caused by the image sensor and the subject.

If the upper limit of this condition is exceeded, the optical low-pass effect becomes too strong, resulting in a blurred image. If the lower limit is exceeded, moire cannot be removed.

Numerical examples of the lens system having the above-mentioned structure for preventing moire are shown below. f = 1.000, F / 5.535, IH = 0.8134, object distance _{= -9.8592 r 1 = ∞ d 1} = 0.2254 n 1 = 1.88300 ν 1 = 40.78 r 2 = 0.5365 d 2 = 0.3521 r 3 = 1.5395 d 3 = 0.9225 n ₂ = 1.64769 ν ₂ ＝ 33.80 r ₄ ＝ -0.9365 d ₄ ＝ 0.1056 r ₅ ＝ ∞ (aperture) d ₅ ＝ 0.0211 r ₆ ＝ ∞ (aspherical surface) d ₆ ＝ 0.2817 n ₃ ＝ 1.52287 ν ₃ ＝ 59.89 r ₇ ＝ ∞ d ₇ = 0.0211 r ₈ = ∞ d ₈ = 1.0563 n ₄ = 1.52000 ν ₄ = 74.00 r ₉ = ∞ d ₉ = 0.8190 r ₁₀ = 1.6405 d ₁₀ = 1.2622 n ₅ = 1.72916 ν ₅ = 54.68 r ₁₁ = -1.6059 d ₁₁ = 0.6232 n ₆ = 1.84666 ν ₆ ＝ 23.78 r ₁₂ ＝ ∞ d ₁₂ ＝ 0.2817 n ₇ ＝ 1.52287 ν ₇ ＝ 59.89 r ₁₃ ＝ ∞ aspherical coefficient E = -0.13388 × 10 ² , F = 0.19179 × 10 ⁴ , G = -0.10971 x 10 ⁶ , H = 0.32108 x 10 ⁷ , I = -0.50865 x 10 ⁸ , J = 0.41464 x 10 ⁹ , K = -0.13616 x 10 ¹⁰ Aspherical surface in the above data (surface with low-pass effect) Is the optical axis z, horizontal scanning of CCD The x, the vertical direction y to the scanning direction, the functional form of the surface z = f (x) × g
Let (x) be expressed by the following equation. f (x) = Ex ⁴ + Fx ⁶ + Gx ⁸ + Hx ¹⁰ + Ix ¹² + Jx ¹⁴ + Kx ¹⁶ g (x) = C (C is a constant) The reference spherical surface of the aspherical surface of the lens system of the above numerical example is a plane. That is, other lens data (values of r, d, n, ν)
If you want to change the shape of only this surface without changing
When this surface is a flat surface, the spherical aberration of the entire system is minimized.

[0077]

According to the present invention, it is possible to obtain an endoscope objective lens having a wide angle, a short overall length, a compact size, a small number of lenses, and an aberration suitable for an image pickup device to be used.

[Brief description of drawings]

FIG. 1 is a sectional view of a first embodiment of the present invention.

FIG. 2 is a sectional view of a second embodiment of the present invention.

FIG. 3 is a sectional view of a third embodiment of the present invention.

FIG. 4 is a sectional view of a fourth embodiment of the present invention.

FIG. 5 is an aberration curve diagram of Example 1 of the present invention.

FIG. 6 is an aberration curve diagram of Example 2 of the present invention.

FIG. 7 is an aberration curve diagram of Example 3 of the present invention.

FIG. 8 is an aberration curve diagram of Example 4 of the present invention.

FIG. 9 is a diagram showing a basic configuration of an objective lens of the present invention.

FIG. 10 is a diagram showing a configuration in which the objective lens of the present invention is provided with a surface having an optical low-pass effect.

FIG. 11 is a diagram for explaining a surface having an optical low-pass effect and the function of the surface.

FIG. 12 is a diagram showing another surface having an optical low-pass effect.

13 is a diagram showing the amount of deviation of the surface shown in FIG. 10 from the reference surface and the distance from the optical axis on the image formation surface.

FIG. 14 is a diagram showing the amount of deviation of the surface shown in FIG. 11 from the reference surface and the distance from the optical axis on the imaging surface.

15 is a diagram showing the intensity and MTF of the spot in the x direction on the surface shown in FIG.

16 is a diagram showing the intensity and MTF of the spot in the x direction on the surface shown in FIG.

FIG. 17 is a diagram showing a surface shape having an optical low-pass effect, which is a combination of FIGS. 12 and 15;

FIG. 18 is a spot intensity distribution and MTF on the surface shown in FIG. 20.
Showing

FIG. 19 is a sectional view of a conventional example.

FIG. 20 is a sectional view of another conventional example.

FIG. 21 is a diagram showing an example of an imaging method.

Claims (1)

[Claims] 1. A first power source having negative power in order from the object side.
A lens, a second lens having a positive power and having a convex surface on the image side, a diaphragm, a filter group arranged immediately after the diaphragm, a biconvex third lens and a plano-concave fourth lens. The following conditions (1) and (2) are provided: a cemented lens in which a lens is cemented, and the cemented lens is in close contact with an image sensor.
Endoscope objective lens that satisfies the requirements. (1) 0.18 <I _m / f _R <0.642 (2) 0.1 <| f ₁ / f ₂ | <2 where f _R is the combined focal length of the third lens and the fourth lens, I _m is the maximum image height of the lens system, and f ₁ and f ₂ are the first
It is the focal length of the lens and the second lens.