JPH03126006A - Image pickup optical system - Google Patents

Abstract

PURPOSE:To prevent the generation of moire fringes by dropping the space frequency characteristics of the case in which the intensity distribution of the image point by the different sizes of the aperture changes and the aperture is small as the optical system has spherical aberrations and the approximate shape of the aberration curve changes with a change in the aperture. CONSTITUTION:An object image is formed on the end face of an image guide 3 by an objective lens 2 of a fiber scope 1 and the transmitted image is magnified by an eyepiece lens 5 and is usually observed by naked eyes. The image pickup optical system 7 is provided in order to form the image of this fiber scope 1 on the photodetecting surface of the image pickup element 8 of the image pickup device 6. The rays passing the aperture part larger than the aperture (a) generates the smaller aberration if the scope 1 of the larger aperture is used. The ratio at which the rays emitted from certain one point on the object gather near the ideal image forming point is higher with the larger aperture when the rays gathering near the image forming point and the rays gathering to the peripheral part thereof are compared if the scope 1 having the aperture of about (a) and the scope 1 having the aperture larger than this aperture are used. The generation of the moires is prevented in this way.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an imaging optical system used for forming an image transmitted by an image guide fiber of an endoscope or the like onto a non-life insurance element. It is.

[Prior Art] In recent diagnosis using an endoscope, a small television camera is connected to the eyepiece of the endoscope and observation is often performed on a television monitor. However, the image guide fibers used in endoscopes, etc. have a regular array structure, and the light-receiving parts of image pickup devices such as CCDs used in television cameras, etc., also have a regular structure. Due to interference, false signals generally called aliasing, moiré, etc. appear on the monitor.

When such moire fringes appear on the monitor, it becomes difficult to see the entire screen (not only that, but there is a risk that the lesion may be overlooked during diagnosis and a misdiagnosis may be made).

In order to remove such moiré fringes, it is sufficient to make the response in the spatial frequency band on the image plane determined by the endoscope's image fiber diameter or the spatial frequency around the Nyquist frequency of the image carrier to zero. Since it is possible to prevent the relatively high spatial frequency components of the image guide fiber from being folded back into the low spatial frequency band, it is possible to prevent the generation of false signals that cause moiré fringes.

However, there are many types of endoscopes, and the diameter of the image fiber and the magnification of the eyepiece often differ depending on the endoscope, so it is impossible to eliminate all responses of a specific spatial frequency on the image plane. Can not.

Further, even if the response of a spatial frequency around the Nyquist frequency of the image pickup device is made zero, since a normal image fiber has a regular structure, the reference spatial frequency has a certain directionality. In other words, since there are several directions that are likely to interfere with the light receiving section of the image sensor, the spatial frequency in each direction must be set to zero in order to prevent the occurrence of moiré fringes.

Therefore, moiré is generally removed by providing a plurality of birefringent plates such as quartz plates and optical low-pass filters such as phase filters in the imaging optical path.

However, in this case as well, it is necessary to match the case where moiré is most likely to occur, and as a result, the number of optical low-pass filters required increases, and the overall size of the optical low-pass filter increases, making it impossible to make the optical system compact.

It also uses many optical low-pass filters, making it very expensive.

Furthermore, as mentioned above, since the structure of an optical low-pass filter is often matched to a filter with strong moiré, when it is combined with a filter with weak moiré, the moire removal effect is too great, resulting in a decrease in resolution.

For this reason, by replacing and using optical low-pass filters with different moiré removal effects depending on each endoscope, optimal resolution can always be obtained. However, this method requires time and effort to replace, and has the disadvantage that a separate optical low-pass filter must be prepared for each endoscope.

Further, as another means for removing moire, there is a method using defocusing as described in, for example, Japanese Patent Application Laid-Open No. 1-1269465. In this method, the image plane is set at a position slightly shifted from the best image plane position, thereby giving an out-of-focus state to the image and removing moiré. This method, for example, uses the birefringence of the crystal to separate one image into two when using a quartz plate as an optical low-pass filter, creating an image with a certain degree of blur as a whole. It's the same.

The amount of blur can be reduced by reducing the response to the Nyquist frequency of the image sensor, or by reducing the response of a specific spatial frequency on the image plane of the image guide fiber. Moiré can be removed because high-order spectra of the fiber structure generated by sampling each time can be suppressed.

However, a normal optical system has a considerable response even in a high spatial frequency region, and in order to reduce the desired spatial frequency, the amount of defocus must be increased, and the The response characteristics are also linear, unlike when using an optical low-pass filter such as a quartz plate. Therefore, when trying to reduce the low spatial frequency to zero in order to remove moiré, the response at lower frequencies also drops, resulting in a decrease in resolution. On the other hand, if the resolution is not reduced, moire removal will not be sufficient.

Furthermore, as mentioned above, since there are many types of endoscopes, there are also problems such as the need to set the defocus amount accordingly.

[Problem to be Solved by the Invention 1] The object of the present invention is to provide a system that can sufficiently remove moiré fringes and provide high resolution when various endoscopes and television cameras are combined, without the use of an optical low-pass filter or the like. An object of the present invention is to provide a small-sized, high-performance imaging optical system that does not cause a decrease in image quality.

[Means for Solving the Problems] The imaging optical system of the present invention is an optical system that is attached to the eyepiece of an endoscope and forms an image containing a specific frequency band on an image sensor. When the aberration curve has a specific aberration, especially spherical aberration, and the approximate shape of the aberration curve changes as the aperture changes, the intensity distribution of the point image changes depending on the size of the aperture, making it difficult to identify the spatial frequency when the aperture is small. It was designed so that it would drop.

FIG. 1 is a schematic diagram of the overall configuration of an apparatus using the imaging optical system of the present invention. For example, in an endoscope, an image transmitted by an image guide fiber is transmitted through the imaging optical system of the present invention to an image sensor. It forms an image on the light receiving surface. In this figure, an object image is formed on the end surface of an image guide 3 by an objective lens 2 of a fiberscope 1. The image transmitted by the image guide fiber 3 is magnified by the eyepiece 5 and usually observed with the naked eye. An imaging optical system 7 is provided to form the image of the fiberscope 1 onto the light receiving surface of the imaging element 8 of the imaging device 6.

The image guide fibers of the endoscope are regularly arranged in a hexagonal close-packed structure. As a result, moiré fringes occur as described above.

Next, removal of moiré fringes by generating a specific aberration in the imaging optical system will be explained.

This is generally referred to as soft focus, and is created by generating specific aberrations, particularly spherical aberrations, in the optical system (sometimes using the image blurring effect).

Spherical aberration exists both on-axis and off-axis, and has the characteristic that it is symmetrical with respect to the optical axis. Generally, when an optical system has spherical aberration, the aberration situation of the light beam is as shown in FIG. 2, considering only the third-order spherical aberration for simplicity. On the image plane near the glass image plane, as shown in FIG. 3, a circular ring formed by intersecting the caustic line and a disk-shaped image L2 formed by the intersection with other light beams are formed. When the image plane is brought to a point where the size of the ring L1 and the image L2 are equal, that is, Fo, the beam diameter (circle of confusion) becomes the smallest as shown in FIG. 3(B).

The image plane at this time is the best image plane in this case, and if there is even a slight deviation forward or backward from this, the beam diameter increases. What is shown in FIG. 2 is a case where the spherical aberration is under-corrected, but the same applies to the case where the spherical aberration is over-corrected as shown in FIG. However, when higher-order terms such as the 5th and 7th orders of spherical aberration are introduced, the shape of the caustic line and the appearance of the spherical aberration curve become different and become more complicated, but the basic idea remains the same.

Next, we will discuss the spherical aberration required for endoscopes.

Typically, the imaging optical system attached to the eyepiece of an endoscope is a front aperture type optical system whose aperture is determined by the aperture diameter of the eyepiece. By the way, a scope with a small aperture generally has a large ocular magnification because the fiber image is small, so the specific spatial frequency due to the fiber structure on the image plane is low (therefore, it is necessary to reduce the low spatial frequency response (MTF)). The amount of blur is thick (this means that when considering the best image plane as the standard, the amount of defocus of the image plane must be large. Conversely, if the aperture is large, the eyepiece magnification is small, so the specific spatial frequency is The amount of blur required to reduce this high frequency response becomes smaller.

As mentioned above, a scope with a small aperture requires a large amount of blur, or a large circle of confusion on the image plane, whereas a scope with a large aperture requires a small circle of confusion on the image plane6. What is required is that the entire screen is blurred and no moiré fringes occur, but that it has a certain level of resolving power. Furthermore, when combined with a scope with a small aperture, the frequency characteristics of the optical system are degraded to some extent, but with a scope with a large aperture, it is necessary that the characteristics are not degraded too much.

Next, a normal spherical aberration curve (lateral aberration) as shown in Figure 5
In the case of , the larger the aperture, the greater the height of the ray of light, and the greater the amount of aberration. Therefore, a scope with a larger aperture has a larger lower response at low spatial frequencies, which causes a reduction in resolution more than the moiré fringe removal effect.

In the case of spherical aberration as shown in FIG. 6, the amount of aberration is maximum at a certain aperture a. In this case, when a scope with a large aperture is used, the amount of aberration generated by light rays passing through an aperture larger than aperture a is small, so a scope with an aperture of approximately a and a scope with an aperture larger than this are used. In this case, when a ray of light emitted from a point on an object passes through each part of the aperture and converges near the ideal imaging point, and another converges around the ideal image point, the one with a larger aperture is considered as a whole, A large proportion of the images gather near the ideal imaging point.

Therefore, even when the aperture is large, the spatial frequency characteristics do not deteriorate significantly, and on the contrary, better characteristics can be obtained than when the aperture is small. In other words, it is necessary to maximize the amount of spherical aberration when using the scope with the smallest aperture of the scope you are actually trying to use, and to minimize the amount of aberration when using a scope with a larger aperture. It is.

As described above, in the present invention, spherical aberration is generated in the optical system and the approximate shape of the aberration curve changes depending on the size of the aperture, so the point image intensity distribution changes depending on the size of the aperture. This is designed to reduce the spatial frequency characteristic when the aperture is small.

It is preferable that the imaging optical system of the present invention has a certain amount of aberration at a small aperture, and spherical aberration whose aberration is sufficiently corrected at a large aperture.

Generally, the spherical aberration that occurs in the case of a positive lens is under-corrected, and the spherical aberration monotonically increases as the ray height increases.

FIG. 5 shows third-order spherical aberration, and in reality, higher-order spherical aberrations are included in addition to this third-order spherical aberration.

If the spherical aberration curve is to have a certain desired shape, higher-order terms such as the 5th and 7th orders become necessary. At this time, in order to change the spherical aberration without affecting aberrations other than the spherical aberration, it is better to use an aspherical lens than to configure the optical system only with spherical lenses.

Generally, the higher the height of the ray of light passing through the optical system, the greater the spherical aberration, and the shorter the focal length of the optical system, the greater the amount of spherical aberration produced. This is because when a ray of light enters a positive single lens, the refraction of the ray gradually increases as it moves from the optical axis to the periphery of the lens.Therefore, spherical aberration occurs at a certain ray height. If you want to further increase the amount of rays, you can increase the refraction of the lens at this ray height, that is, increase the refraction of the lens surface. In order to achieve this, the radius of curvature of this surface can be made smaller, but in that case, the amount of aberration for light rays at other ray heights will also change.
Therefore, in order to obtain the desired amount of aberration for the rays at each ray height, it is preferable to use an aspheric surface.
In other words, by using an aspherical surface, higher-order aberrations are more likely to occur than with a normal spherical surface. Furthermore, in order to generate high-order aberrations using only a spherical lens, the structure of the entire lens system and the shape of each lens must change significantly, which may impair the machinability of the lens and make it difficult to correct other aberrations. .

Increasing the refractive effect here means, in the case of a positive lens, increasing its converging effect, or the positive power of the lens, and in the case of a negative lens, increasing its diverging effect, or the negative power of the lens. It is to be.

Further, as described above, the amount of spherical aberration generated is related to the focal length of the optical system, the refractive effect of the lens, and the amount of deviation from the reference spherical surface at each ray height when an aspherical surface is provided.

Note that the reference spherical surface is a spherical surface that touches an aspherical surface near the optical axis. Also, the height is 1.5 mm with respect to the optical axis. The amount of deviation Δ2 from the spherical surface at the beginning is positive when the aspherical surface deviates from the reference spherical surface in the direction in which the light ray travels, and negative when it deviates in the opposite direction. Here, even if the amount of deviation is a negative value with the same sign, the change in the lens power may be different. For example, as shown in Figure 7, although the amount of deviation is negative, the power of the lens in the peripheral area becomes weaker. As shown in FIG. 8, although the deviation is negative, the power of the lens in the peripheral area becomes stronger. This also applies to negative lenses. In other words, depending on whether the aspheric surface is provided on the object side or the image side of the lens, and whether the reference surface is a surface with a concave surface facing the object side or a surface with a convex surface, even if the direction of deviation is the same, The change in lens power is not necessarily the same (even if the sign of ΔZ is the same) l/X.

Generally, an aspherical surface can be expressed by the following formula.

z = Cy"/(1+ r"■v?) + By"
+ Ey' + Fy' + ...-Here, z and y have the optical axis as the Z axis, the direction of the image as the positive direction, and the intersection of the aspherical surface and the optical axis as the origin, and the direction perpendicular to the Z axis as the y axis. , C is the reciprocal of the radius of curvature of the circle that touches this aspherical surface near the optical axis, P is the parameter representing the shape of the aspherical surface, and parameters B, E, F, ... are quadratic, 4 These are the aspherical coefficients of order, sixth order, etc. If P=l and B, E, F, . . . are all 0, the above equation represents a spherical surface.

When an aspherical surface is provided in the optical system of the present invention, it is desirable that the following condition fll be satisfied.

fil X 10-5<lΔ21/f<5
X 10 - where Δ2 is the amount of deviation of the aspheric surface from the reference spherical surface in the optical axis direction at the line height of the marginal ray passing through the aspheric surface when the aperture is maximum, and f is the focal length of the entire system. .

As mentioned above, the amount of spherical aberration generated generally increases as the height of the ray increases, but in the present invention, it is necessary to suppress the spherical aberration especially when the aperture is large, that is, at a place where the height of the ray is high.

If the lower limit of the condition fil is exceeded, the correction of spherical aberration by the aspheric surface will not be sufficient. Moreover, if the upper limit of this condition is exceeded, the aberration correction will be performed too much, which is not preferable.

[Example] Next, embodiments of the imaging optical system of the present invention will be described.

Example 1 f=1. ooOF15.3 2ω = 12.966°I H = 0.112 rl = beauty (aperture) d1 = 0. Mouth l mouth 1 r2=■ da= 0.0338 jl,= 1.5163
3r3 = ■ d, = 0.1387 r4 = ■ d4 = 0.0274 Q, = 1.51633r
5=■ d, = 0.0947 1”6= -o, 5160 (Aspherical surface) d, = 0
．． 0338 71. = 1.65160ν, =
64.15 ν2 =64.15 ν, =58.52 r, = 0.2372 d7=0.0609 n4=1.71736
v, =29.51ra=0.4516 d,t=0.1827 re=1.7262 do” 0.1218 n-=1.73400
1/6 = 51.49r. =-0,53
82 d. =0.0169 r++ ”-51,7006 d,, =0.169I n5=1.74400
v,=44.73r+z=-0,2970 d,2=0.044On-=1.78472
vt = 25.71 "+, = -1,4354 d, 3 = 0.0271 rl == o.

d, 4”0.0605 na=1.51633
v,=64.15rls :OO Aspheric coefficient P=1.0000, B=0.10209E=
0.29207 x 10, F = -0,18
885x 10”Δ Z =0.000948 Example 2 f = 1.0000 F/14.2I H
= 0.071 2 ω = 7.92 @ r, = luster (aperture) d, = 0.0072 r2 = ■ d,: 0.0362 old = 1.51633rs
= ■ d, = 0.1183 r4 = 0.2077 (aspherical surface) d 4 = 0, 0821 n x = 1,
71300r5 knee 2.3524 d, = 0.0644 ra = -0,3082 d, = 0.0532 Q3 = 1.75520
ry=0.1928d? = 0.1193 ra=0.7072 da= 0.0687 n-= 1.59270
=64.15 ν, =53.84 ν, =27.51 ν4 =35.29 re=-0.4648 r5: 3.1152 d. = 0.1132 d, = Q. 0643 rs=-0.2677 d+o=0.0362 ns=1.51633 ν5=64.15 da=0.053t nz=1.75520 ν3=27.51 ry”0.1876 Aspheric coefficient d,=o. 1192 p = i.oooo B = 0.14089 r, = 0.7285 E = 0.55469 ×10 F = -0.37135x 10' ds:o.0687 n4 = 1.59270 ν4 =35.29 Δ Z = −0.000185 r.=−0.4113 Example 3 d.=0.1131 f=1.000 F/14.2 2 ω=7.92” r+o ■ H=0.071 d,.

= 0.0361 ns = 1.51633 ν5 = 64.15 (aperture) r++ d. =0.0072 Aspherical coefficient r2=■ P = 1.0000 B = 0.38924 × 104 r3= (3) Δ Z = 0.000050 d. =0.1182 Example 4 r. = 0.2124 (aspherical surface) f = 1.0000 F/2.1 2 ω=13° d. = 0.0820 nt = 1.71300 ν2 : 53.84 ■ H = 0.113 r+” ■ (Aperture) = -1.1935 d+ = 0.0263 dll = 0.0392 r2 = Flash r + t = 3.1698 d2 = 0.0876 = 1.51633 ν = 64.15 d+2 = 0.0788 11. = 1.69680 ν6 = 55.52 rs = (fund) r+s = -1.4952 dz = 0.1927 d13: 0. 2134 r4= ■ r+4 d.= 0.0876 n,= 1.51633 ν2 = 64.15 d+4 = 0.0964 nt= 1.52000 νfu= 74.00 rs=■ r+s ds”0.0964 dl8: 0 .0175 ra=0.6745 (aspherical surface) r+a d. = 0.1734 n-= 1.78590 ν3 = 44.18 d+8 = 0.1209 jla= 1.54869 ν8 = 45.55 rt=5.9213 r1te d,= 0.0436 aCt =0.1577 n9= 1.54869 ν9 =45.55 r. =-1.1230 r+a d. = 0.0701 n4 = 1.78472 ν4 = 25.71 dlll : 0.1323 neo = 1.54869 ν+o=45.55 ro=0.6375 r19 d. = 0.0816 dl9 = 0.0175 rIo = 3.4100 rgo dlG = 0.1165 jl, = 1.88300 ν5 = 40.78 d2o: 0.3153 nl+ = 1.80610 ν++ = 40.95 "2 ra: o.7068 (Aspheric surface) (1+: 0.0350 n+z = 1.51633 ?1■= 64.15 d.= 0.1734 ns” 1.78590 ν3 =44.18 r22 rt=5.8596 Aspheric coefficient d, = 0.0438 P = t.oooo B = 0.76013 x lO~ rlI: 1.6385 E = = 0.35469 F = 0.63098 d. = 0.0700 b, = 1.78470 ν4 = 28 .22 △ Z =: 0.005588 r*=0.7826 Example 5 d.= 0.0832 f = 1.000 F/2.1 2 ω=13.Ol゜r+o =2.8652 ■ H=0 .ll3 dIa = 0.1165 n, = 1.88300 ν5 = 40.78 (aperture) = -1.2074 d, = 0.0263 dll = 0.0394 r2 = ■ rI2 = -7.6465 d2 = 0. 0876 n,= 1.51633 ν1 =64.15 d+z = 0.0788 ns= 1.70154 ν6 =41.24 rz= ■ r+s = 1.2684 dz=o.I926 d+z = 0.2097 r4= ■ rI4 d 4 = 0.0876 Q2= 1.51633 ν2 =64.15 dl4: G.0963 nt= 1.52000 νfu= 74.00 ",= ■ rl6 ds"0.0963 dl+s = 0.0175 rs"■ dte = 0.4106 na=1.54869 ν8 =45.55 da= o.1926 r+y r4= 00 d+7 =0.0175 d.= 0.0876 n!= 1.51633 ν2 =64.15 rl8 rs=■ d+a = 0.3152 fi,= 1.80610 ν9 = 40.95 d.= 0.0963 ra=0.6241 dI9: 0.0350 = 1.51633 ν1.=64.15 d.= 0.1734 ns=1. 78590 ν: 44.18 r20 rt=-1.9996 Aspheric coefficient d,= 0.0438 P=t. oooo E=0.30802 ra=-0.8841 F=o. 16161 ω = 12.978' r+o = -5.4879 ■ H = 0.ll3 dlo = 0.1165 ns = 1.88300 ν5 = 40.78 (aperture) r++ = -1.0629 d. = 0.0263 dll = 0.0394 r2 = (fund) rl1 = -29.7088 d. = 0.0876 nl = 1.51633 ν1 = 64.tS d+2 = 0.0787 n,: IJ9700 ν6 = 48.51 r+3 = -0,9127 However, rl+r!+... is the radius of curvature of each lens surface,
d.

d2. ... is the wall thickness and air spacing of each lens, n +
+nz, . . . are the refractive index of each lens, and si1. C2.・
. . is the number for each lens.

Embodiment 1 has a two-group configuration consisting of a negative lens group and a positive lens group, as shown in FIG. In this embodiment, the first surface (r6) of the negative lens group of the optical system is made an aspherical surface. The amount of spherical aberration generated is greater when the ray height is higher, but by providing an aspherical surface near the aperture stop as in this example, the rays are symmetrical and astigmatism is particularly prevented from occurring as much as possible. . In other words, if astigmatism, field curvature, etc. are large, the circle of confusion at the image plane position will become large, no matter how appropriate the spherical aberration is.
It becomes impossible to obtain the desired moire removal effect and resolution.

In this example, the spherical aberration is 0 at the maximum aperture, and the spherical aberration becomes maximum when the aperture ratio is approximately 0.7.

Embodiments 2 and 3 are triplet quive optical systems consisting of a positive lens group, a negative lens group, and a positive lens group as shown in FIGS. 10 and 11, respectively.

In Example 2, an aspherical surface is provided on the first surface (r4) of the optical system, and negative spherical aberration occurs, and the shape of the aspherical surface gradually becomes looser as it moves away from the optical axis.

Example 3 has a configuration similar to Example 2, but the shape of the aspherical surface is opposite to Example 2 because spherical aberration is generated exactly. In other words, it gradually becomes tighter as you move away from the optical axis.

Embodiment 4.5 has a four-element configuration in four groups: a positive lens group, a negative lens group, and a lens group with positive lens group pressure, as shown in FIGS. 12 and 13, respectively, and the first surface of the optical system is ('6) is an aspherical surface.

Example 4 generates negative spherical aberration, and Example 5
conversely causes spherical aberration.

In any of the examples, 0.65 to 0.7 with respect to the maximum aperture
The spherical aberration is maximized at a certain point.

Next, Example 6 is as shown in FIG. 1.4, and is composed only of spherical lenses without using an aspherical surface.

In these embodiments, the spherical aberration is maximized at about 0.7 of the maximum aperture, but if necessary, as shown in FIG. Alternatively, higher-order spherical aberration may be generated, for example, as shown in FIGS. 22 and 23.

When using a scope with a smaller aperture, the aperture when the maximum aberration of the spherical aberration curve occurs may be set lower accordingly.

In each of the above embodiments, the thin parallel plane roots placed before and after the optical system are cover glasses. In addition, the parallel plane plate placed on the image side in Examples 4 and 5 is a filter that cuts infrared light that is provided so that the image sensor has sensitivity in the infrared region, and a filter that cuts unnecessary laser light during laser treatment. It shows filters etc. for cutting.

Of course, it is also possible to use the imaging optical system of the present invention in combination with an optical low-pass filter, and it is also possible to use it in combination with defocusing.

Further, the imaging optical system of the present invention can be handled as a television camera integrated with an imaging device, or can be configured as an attacher optical system connected to the imaging device side of a television camera or a television camera that includes an imaging device and an optical system. You can.

Of course, a configuration may be adopted in which no aspherical surface is used, or a configuration in which two or more aspherical surfaces are used to correct other aberrations such as astigmatism and field curvature may be adopted.

Further, the aspherical lens used is preferably formed by molding optical glass from the viewpoints of durability, workability, cost, etc. as an endoscope.

Materials such as plastic may be used as necessary.

[Effect of the Invention J According to the present invention, when an image is formed by an image guide fiber such as an endoscope on an image sensor, even when used in combination with various endoscopes of different types, the optical The occurrence of moiré can be prevented without using a low-pass filter, and a compact and good imaging optical system can be achieved without reducing resolution.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the overall configuration of an apparatus equipped with the imaging optical system of the present invention, Figure 2 is a diagram showing how the optical system converges spherical aberrations due to insufficient correction, and Figure 3 is a diagram showing the state of ray convergence by the optical system due to the confusion of the optical system. Figure 4 is a diagram showing the convergence of a light beam in an optical system with overcorrected spherical aberration, Figure 5 is a schematic diagram of normal spherical aberration (lateral aberration), and Figure 6 is a diagram showing other spherical aberrations. Schematic diagrams of aberrations; FIGS. 7 and 8 are diagrams showing the relationship between an aspherical surface and a reference spherical surface; FIGS. 9 to 14 are cross-sectional views of Examples 1 to 6 of the present invention, respectively; FIG. 15 Figures 20 to 20 show the above embodiments 1 to 6.
The aberration curve diagrams of FIGS. 21 to 23 are schematic diagrams showing other examples of spherical aberration situations generated in the optical system of the present invention.

Claims (1)

[Claims] An optical system that is attached to the eyepiece of an endoscope and forms an image containing a specific spatial frequency band on an image sensor.
By generating specific aberrations, especially spherical aberrations, the approximate shape of the aberration curve changes as the aperture of the optical system changes, and the point image intensity distribution changes depending on the size of the aperture, thereby changing the spatial frequency characteristics when the aperture is small. Imaging optical system designed to be dropped.