JP2017211581A - Image forming optical system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming optical system capable of reducing entire cost.SOLUTION: A first group lens L11 and a second group lens L12, or the second group lens L12 and a third group lens L13 are disposed so that a difference between an image side sag amount and an object side sag amount between the neighboring the first group lens L11 and the second group lens L12, or between the second group lens L12 and the third group lens L13 is equal to the distance between the optical axes on the neighboring lens surfaces.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an imaging optical system that generates a subject image on a two-dimensional detector surface.

As a conventional imaging optical system, for example, as disclosed in Patent Document 1, there is a lens relating to an infrared camera lens. In Patent Document 1, chalcogenide glass, Ge (germanium), ZnS (zinc sulfide) is used as a lens material in an infrared camera lens used in a two-dimensional detector having a wavelength range of 3 to 5 μm, 8 to 14 μm, and a pixel pitch of 17 μm. In addition, a camera lens having a two- to three-lens configuration using a high-order aspheric surface and a diffractive lens surface is shown which obtains good imaging performance.
The lens disclosed in Patent Document 1 is a two-group two-lens camera lens, which is an optical system having a focal length of 120 mm and an F value of 1.3. The first group lens uses chalcogenide glass as a material, and the first surface is composed of an 8th-order aspheric surface, the second surface is composed of a 14th-order aspheric surface and a diffractive lens surface. The second group lens is made of germanium, the third surface is spherical, and the fourth surface is a 10th aspherical surface. With only two lenses, the desired imaging performance can be achieved. Yes.

International Publication No. 2013/098180

  In general, an imaging optical system such as a camera lens uses a plurality of lenses in order to correct aberrations that degrade imaging performance. There are monochromatic aberrations and chromatic aberrations, and spherical and aspherical surfaces are used to correct monochromatic aberrations. Depending on the case, an aspherical surface has higher aberration correction capability than a spherical surface, and an aspherical surface can correct aberrations that cannot be corrected without using a plurality of spherical surfaces. In addition, an aberration that cannot be corrected in principle with a spherical surface may be corrected with an aspherical surface. In particular, in an imaging optical system having a bright F value of 1.3 as shown in Patent Document 1, the effect of using an aspheric surface is significant.

  To correct chromatic aberration, it is necessary to use a combination of two or more refractive lenses with positive and negative power using at least two types of lens materials with different dispersion (rate of change in refractive index due to wavelength difference). is there. On the other hand, the diffractive lens surface has a concentric saw-like step structure centered on the optical axis, and its dispersion characteristics are very different from the dispersion of a refractive surface such as a spherical surface. When combined, it may have a high ability to correct chromatic aberration. In some cases, the number of lenses used for chromatic aberration correction can be reduced compared to the case of using a refractive lens utilizing this characteristic. In this way, aspherical lenses and diffractive lens surfaces can achieve high imaging performance while reducing the number of lenses used compared to the case of using only spherical lenses. In some cases, the cost can be reduced by reducing the number of lenses.

  However, if an aspherical lens is used to reduce the number of lenses for the purpose of compacting the imaging optical system and reducing the cost, the amount of aberration corrected by the aspherical surface increases. Aspherical. In general, an aspherical surface is more difficult to manufacture than a spherical surface, so the manufacturing cost per surface is higher than that of a spherical surface. Further, the higher the order of the aspherical surface, the more difficult it becomes to manufacture and the manufacturing cost increases. Therefore, in order to reduce the number of lenses in the imaging optical system, the use of a higher-order aspheric lens increases the manufacturing cost, and the total cost of the imaging optical system is different from the case where a plurality of spherical lenses are used. It may become high. Further, if the number of lenses of the imaging optical system is reduced, the curvature of the lens surface is increased regardless of whether the lens is a spherical lens or an aspheric lens, so that the lens becomes thick. Therefore, even if the number of lenses is reduced, the lens becomes thick, and the cost of the lens material may not be reduced compared to the case where a plurality of thin lenses are used.

In addition, higher-order aspherical surfaces and diffractive lens surfaces may be molded using a precision mold in order to reduce manufacturing costs and ensure stable quality. Since chalcogenide glass has a lower transition point (temperature at which it softens when heated) than other infrared lens materials such as Ge and ZnS, it is easy to mold using a precision mold. The chalcogenide glass lens is used. However, since precision dies are very expensive, a large number of lenses are required to depreciate initial costs.
For this reason, when the number of production is small and the cost amortization of the precision mold cannot be expected, high-order aspherical surfaces and diffractive lens surfaces are manufactured by grinding. In such a grinding process, the surface is processed with a diamond tool or the like, but grinding marks tend to remain on the processed surface as compared with a polished spherical surface having a very smooth processed surface. The higher the aspherical surface, the more difficult the processing becomes, and the more easily the surface becomes a non-smooth surface with grinding marks, which causes a decrease in optical performance. Therefore, there are cases where the use of a plurality of spherical surfaces is superior in terms of cost and quality over higher-order aspherical surfaces.

  On the other hand, by increasing the number of lenses, it is possible to achieve a desired imaging performance without using a higher-order aspheric lens, but in this case, for example, a large number of spacers are required to hold the lens. For example, it is difficult to reduce the cost even with such a configuration.

  The present invention has been made to solve the above-described problems, and an object thereof is to obtain an imaging optical system capable of reducing the overall cost.

  The imaging optical system according to the present invention includes first, second, and third group lenses, and image side sag amount and object side sag amount of adjacent lenses between the first group and the second group or between the second group and the third group. Is equal to the distance between the optical axes of adjacent lens surfaces.

  In the imaging optical system of the present invention, the difference between the image side surface sag amount and the object side surface sag amount between adjacent lenses between the first group and the second group or between the second group and the third group is the distance between the optical axes of the adjacent lens surfaces. Since they are equal, the cost of the entire imaging optical system can be reduced.

It is a block diagram which shows the imaging optical system by Embodiment 1 of this invention. It is explanatory drawing which shows the optical path based on the power and Abbe number of each lens group of the imaging optical system by Embodiment 1 of this invention. It is a block diagram which shows the comparative example of an imaging optical system. It is explanatory drawing which shows the optical system specification of the imaging optical system by Embodiment 1 of this invention, and a comparative example. It is explanatory drawing which shows the lens data of the imaging optical system by Embodiment 1 of this invention. It is explanatory drawing which shows MTF on the optical axis and image circle end of the imaging optical system by Embodiment 1 of this invention and a comparative example. FIG. 7A is an explanatory diagram showing two lenses in two groups, and FIG. 7B is an explanatory diagram showing three lenses in three groups. It is explanatory drawing which shows the relationship between the 1st group lens and 2nd group lens of the imaging optical system by Embodiment 1 of this invention. It is a block diagram which shows the imaging optical system by Embodiment 2 of this invention. It is explanatory drawing which shows the lens data of the imaging optical system by Embodiment 2 of this invention. It is explanatory drawing which shows MTF on the optical axis and image circle end of the imaging optical system by Embodiment 2 of this invention and a comparative example. It is a block diagram which shows the imaging optical system by Embodiment 3 of this invention. It is explanatory drawing which shows the lens data of the imaging optical system by Embodiment 3 of this invention. It is explanatory drawing which shows MTF on the optical axis and image circle end of the imaging optical system by Embodiment 3 of this invention and a comparative example. It is a block diagram which shows the imaging optical system by Embodiment 4 of this invention. It is explanatory drawing which shows the lens data of the imaging optical system by Embodiment 4 of this invention. It is explanatory drawing which shows MTF on the optical axis and image circle end of the imaging optical system by Embodiment 4 of this invention and a comparative example.

ここで、ｎ_０ｉ、ｎ_１ｉ、ｎ_２ｉは以下の値を示している。
ｎ_ｏｉ：ｉ群の設計波長における屈折率
ｎ_１ｉ−ｎ_２ｉ：ｉ群の色消しを行う波長間の屈折率差 Embodiment 1 FIG.
FIG. 1 is a block diagram showing an imaging optical system according to Embodiment 1 of the present invention.
The imaging optical system shown in the figure is configured by an optical arrangement including a first group lens L11, a second group lens L12, and a third group lens L13. In the imaging optical system, chalcogenide glass is used for the first group lens L11 and the second group lens L12, and germanium is used for the third group lens L13.
FIG. 2 is an explanatory diagram showing an optical path based on the power and Abbe number of the first lens unit L11 to the third lens unit L13. In FIG. 2, the Abbe numbers of the first group, the second group, and the third group are denoted by ν ₁ , ν ₂ , and ν ₃ , respectively. The Abbe number is a parameter representing the difference in refractive index depending on the wavelength of the lens material, and is defined by the following equation (1).

Here, n _0i , n _1i , and n _2i indicate the following values.
n _oi : Refractive index at the design wavelength of the i group n _1i −n _2i : Refractive index difference between wavelengths at which the i group is achromatic

ここで、（２）式中のｈ_１、ｈ_２、ｈ_３、ｈ_ｐ１、ｈ_ｐ２、ｈ_ｐ３は以下の（３）式、（４）式で与えられる近軸光線追跡より得られる。

The distance between the first group and the second group is d ₁ ′, the distance between the second group and the third group is d ₂ ′, the image paraxial ray incident height is h ₁ , h ₂ , h ₃ , and the pupil paraxial ray incident height is h. _{Let p1} , _hp2 , and _hp3 . At this time, the total system power φ, the longitudinal chromatic aberration coefficient L, and the lateral chromatic aberration coefficient T are given by the following equation (2).

Here, h ₁ , h ₂ , h ₃ , h _p1 , h _p2 , and h _p3 in the equation (2) are obtained by paraxial ray tracing given by the following equations (3) and (4).

（５）式が正になる様にν_１、ν_２、ν_３、ｄ_１’、ｄ_２’を与える。その後、所望の結像性能が得られる様に単色収差補正を行い、薄い球面レンズ、低次の非球面で実現可能な解を得るν_１、ν_２、ν_３、ｄ_１’、ｄ_２’を求める。
なお、（１）式から（６）式は、１群レンズＬ１１〜３群レンズＬ１３が単レンズの場合に限定される式ではなく、レンズが密集したレンズ群が三つの群に分けられる場合にも適用できる。 In the above formulas (1), (2), and (3), φ, ν ₁ , ν ₂ , ν ₃ , d ₁ ′, d ₂ ′ are given, and when L = T = 0, φ ₁ , φ ₂ , solving simultaneous equations for phi _3, the underbarrel real solutions, the entire system power phi, longitudinal chromatic aberration and lateral chromatic aberration is determined power arrangement zero, the diffractive lens surface in the imaging optical system at this point No need to use it.
Note that the condition that φ ₁ , φ ₂ , and φ ₃ have real solutions is the following equation (5).
b ² -4 × a × c ≧ 0 (5)
Here, a, b, and c in the equation (5) are given by the following equation (6).

(5) ν ₁ , ν ₂ , ν ₃ , d ₁ ′, d ₂ ′ are given so that the expression becomes positive. Thereafter, monochromatic aberration correction is performed so as to obtain a desired imaging performance, and a solution that can be realized with a thin spherical lens and a low-order aspheric surface is obtained ν ₁ , ν ₂ , ν ₃ , d ₁ ′, d ₂ ′. Ask for.
The formulas (1) to (6) are not limited to the case where the first group lens L11 to the third group lens L13 are single lenses, but when the lens group with dense lenses is divided into three groups. Is also applicable.

When each lens group is housed in the lens barrel during the production of the imaging optical system, the lens may be fixed by disposing a spacer between the lenses. At this time, the necessary number of spacers is 1 in 2 groups 2 and 2 in 3 groups 3 sheets configuration. Since the present invention has a three-group configuration, the number of spacers is increased and the cost of mechanical parts is increased compared to the prior art.
In order to prevent this increase in the number of spacers, in the present invention, the difference between the sag amount on the image side of the first group and the sag on the object side of the second group is the distance d1 ′ between the optical axes of the first and second groups (indicated as D11 in FIG. 1). To be equal). With this configuration, a spacer between the first group and the second group is unnecessary, and an increase in cost due to an increase in the number of mechanical parts due to the third group can be prevented. This configuration is not limited to between the first group and the second group, and depending on the aberration correction solution, it can be performed between the second group and the third group.

FIG. 3 shows an optical path diagram of the imaging optical system reproduced based on the lens data of Patent Document 1 as a comparative example of the imaging optical system of the present embodiment shown in FIG. The comparative example includes a first group lens L21 and a second group lens L22, and chalcogenide glass is used for the first group lens L21 and germanium is used for the second group lens L22. FIG. 4 shows the optical system specifications of the imaging optical system of the present embodiment and the comparative example.
As shown in FIG. 3, the configuration of the comparative example is a two-group, two-lens configuration and uses fewer lenses than the present embodiment shown in FIG. However, since the number of lenses is small, the curvature radii of R21 of the first surface and R22 of the second surface of the first lens unit L21 are R11 of the first surface, R12 of the second surface, and the third surface of this embodiment. R13 is smaller than R14 on the fourth surface, and it can be seen that the lens L21 is thicker. From this, it can be seen that the amount of chalcogenide glass is not significantly different between the optical systems of FIGS.

FIG. 5 shows lens data of the imaging optical system of the present embodiment. As shown in FIG. 5, in the present embodiment, R11 of the first surface to R15 of the fifth surface are spherical surfaces, and R16 of the sixth surface is a sixth-order aspheric surface. On the other hand, in the comparative example, R21 on the first surface is an 8th-order aspheric surface, R22 on the second surface is a 14th-order aspherical surface and a diffractive lens surface, R23 on the third surface is a spherical surface, and R24 on the fourth surface is a 10th-order aspheric surface. It is.
As shown in FIG. 5, in the imaging optical system of the present embodiment, most of the surfaces are spherical surfaces, and the only aspheric surface is a sixth-order aspheric surface. The order of the aspheric surface is lower than that of the comparative example. That is, the lens constituting the imaging optical system of the present embodiment uses one more lens than the comparative example, but has excellent processability and low cost. In addition, since a molding process using a precision mold such as a diffractive lens surface is not necessarily required, the initial cost is smaller than that of the comparative example, and the present invention can be employed even when the number of production is small.

  6 shows MTF (Modulation Transfer Function) on the optical axis and at the edge of the image circle of FIG. 4 in the imaging optical system of the comparative example and the present embodiment. MTF is a parameter for evaluating the imaging performance. As shown in FIG. 6, it can be seen that the present embodiment has an imaging performance equivalent to or higher than that of the comparative example.

Next, the spacer used for lens fixation is demonstrated. FIG. 7 is an explanatory diagram showing two lenses in two groups and three lenses in three groups.
In general, when a lens is fixed in a lens barrel, a part called a spacer may be used between the lenses. As shown in FIG. 7, since the comparative example has two groups and two elements, the required spacer is one of S1. On the other hand, in the case of the 3 group 3 sheet configuration, the required spacers are S2 and S3, and the cost of the machine parts is increased as compared with the 2 group 2 sheet configuration. Therefore, the imaging optical system of the present embodiment has the following configuration.

FIG. 8 is an explanatory diagram showing the relationship between the first group lens L11 and the second group lens L12 in the lens barrel 100 in the imaging optical system of the present embodiment. As illustrated, the edge portion of the first group lens L11 and the edge portion of the second group lens L12 are in close contact at E1. With this configuration, the spacer S2 shown in FIG. 7B becomes unnecessary. This is because the difference between the image side sag amount Z ₁ ′ of the first group lens and the object side sag amount Z ₂ ′ of the second group lens coincides with the distance D11 between the optical axes of the lens surfaces of the first group lens L11 and the second group lens L12. It means that
With this configuration, a spacer between the first group and the second group is unnecessary, and an increase in cost due to an increase in the number of mechanical parts due to the third group can be prevented.

  As described above, according to the imaging optical system of Embodiment 1, the first group, the second group, and the third group of lenses are provided, and the adjacent lenses between the first group and the second group or between the second group and the third group are arranged. Since the difference between the image side surface sag amount and the object side surface sag amount is made equal to the distance between the optical axes of the adjacent lens surfaces, the following effects can be obtained. That is, it is possible to use a thin spherical lens or a low-order aspheric lens excellent in manufacturing cost and quality without using a diffractive lens surface or a high-order aspheric surface. In addition, the number of lenses to be used is increased by one as compared with the conventional imaging optical system, but the spacers between adjacent lenses between the first group and the second group or between the second group and the third group can be eliminated. It is possible to achieve a low-cost imaging optical system without increasing the number of lens barrel mechanical parts while obtaining imaging performance equivalent to that of the imaging optical system. Furthermore, it is possible to cope with a case where the number of production in which a precision mold cannot be used is small.

  Further, according to the imaging optical system of Embodiment 1, the lens materials of the first group and the second group are made of chalcogenide glass, and the lens material of the third group is made of germanium. The cost as an image optical system can be reduced.

  Further, according to the imaging optical system of the first embodiment, since the used wavelength range is 8 μm to 12 μm, the cost can be reduced as an infrared camera lens.

Embodiment 2. FIG.
FIG. 9 shows an optical path diagram of the imaging optical system in the second embodiment. The optical system specifications are as shown in FIG. 4, and the first group lens L31 and the second group lens L32 use chalcogenide glass, and the third group lens L33 uses silicon. In FIG. 9, R31 to R36 indicate the first to sixth surfaces of the lens.
FIG. 10 shows lens data of the imaging optical system according to the second embodiment of the present invention. As shown in FIG. 10, in the imaging optical system of Embodiment 2, R35 on the first surface to R35 on the fifth surface are spherical surfaces, and R36 on the sixth surface is a sixth-order aspheric surface.
In the second embodiment, as in the first embodiment, most of the surfaces are spherical surfaces, and the only aspheric surface is a sixth-order aspheric surface. The order of the aspheric surfaces is lower than that of the comparative example shown in FIG. That is, the lens constituting the imaging optical system of Embodiment 2 uses one more lens than the comparative example, but has excellent processability and low cost. In addition, since a molding process using a precision mold such as a diffractive lens surface is not necessarily required, the initial cost is smaller than that of the comparative example, and the present invention can be employed even when the number of production is small.

FIG. 11 shows MTFs on the optical axis and at the image circle end in FIG. 4 in the comparative example and the second embodiment. As shown in FIG. 11, it can be seen that the imaging optical system of Embodiment 2 has an imaging performance equal to or higher than that of the comparative example.
Similarly to the first embodiment, the edge portion of the first group lens L31 and the edge portion of the second group lens L32 are in close contact at E3. That is, the difference between the sag amount of the image side surface (second surface) R32 and the sag amount of the object side surface (third surface) R33 of the adjacent lenses between the first group lens L31 and the second group lens L32 is the optical axis of these lens surfaces. It is comprised so that it may become equal to the distance D31. With such a configuration, it is not necessary to use a spacer between the first group lens L31 and the second group lens L32, and an increase in component costs can be suppressed.

  In the second embodiment, silicon is used for the lens material. Silicon absorbs in the wavelength range shown in FIG. For this reason, when the absorption by the three groups of silicon lenses cannot be ignored, it is practical to reduce the lens data so that the focal length is reduced to a level that does not affect the absorption of the silicon lenses.

  As described above, according to the imaging optical system of the second embodiment, the lens materials of the first group and the second group are made of chalcogenide glass, and the lens material of the third group is made of silicon. The cost of the imaging optical system can be reduced while holding the lens.

Embodiment 3 FIG.
FIG. 12 shows an optical path diagram of the imaging optical system in the third embodiment. The optical system specifications are as shown in FIG. 4, and the first group lens L41 and the second group lens L42 are made of germanium, and the third group lens L43 is made of silicon. In FIG. 12, R41 to R46 indicate the first to sixth surfaces of the lens.
FIG. 13 shows lens data of the imaging optical system according to Embodiment 3 of the present invention. As shown in FIG. 13, in the imaging optical system of Embodiment 3, R41 of the first surface to R45 of the fifth surface are spherical surfaces, and R46 of the sixth surface is a fourth-order aspherical surface.
In the third embodiment, as in the first embodiment, most of the surfaces are spherical surfaces, and the only aspheric surface is a fourth-order aspheric surface. The order of the aspheric surfaces is lower than that of the comparative example shown in FIG. In other words, the lens constituting the imaging optical system of Embodiment 3 uses one more lens than the comparative example, but has excellent processability and low cost. In addition, since a molding process using a precision mold such as a diffractive lens surface is not necessarily required, the initial cost is smaller than that of the comparative example, and the present invention can be employed even when the number of production is small.

FIG. 14 is an MTF on the optical axis and at the edge of the image circle in FIG. 4 in the comparative example and the third embodiment. As shown in FIG. 14, it can be seen that the imaging optical system of Embodiment 3 has an imaging performance equivalent to or higher than that of the comparative example.
In the imaging optical system according to the third embodiment, as in the first embodiment, the edge portion of the first lens group L41 and the edge portion of the second lens group L42 are in close contact with each other at E4. That is, the difference between the sag amount of the image side surface (second surface) R42 and the sag amount of the object side surface (third surface) R43 of the adjacent lenses between the first group lens L41 and the second group lens L42 is the optical axis of these lens surfaces. It is configured to be equal to the distance D41. With such a configuration, it is not necessary to use a spacer between the first group lens L41 and the second group lens L42, and an increase in component costs can be suppressed.

  In the third embodiment, as in the second embodiment, silicon is used for the lens material. Silicon absorbs in the wavelength range shown in FIG. For this reason, when the absorption by the three groups of silicon lenses cannot be ignored, it is practical to reduce the lens data so that the focal length is reduced to a level that does not affect the absorption of the silicon lenses.

  As described above, according to the imaging optical system of Embodiment 3, the lens materials of the first and second groups are made of germanium, and the lens material of the third group is made of silicon. The cost as an imaging optical system can be reduced while holding.

Embodiment 4 FIG.
FIG. 15 shows an optical path diagram of the imaging optical system in the fourth embodiment. The optical system specifications are as shown in FIG. 4, and the first group lens L51 and the third group lens L53 use silicon, and the second group lens L52 uses germanium. In FIG. 15, R51 to R56 denote the first to sixth surfaces of the lens.
FIG. 16 shows lens data of the imaging optical system according to Embodiment 4 of the present invention. As shown in FIG. 16, in the imaging optical system of Embodiment 3, R51 of the first surface to R55 of the fifth surface are spherical surfaces, and R56 of the sixth surface is a fourth-order aspherical surface.
As in the first embodiment of the present invention, in the fourth embodiment, most of the surfaces are spherical surfaces, and the only aspheric surface is a fourth-order aspheric surface, and the degree of the aspheric surface is lower than that of the comparative example. That is, the lens constituting the imaging optical system of Embodiment 4 uses one more lens than the comparative example, but has excellent processability and low cost. In addition, since a molding process using a precision mold such as a diffractive lens surface is not necessarily required, the initial cost is smaller than that of the comparative example, and the present invention can be employed even when the number of production is small.

FIG. 17 shows MTFs on the optical axis and at the image circle end in FIG. 4 in the comparative example and the fourth embodiment. As shown in FIG. 17, it can be seen that the imaging optical system of Embodiment 4 obtains imaging performance equivalent to that of the comparative example.
In the imaging optical system of the fourth embodiment, as in the first embodiment, the edge portion of the first group lens L51 and the edge portion of the second group lens L52 are in close contact with each other at E5. That is, the difference between the sag amount of the image side surface (second surface) R52 and the sag amount of the object side surface (third surface) R53 of the adjacent lenses between the first group lens L51 and the second group lens L52 is the optical axis of these lens surfaces. It is configured to be equal to the distance D51. With such a configuration, it is not necessary to use a spacer between the first group lens L51 and the second group lens L52, and an increase in component costs can be suppressed.

  In the fourth embodiment, silicon is used for the lens material, as in the second and third embodiments. Silicon absorbs in the wavelength range shown in FIG. Therefore, if the absorption by the 1st and 3rd group silicon lenses cannot be ignored, the lens data can be scaled down to a focal length that has a thickness that does not affect the absorption of the silicon lens. It is.

  As described above, according to the imaging optical system of Embodiment 4, the lens materials of the first group and the third group are made of silicon, and the lens material of the second group is made of germanium. The cost as an imaging optical system can be reduced while holding.

  In the first to fourth embodiments, the edge portions of the first group lenses L11, L31, L41, and L51 and the edge portions of the second group lenses L12, L32, L42, and L52 are E1, E3, E4, and E5. Although it is configured to be in close contact, this configuration is not limited to between the first group lens L11, L31, L41, L51 and the second group lens L12, L32, L42, L52, and depending on the aberration correction solution, the second group lens L12, It is also possible to carry out between L32, L42, L52 and the third lens group L13, L33, L43, L53.

  In the present invention, within the scope of the invention, any combination of the embodiments, or any modification of any component in each embodiment, or omission of any component in each embodiment is possible. .

  L11, L31, L41, L51 first lens group, L12, L32, L42, L52 second lens group, L13, L33, L43, L53 third lens group, R11, R31, R41, R51 first surface, R12, R32, R42, R52 2nd surface, R13, R33, R43, R53 3rd surface, R14, R34, R44, R54 4th surface, R15, R35, R45, R55 5th surface, R16, R36, R46, R56 6th surface, D1 , D3, D4, D5 Distance between optical axes of the lens surfaces of the first group lens and the second group lens, and edge portions of E1, E3, E4, E5 first group lens and the second group lens.

Claims (6)

  A first group, a second group, and a third group of lenses, and the difference between the image side sag amount and the object side sag amount between adjacent lenses between the first group and the second group or between the second group and the third group An imaging optical system characterized by being equal to the distance between the optical axes of the mating lens surfaces.   2. The imaging optical system according to claim 1, wherein the lens materials of the first group and the second group are chalcogenide glass, and the lens materials of the third group are germanium.   2. The imaging optical system according to claim 1, wherein the lens materials of the first group and the second group are chalcogenide glass, and the lens materials of the third group are silicon.   2. The imaging optical system according to claim 1, wherein the lens material of the first group and the second group is germanium, and the lens material of the third group is silicon.   2. The imaging optical system according to claim 1, wherein the lens material of the first group and the third group is silicon, and the lens material of the second group is germanium.   The imaging optical system according to any one of claims 2 to 5, wherein a usable wavelength range is 8 µm to 12 µm.

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