JPH0949968A - Lens system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lens system for electronic image-pickup device whose exit angle of a principal light ray is nearly parallel to the optical axis even while the system has a retro-focus form. SOLUTION: This system is composed of two negative and positive groups GN, GP, has an aperture diaphragm AP in between, the positive group GP includes three lens components, a positive lens component on the closest side to an image is composed of a doublet of a positive lens and a negative lens or a positive single lens, the positive lens component is a rear focus lens moving for focusing and setting conditions of the focal distance of the negative group GN, the reduced air distance on the optical axis between the aperture diaphragm and the surface on the side closest to the object of the positive group, a shape factor of a lens component moving for focusing, its focal distance, a distance on the optical axis between the surface of the group moving for focusing closest to the object and the surface closest to the image among lenses on the side closer to the object at the time of focusing on an object at infinity are satisfied.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a lens system,
In particular, the present invention relates to a photographing lens for an electronic camera that requires a long back focus with respect to the focal length in order to insert many optical elements such as prisms and filters between the imaging lens system and the electronic image pickup element.

[0002]

2. Description of the Related Art Conventionally, as in a single-lens reflex camera, since a quick return mirror is inserted between an imaging lens system and a silver salt film, a back focus is long even for a taking lens having a short focal length. A so-called retrofocus type lens system is usually used so as to ensure the above. However, when an electronic image pickup device is used instead of the silver salt film, it is desired that the light exit angle from the imaging lens system be as perpendicular to the image pickup device as possible. On the other hand, the retrofocus type lens system for a single-lens reflex camera has a large ray exit angle with respect to an off-axis object point. Therefore, it is difficult to use it as it is in a camera using an electronic image pickup device. Further, since the focal length is particularly short as compared with a lens system for a single-lens reflex camera, even if the whole payout method is used for focusing, the focus stroke is too small and the payout accuracy is severe.

[0003]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and an object thereof is to provide a chief ray (aperture stop and An optimal lens system is provided for the shape and configuration of the lens element and the setting of the position of the aperture stop such that the exit angle of an off-axis ray passing through the intersection with the optical axis is substantially parallel to the optical axis. Further, the present invention provides a retrofocus type lens system in which the entire extension method in which the focus stroke is determined by the focal length is stopped and the extension accuracy is relaxed by making good use of the partial extension method.

[0004]

In order to solve the above problems, the lens system of the present invention is composed of two groups, a negative group and a positive group, in order from the object side, and has an aperture stop in the middle. However, the positive group includes three positive lens components, of which the positive lens component closest to the image side is composed of a doublet of a positive lens and a negative lens, and the positive lens component is moved for focusing. The rear focus lens according to the present invention is characterized by satisfying the following conditions. −20 <f ₁ / f <−1 (1) 0.14 <D _AS /f<0.8 (2) 0 <(r _R1 + r _R2 ) / (r _R1 −r _R2 ) <0.9 ・ ・ ・ (3) 2.5 <f _R / f <10 ・ ・ ・ (4) 0.12 <D ₂₃ / f <1 ・ ・ ・ (5) where f is the focus of the entire system The distance, f ₁ is the focal length of the negative group, D _AS is the air-equivalent distance on the optical axis of the aperture stop and the surface of the positive group closest to the object side, and r _R1 is the maximum of the positive lens component closest to the image side. The radius of curvature of the object-side surface, r _R2 is the radius of curvature of the most image-side surface of the most image-side positive lens component, f _R is the focal length of the lens group moved for focusing, and D ₂₃ is the focus It is the distance at the time of focusing on an object point at infinity on the optical axis between the most object-side surface of the group moved for focusing and the most image-side surface of the lens on the object side.

Another lens system of the present invention is composed of two groups, a negative group and a positive group, in order from the object side, and further has an aperture stop in the middle thereof, and the positive group has three positive lens components. In the rear focus lens, in which the most positive lens component on the image side is composed of a single lens, and the positive lens component is moved for focusing, the following conditions are satisfied: Is. −20 <f ₁ / f <−1 (1) 0.14 <D _AS /f<0.8 (2) −2.0 <(r _R1 + r _R2 ) / (r _R1 − r _R2 ) <0 ・ ・ ・ (3 ′) 2.5 <f _R / f <10 ・ ・ ・ (4) 0.12 <D ₂₃ / f <1 ・ ・ ・ (5) where f is the whole system , F ₁ is the focal length of the negative group, D _AS is the air-equivalent distance on the optical axis of the aperture stop and the surface of the positive group closest to the object side, and r _R1 is the positive lens component closest to the image side. Is the radius of curvature of the surface closest to the object, r _R2 is the radius of curvature of the surface closest to the image of the positive lens component closest to the image, and f _R is the focal length of the group moved for focusing, D ₂₃ Is the distance at the time of focusing on an object point at infinity on the optical axis between the most object-side surface of the group moved for focusing and the most image-side surface of the lens on the object side.

In this case, the negative group is, in order from the object side,
The positive group is composed of two lenses, a positive lens and a negative lens.
It is composed of a total of four negative lenses and three positive lenses in order from the object side. Among the three positive lenses, only one positive lens closest to the image side is movable for focusing. It is desirable to satisfy the condition of. 0.1 <n ₁ −n ₂ <0.4 (6) ν ₁ <35 (7) In the range of 1.65 ≦ n _R <1.80, −2.0 <(r _{R1 + r R2) / (r} R1 -r R2) <- at 0.75 · (range of _{3'-1) 1.55 ≦ n R} <1.65, -0.95 <(r R1 + r R2) / (R _R1 −r _R2 ) <− 0.25 · (3′−2) 1.45 ≦ n _R <1.55 −0.45 <(r _R1 + r _R2 ) / (r _R1 −r _R2 ) <0 ··· (3′-3) where n _R is the medium refractive index of the _one positive lens closest to the image side, n ₁ is the medium refractive index of the negative lens positive lens, and n ₂
Is the medium refractive index of the negative lens of the negative group, and ν ₁ is the medium Abbe number of the positive lens of the negative group.

The reason and operation of the above configuration will be described below. The lens system of the present invention is composed of two groups, a negative group and a positive group, in order from the object side, and further has an aperture stop in the middle, and the positive group includes three positive lens components. The positive lens component on the side is composed of a doublet consisting of a positive lens and a negative lens, and this positive lens component is a rear focus type retrofocus lens that is moved for focusing. Is for air-converted lengths of 1.5 or more. Therefore, in the case of −20 <f ₁ / f <−1 (1), if the upper limit value of −1 is exceeded, the back focus becomes short, which is outside the scope of the present invention. On the other hand, the lower limit −
If it exceeds 20, the back focus tends to be long, but
Since it is difficult to correct each aberration and the imaging performance cannot be improved with a small number of constituents, this is also excluded from the object. The condition (1) is more preferably in the range of −4 <f ₁ /f<−1.1 (1 ′). When the positive lens component closest to the image side is composed of a doublet, it is best to satisfy the range of −2.1 <f ₁ /f<−1.2 (1 ″). On the other hand, when the positive lens component closest to the image side is composed of a single lens, it is best to satisfy the range of −3.3 <f ₁ /f<−1.8 (1 ″ ′). is there.

Next, the position of the aperture stop is specified under the following conditions so that the emission angle of the principal ray (the off-axis ray passing through the intersection of the aperture stop and the optical axis) can be made substantially parallel to the optical axis. 0.14 <D _AS /f<0.8 (2) D _AS is the air-equivalent distance on the optical axis of the aperture stop and the surface of the positive group closest to the object side. Under this condition (2), the chief ray exit angle can be made substantially parallel to the optical axis. The lower limit of 0.14
Beyond, like a conventional retrofocus lens,
The exit angle of the chief ray becomes large and the object of the present invention cannot be achieved. On the other hand, when the upper limit of 0.8 is exceeded, the ray height in the positive group having a large refractive power becomes too high, and it becomes difficult to correct the off-axis aberration. In this way, when the chief ray exit angle is set to be substantially parallel to the optical axis, the shape of the positive lens component closest to the image side is on the axis when the radius of curvature of the object side surface is reduced. It is preferable for both aberration and off-axis aberration. It should be noted that the range of 0.15 <D _AS /f<0.7 (2 ′) is more preferable. Further, the range of 0.25 <D _AS /f<0.6 (2 ") is the best.

Next, regarding the shape of the positive lens component closest to the image side, the condition is different between the doublet and the single lens, and when doublet, 0 <(r _R1 + r _R2 ) / (r _R1- r _R2 ). <0.9 ... (3) is satisfied, and when a single lens is used, −2.0 <(r _R1 + r _R2 ) / (r _R1 −r _R2 ) <0 ... (3 ′) is satisfied. It is necessary. Where r _R1 is the radius of curvature of the most object-side surface of the most image-side positive lens component, and r _R2
Is the radius of curvature of the most image-side surface of the most image-side positive lens component. Upper limit value 0.9 of equation (3) or equation (3 ')
Alternatively, when it exceeds 0, it is effective in the case where the exit angle of the principal ray is large as in the conventional retrofocus lens, and particularly in the case where the exit angle of the principal ray is substantially parallel to the optical axis as in the present invention. Not good for aberrations. On the other hand, if the lower limit value of each equation is 0 or -2.0, it is not preferable for off-axis aberrations. When doublet, 0 <(r _R1 + r _R2 ) / (r _R1 −r _R2 ) <0.5, and when single lens, −1.3 <(r _R1 + r _R2 ) / (r _R1 −r _R2 ). A range of <-0.1 is more preferable. Furthermore, in the case of a single lens, it is even more preferable that the range is −1.1 <(r _R1 + r _R2 ) / (r _R1 −r _R2 ) <− 0.15.

Next, the present invention provides a method of lengthening the focusing stroke of an optical system having an extremely short focal length to relax the feeding accuracy. It is a rear focus lens that satisfies the following conditions for one or two positive lens components closest to the image among the above three positive lens components of the positive group. 2.5 <f _R / f <10 (4) 0.12 <D ₂₃ / f <1 (5) where f _R is the focal length of the group that is movable for focusing. ,
D ₂₃ is the distance at the time of focusing on an object point at infinity on the optical axis between the most object-side surface of the group moved for focusing and the most image-side surface of the lens on the object side of the group. .

If the focal length of the group that is movable for focusing is as long as possible, the focusing stroke becomes longer and the payout accuracy may be slower. However, if the upper limit of 10 in the expression (4) is exceeded, not only the focusing stroke becomes too long, but also the distribution of the refracting power imposes a burden on the object side, which is disadvantageous in aberration correction. When the lower limit of 2.5 in the equation (4) is exceeded, the focusing stroke becomes too short and the feeding accuracy becomes severe, which defeats the purpose of the present invention. On the other hand, due to the lengthening of the focusing stroke, infinity on the optical axis between the most object-side surface of the lens that is movable for focusing and the most image-side surface of the lens closer to the object side It is necessary to secure a large distance when focusing on an object point. When the lower limit of 0.12 in the equation (5) is exceeded, the movable distance is too short to focus on the near object point. Further, the larger this distance, the more effective it is to make the chief ray exit angle smaller with respect to the optical axis. On the other hand, when the upper limit of 1 is exceeded, the total length becomes long, which is not preferable. It should be noted that the range of 2.7 <f _R / f <5 (4 ′) 0.12 <D ₂₃ /f<0.5 (5 ′) is more preferable. In the case of the focal length of the embodiment of the present invention, 2.7 <f _R /f<3.2 (4 ″) 0.13 <D ₂₃ /f<0.2 (5 ″) ) Is the best range.

Next, the negative group is composed of two lenses, a positive lens and a negative lens in order from the object side, and the positive group is a total of three negative lenses and three positive lenses in order from the object side. If the configuration is made up of four lenses and only one or two positive lenses on the image side of the three positive lenses are movable for focusing, if the following conditions are satisfied, then the total number of components Is 6
It is possible to provide a lens system for an electronic image pickup device having a small number of sheets, a long back focus, and good imaging performance. In this case, it is better to change the range of the shape factor of the positive lens closest to the image side according to the refractive index of the glass material. The refractive index is n
_{When R} , in the range of 1.65 ≦ n _R <1.80, −2.0 <(r _R1 + r _R2 ) / (r _R1 −r _R2 ) <− 0.75 ... (3′-1 ) In the range of 1.55 ≦ n _R <1.65, −0.95 <(r _R1 + r _R2 ) / (r _R1 −r _R2 ) <− 0.25 ·· (3′−2) 1.45 When ≦ n _R <1.55: −0.45 <(r _R1 + r _R2 ) / (r _R1 −r _R2 ) <0 ··· (3′−3) is satisfied.

When only the positive lens is movable for focusing, it is preferable that the positive lens closest to the image side satisfies the above range. As in the previous case, when the upper limit values are exceeded, it is effective when the exit angle of the chief ray is large as in the conventional retrofocus lens, and the exit angle of the chief ray with respect to the optical axis is the same as in the present invention. Especially when they are substantially parallel to each other, it is not preferable for axial aberration. Further, if the respective lower limit values are exceeded, it is rather undesirable for off-axis aberrations. Incidentally, in such a lens system in which the chief ray exit angle is substantially parallel to the optical axis, the Petzval sum tends to take a large positive value, and the off-axis imaging performance is likely to deteriorate. To correct this, it is well known that the positive lens has a high refractive index and the negative lens has a low refractive index. However, when this is performed in the positive group, spherical aberration, coma aberration,
Since the chromatic aberration is aggravated, the Petzval sum can be corrected without deterioration of other aberrations when the negative group is used. Therefore, 0.1 <n ₁ −n ₂ <0.4 (6). If the lower limit of 0.1 in the equation (6) is exceeded, the Petzval sum cannot be sufficiently corrected, and the off-axis performance tends to deteriorate. The upper limit of 0.4 does not have to be set within the range of ordinary glass materials, but theoretically the Petzval sum deteriorates in the opposite direction. When the refractive index of the positive lens is 2 or more, theoretically, the centering accuracy is likely to be deteriorated. However, n ₁ is the medium refractive index of the positive lens of the negative group, and n ₂ is the medium refractive index of the negative lens of the negative group. On the other hand, the Abbe number of the positive lens in the negative group should preferably satisfy ν ₁ <35 (7). When the upper limit value of 35 is exceeded, lateral chromatic aberration cannot be completely corrected. The lower limit does not have to be set for a glass material that actually exists (23 <ν ₁ ). It is more preferable to set 0.14 <n ₁ −n ₂ <0.35 (6 ′) ν ₁ <28 (7 ′).

[0014]

Embodiments 1 to 6 of the lens system of the present invention will be described below. 1 to 6 show lens cross sections of Examples 1 to 6 when focusing on an object point at infinity, respectively. In each figure, GN is a negative group, AP is an aperture stop, GP is a positive group, P1 to P4 are depolarizers, low-pass filters,
A plane parallel plate such as a half prism, P5 is a cover glass.

As shown in FIG. 1, in the lens structure of the first embodiment, the negative group GN is composed of a negative meniscus lens having a convex surface facing the object side, a biconvex lens, and a negative meniscus lens having a convex surface facing the object side. Through the aperture stop AP, the positive group GP is
A biconvex lens, a biconcave lens, a positive meniscus lens with a convex surface facing the image side, and a cemented lens of a biconvex lens and a negative meniscus lens with a convex surface facing the image side.
3 and the cover glass P5 are arranged on the image side of the positive group GP. Focusing is performed by extending the cemented lens closest to the image side of the positive lens group GP. The amount of extension to the object side is 0.90 mm for an object distance of 20 cm.

In the lens structure of the second embodiment, as shown in FIG. 2, the negative group GN is composed of a positive meniscus lens having a convex surface facing the object side and a negative meniscus lens having a convex surface facing the object side. The positive group GP includes a cemented lens of a concave plano lens and a plano-convex lens, a positive meniscus lens having a convex surface facing the image side, and a biconvex lens and a negative meniscus having a convex surface facing the image side with a plane-parallel plate P1 interposed therebetween. The plane-parallel plates P2 to P3 and the cover glass P5 are formed of cemented lenses, and are arranged on the image side of the positive group GP. Focusing is performed by extending the cemented lens closest to the image side of the positive lens group GP. The amount of extension to the object side is 0.91 for an object distance of 20 cm
mm.

In the lens configuration of the third embodiment, as shown in FIG. 3, the negative group GN is composed of a positive meniscus lens having a convex surface directed toward the object side and a negative meniscus lens having a convex surface directed toward the object side. Through, the positive group GP is composed of a cemented lens of a positive meniscus lens having a convex surface facing the image side and a negative meniscus lens having a convex surface facing the image side, a positive meniscus lens having a convex surface facing the image side, and a biconvex lens, Parallel plane plate P1
P3 and the cover glass P5 are arranged on the image side of the positive group GP. Focusing is performed by extending the biconvex lens closest to the image side of the positive group GP. The amount of extension to the object side is 0.81 mm for an object distance of 20 cm.

As shown in FIG. 4, in the lens structure of the fourth embodiment, the negative group GN is composed of a positive meniscus lens having a convex surface facing the object side and a negative meniscus lens having a convex surface facing the object side. Through, the positive group GP includes a negative meniscus lens having a convex surface facing the image side, a positive meniscus lens having a convex surface facing the image side, a biconvex lens, and a positive meniscus lens having a convex surface facing the object side. P1 to P3 and the cover glass P5 are arranged on the image side of the positive group GP. Focusing is performed by extending the positive meniscus lens closest to the image side of the positive group GP. The amount of extension to the object side is 0.85 mm for an object distance of 20 cm.

In the lens construction of the fifth embodiment, as shown in FIG. 5, the negative group GN is composed of a positive meniscus lens having a convex surface directed toward the object side and a negative meniscus lens having a convex surface directed toward the object side. Through, the positive group GP is a biconcave lens,
A positive meniscus lens with a convex surface facing the image side, a biconvex lens,
It is composed of a biconvex lens, and the plane-parallel plates P1 to P4 and the cover glass P5 are arranged on the image side of the positive group GP. Focusing is performed by extending the biconvex lens closest to the image side of the positive group GP. The amount of extension to the object side is 0.82 mm for an object distance of 20 cm.

In the lens construction of the sixth embodiment, as shown in FIG. 6, the negative group GN is composed of a biconvex lens and a negative meniscus lens having a convex surface facing the object side. , A biconcave lens, a positive meniscus lens having a convex surface facing the image side, a biconvex lens, and a biconvex lens, and the plane-parallel plates P1 to P4 and the cover glass P5 are arranged on the image side of the positive group GP. Focusing is performed by extending the biconvex lens closest to the image side of the positive group GP. The amount of extension to the object side is 0.82 mm for an object distance of 20 cm.

Numerical data of each of the above-mentioned embodiments will be shown below. Symbols are other than the above, f is the focal length of the entire system, F _NO is the F number, ω is the half angle of view, f _B is the back focus, and r ₁ , R
₂ ... curvature radius of each lens surface, d _1, d ₂ ... the spacing between the lens surfaces, n _d1, n _d2 ... d-line refractive index of each lens,
ν _d1 , ν _d2 ... are Abbe numbers of the respective lenses.

Example 1 f = 12,000, F _NO = 2.8, ω = 20.6 °, f _B /f=1.566 r ₁ = 17.5037 d ₁ = 0.900 n _d1 = 1.51633 ν _d1 = 64.15 r ₂ = 7.8220 d ₂ = 1.240 r ₃ = 42.0901 d ₃ = 1.900 n _d2 = 1.69895 ν _d2 = 30.12 r ₄ = -39.7078 d ₄ = 0.600 r ₅ = 29.1518 d ₅ = 0.800 n _d3 = 1.51633 ν _d3 = 64.15 r ₆ = 6.2343 d ₆ = 4.000 r ₇ = ∞ _{(stop) d 7 = 6.000 r 8 =} 13.9321 d 8 = 2.900 n d4 = 1.80610 ν d4 = 40.95 r 9 = -16.6901 d 9 = 0.010 r 10 = -16.4839 d 10 = 0.900 n d5 = 1.74950 ν d5 = 35.27 r ₁₁ = 13.1922 d ₁₁ = 0.830 r ₁₂ = -50.4005 d ₁₂ = 2.100 n _d6 = 1.77250 ν _d6 = 49.60 r ₁₃ = -11.1976 d ₁₃ = 2.000 r ₁₄ = 41.7761 d ₁₄ = 3.300 n _d7 = 1.51633 ν _d7 = 64.15 r ₁₅ = -9.1587 d ₁₅ = 1.000 n _d8 = 1.84666 ν _d8 = 23.78 r ₁₆ = -16.9185 d ₁₆ = 2.000 r ₁₇ = ∞ d ₁₇ = 5.000 n _d9 = 1.54771 ν _d9 = 62.83 r ₁₈ = ∞ d ₁₈ = 2.240 n _d10 = 1.54771 ν _d10 = 62.83 r ₁₉ = ∞ d ₁₉ = 10.000 nd ₁₁ = 1.51633 ν _d11 = 64.15 r ₂₀ = ∞ d ₂₀ = 3.610 r ₂₁ = ∞ d ₂₁ = 0.750 n _d12 = 1.48749 ν _d12 = 70.20 r ₂₂ = ∞.

Example 2 f = 12.003, F _NO = 2.8, ω = 20.6 °, f _B /f=1.296 r ₁ = 34.0111 d ₁ = 2.000 n _d1 = 1.84666 ν _d1 = 23.78 r ₂ = 391.9139 d ₂ = 0.200 r ₃ = 20.1738 d ₃ = 1.200 n _d2 = 1.51742 ν _d2 = 52.41 r ₄ = 5.3841 d ₄ = 7.000 r ₅ = ∞ (aperture) d ₅ = 4.500 r ₆ = -13.0319 d ₆ = 0.900 n _d3 = 1.64769 ν _d3 = 33.80 r ₇ = ∞ d ₇ = 3.100 n _d4 = 1.77250 ν _d4 = 49.60 r ₈ = -11.7141 d ₈ = 0.150 r ₉ = -54.8602 d ₉ = 1.800 n _d5 = 1.78590 ν _d5 = 44.19 r ₁₀ = -19.5142 d ₁₀ = 0.200 r ₁₁ = ∞ d ₁₁ = 5.000 n _d6 = 1.54771 ν _d6 = 62.83 r ₁₂ = ∞ d ₁₂ = 2.000 r ₁₃ = 42.2244 d ₁₃ = 3.600 n _d7 = 1.72000 ν _d7 = 50.25 r ₁₄ = -10.7376 d ₁₄ = 1.000 n _d8 = 1.84666 ν _d8 = 23.78 r ₁₅ = -34.7936 d ₁₅ = 2.000 r ₁₆ = ∞ d ₁₆ = 2.240 _{nd 9} = 1.54771 ν _d9 = 62.83 r ₁₇ = ∞ d ₁₇ = 10.000 n _d11 = 1.51633 ν _d11 = _{64.15 r 18 = ∞ d 18 =} 3.610 r 19 = ∞ d 19 = 0.750 n d12 = 1.48749 ν d12 = 70.20 ₂₀ = ∞.

Example 3 f = 1.005, F _NO = 2.8, ω = 20.6 °, f _B /f=1.564 r ₁ = 21.2963 d ₁ = 2.000 n _d1 = 1.84666 ν _d1 = 23.78 r ₂ = 76.7949 d ₂ = 0.200 r ₃ = 21.4802 d ₃ = 1.200 n _d2 = 1.48749 ν _d2 = 70.20 r ₄ = 5.7893 d ₄ = 7.000 r ₅ = ∞ (aperture) d ₅ = 3.000 r ₆ = -9.1368 d ₆ = 6.000 n _d3 = 1.74400 ν _d3 = 44.79 r ₇ = -5.3343 d ₇ = 0.900 n _d4 = 1.84666 ν _d4 = 23.78 r ₈ = -10.4492 d ₈ = 0.500 r ₉ = -58.9796 d ₉ = 2.000 n _d5 = 1.69680 ν _d5 = 55.53 r ₁₀ = -16.9237 d ₁₀ = 1.800 r ₁₁ = 32.1871 d ₁₁ = 2.100 n _d6 = 1.69680 ν _d6 = 55.53 r ₁₂ = -78.1698 d ₁₂ = 2.000 r ₁₃ = ∞ d ₁₃ = 5.000 n _d7 = 1.54771 ν _d7 = 62.83 r ₁₄ = ∞ d ₁₄ = 2.240 n _d8 = 1.54771 ν _d8 = 62.83 r ₁₅ = ∞ d ₁₅ = 10.000 n _d9 = 1.51633 ν _d9 = 64.15 r ₁₆ = ∞ d ₁₆ = 3.610 r ₁₇ = ∞ d ₁₇ = 0.750 n _d10 = 1.48749 ν _d10 = 70.20 r ₁₈ = ∞.

Example 4 f = 12,000, F _NO = 2.8, ω = 20.6 °, f _B /f=1.565 r ₁ = 16.1929 d ₁ = 2.400 n _d1 = 1.84666 ν _d1 = 23.78 r ₂ = 53.8295 d ₂ = 0.150 r ₃ = 15.9084 d ₃ = 1.000 n _d2 = 1.69680 ν _d2 = 55.53 r ₄ = 5.3300 d ₄ = 10.000 r ₅ = ∞ (aperture) d ₅ = 2.000 r ₆ = -9.8812 d ₆ = 0.900 n _d3 = 1.84666 ν _d3 = 23.78 r ₇ = -476.4836 d ₇ = 0.180 r ₈ = -31.3409 d ₈ = 1.900 n _d4 = 1.69680 ν _d4 = 55.53 r ₉ = -10.0962 d ₉ = 0.150 r ₁₀ = 6482.1895 d ₁₀ = 2.300 n _d5 = 1.69680 ν _{d5 = 55.53 r 11 = -11.4666 d} 11 = 2.000 r 12 = 24.8349 d 12 = 1.700 n d6 = 1.69680 ν d6 = 55.53 r 13 = 1833.2522 d 13 = 2.000 r 14 = ∞ d 14 = 5.000 n d7 = 1.54771 ν d7 = 62.83 r ₁₅ = ∞ d ₁₅ = 2.240 n _d8 = 1.54771 ν _d8 = 62.83 r ₁₆ = ∞ d ₁₆ = 10.000 n _d9 = 1.51633 ν _d9 = 64.15 r ₁₇ = ∞ d ₁₇ = 3.610 r ₁₈ = ∞ d ₁₈ = 0.750 n _d10 = 1.48749 ν _d10 = 70.20 r ₁₉ = ∞.

Example 5 f = 12,000, F _NO = 2.8, ω = 20.6 °, f _B /f=1.641 r ₁ = 14.8369 d ₁ = 2.400 n _d1 = 1.84666 ν _d1 = 23.78 r ₂ = 54.8349 d ₂ = 0.300 r ₃ = 19.4900 d ₃ = 1.000 n _d2 = 1.69680 ν _d2 = 55.53 r ₄ = 5.0060 d ₄ = 6.500 r ₅ = ∞ (aperture) d ₅ = 4.500 r ₆ = -13.3748 d ₆ = 0.900 n _d3 = 1.84666 ν _d3 = 23.78 r ₇ = 160.8036 d ₇ = 0.250 r ₈ = -33.9182 d ₈ = 2.000 n _d4 = 1.69680 ν _d4 = 55.53 r ₉ = -10.9615 d ₉ = 0.150 r ₁₀ = 179.5907 d ₁₀ = 3.000 n _d5 = 1.56873 ν _d5 = 63.16 r ₁₁ = -10.8699 d ₁₁ = 1.600 r ₁₂ = 29.2589 d ₁₂ = 2.100 n _d6 = 1.56873 ν _d6 = 63.16 r ₁₃ = -64.5253 d ₁₃ = 2.000 r ₁₄ = ∞ d ₁₄ = 5.000 n _d7 = 1.54771 ν _d7 = 62.83 r ₁₅ = ∞ d ₁₅ = 1.600 n _d8 = 1.51633 ν _d8 = 64.15 r ₁₆ = ∞ d ₁₆ = 2.240 n _d9 = 1.54771 ν _d9 = 62.83 r ₁₇ = ∞ d ₁₇ = 13.000 n _d10 = 1.51633 ν _d10 = 64.15 r ₁₈ = ∞ d ₁₈ = 1.500 r ₁₉ = ∞ d ₁₉ = 0.750 n _d11 = 1.48749 ν _d11 = 70.2 0 r ₂₀ = ∞.

Example 6 f = 12,000, F _NO = 2.8, ω = 20.6 °, f _B /f=1.642 r ₁ = 19.7495 d ₁ = 2.200 n _d1 = 1.84666 ν _d1 = 23.78 r ₂ = 298.3938 d ₂ = 0.600 r ₃ = 47.3674 d ₃ = 0.900 n _d2 = 1.51633 ν _d2 = 64.15 r ₄ = 4.9415 d ₄ = 4.500 r ₅ = ∞ (aperture) d ₅ = 6.500 r ₆ = -15.1388 d ₆ = 0.900 n _d3 = 1.84666 ν _d3 = 23.78 r ₇ = 104.4931 d ₇ = 0.250 r ₈ = -36.0058 d ₈ = 2.200 n _d4 = 1.56873 ν _d4 = 63.16 r ₉ = -10.4558 d ₉ = 0.150 r ₁₀ = 107.1696 d ₁₀ = 3.100 nd _d = 1.56873 ν _d5 = 63.16 r ₁₁ = -11.4135 d ₁₁ = 1.600 r ₁₂ = 30.9530 d ₁₂ = 2.300 n _d6 = 1.51633 ν _d6 = 64.15 r ₁₃ = -44.2824 d ₁₃ = 2.000 r ₁₄ = ∞ d ₁₄ = 5.000 n _d7 = 1.54771 ν _d7 = 62.83 r ₁₅ = ∞ d ₁₅ = 1.600 n _d8 = 1.51633 ν _d8 = 64.15 r ₁₆ = ∞ d ₁₆ = 2.240 n _d9 = 1.54771 ν _d9 = 62.83 r ₁₇ = ∞ d ₁₇ = 13.000 n _d10 = 1.51633 ν _d10 = 64.15 r ₁₈ = ∞ d ₁₈ = 1.500 r ₁₉ = ∞ d ₁₉ = 0.750 n _d11 = 1.48749 ν _d11 = 70. 20 r ₂₀ = ∞.

7 to 8 are aberration charts of Example 1 when focusing on an object point at infinity and an object point at a short distance (object distance of 20 cm), respectively. In the figure, (a) is spherical aberration, (b) is astigmatism, (c) is distortion, and (d) is lateral chromatic aberration. Similar aberration diagrams for Examples 2 to 6 are shown in FIGS. 9 to 1, respectively.
0, FIGS. 11 to 12, 13 to 14, and 15 to 1
6, shown in FIGS.

Next, the conditional expressions (1)-
(5) (Regarding conditional expression (3), conditional expression (3 ′) may be used instead, but conditional expression (3)
And ) Values are shown in the following table. *: 0.452 without parallel plane plate (filter).

[0030]

By using the lens system of the present invention described in detail above, it is possible to obtain a lens system for an electronic image pickup device which has a long back focus, a small light exit angle, and good imaging performance. Further, even if the focal length is extremely short, a long focus stroke can be taken, and the focus accuracy can be relaxed.

[Brief description of drawings]

FIG. 1 is a lens sectional view of a lens system of Example 1 of the present invention when focusing on an object point at infinity.

FIG. 2 is a lens cross-sectional view of a lens system of Example 2 of the present invention when focused on an object point at infinity.

FIG. 3 is a lens cross-sectional view of a lens system of Example 3 of the present invention when an object point at infinity is focused.

FIG. 4 is a lens cross-sectional view of a lens system of Example 4 of the present invention when focused on an object point at infinity.

FIG. 5 is a lens sectional view of a lens system of Example 5 of the present invention when focusing on an object point at infinity.

FIG. 6 is a lens cross-sectional view of a lens system of Example 6 of the present invention when focused on an object point at infinity.

FIG. 7 is an aberration diagram for Example 1 upon focusing on an object point at infinity.

FIG. 8 is an aberration diagram for Example 1 upon focusing on an object point at a short distance.

FIG. 9 is an aberration diagram for Example 2 upon focusing on an object point at infinity.

FIG. 10 is an aberration diagram for Example 2 upon focusing on an object point at a short distance.

FIG. 11 is an aberration diagram for Example 3 upon focusing on an object point at infinity.

FIG. 12 is an aberration diagram of Example 3 upon focusing on a short-distance object point.

FIG. 13 is an aberration diagram for Example 4 upon focusing on an object point at infinity.

FIG. 14 is an aberration diagram for Example 4 upon focusing on an object point at a short distance.

FIG. 15 is an aberration diagram for Example 5 upon focusing on an object point at infinity.

FIG. 16 is an aberration diagram for Example 5 upon focusing on an object point at a short distance.

FIG. 17 is an aberration diagram for Example 6 upon focusing on an object point at infinity.

FIG. 18 is an aberration diagram for Example 6 upon focusing on an object point at a short distance.

[Explanation of symbols]

 GN ... Negative group AP ... Aperture stop GP ... Positive group P1 to P4 ... Parallel plane plate P5 ... Cover glass

Claims (3)

[Claims] 1. A lens unit comprising, in order from the object side, two groups, a negative group and a positive group, and further having an aperture stop in the middle, the positive group includes three positive lens components, and the most image side among them. A lens system characterized by satisfying the following conditions in a rear focus lens in which the positive lens component of is composed of a doublet of a positive lens and a negative lens, and the positive lens component is moved for focusing. −20 <f ₁ / f <−1 (1) 0.14 <D _AS /f<0.8 (2) 0 <(r _R1 + r _R2 ) / (r _R1 −r _R2 ) <0.9 ・ ・ ・ (3) 2.5 <f _R / f <10 ・ ・ ・ (4) 0.12 <D ₂₃ / f <1 ・ ・ ・ (5) where f is the focus of the entire system The distance, f ₁ is the focal length of the negative group, D _AS is the air-equivalent distance on the optical axis of the aperture stop and the surface of the positive group closest to the object side, and r _R1 is the maximum of the positive lens component closest to the image side. The radius of curvature of the object-side surface, r _R2 is the radius of curvature of the most image-side surface of the most image-side positive lens component, f _R is the focal length of the lens group moved for focusing, and D ₂₃ is the focus It is the distance at the time of focusing on an object point at infinity on the optical axis between the most object-side surface of the group moved for focusing and the most image-side surface of the lens on the object side. 2. The optical system is composed of two groups, a negative group and a positive group, arranged in order from the object side, and further has an aperture stop in the middle, the positive group including three positive lens components, and the most image side among them. A rear focus lens in which the positive lens component of is composed of a single lens, and the positive lens component is moved for focusing, a lens system characterized by satisfying the following conditions. −20 <f ₁ / f <−1 (1) 0.14 <D _AS /f<0.8 (2) −2.0 <(r _R1 + r _R2 ) / (r _R1 − r _R2 ) <0 ・ ・ ・ (3 ′) 2.5 <f _R / f <10 ・ ・ ・ (4) 0.12 <D ₂₃ / f <1 ・ ・ ・ (5) where f is the whole system , F ₁ is the focal length of the negative group, D _AS is the air-equivalent distance on the optical axis of the aperture stop and the surface of the positive group closest to the object side, and r _R1 is the positive lens component closest to the image side. Is the radius of curvature of the surface closest to the object, r _R2 is the radius of curvature of the surface closest to the image of the positive lens component closest to the image, and f _R is the focal length of the group moved for focusing, D ₂₃ Is the distance at the time of focusing on an object point at infinity on the optical axis between the most object-side surface of the group moved for focusing and the most image-side surface of the lens on the object side. 3. The negative group is composed of two positive lenses and negative lenses in order from the object side, and the positive group is composed of four negative lenses and three positive lenses in order from the object side. The lens system according to claim 2, wherein only one of the three positive lenses closest to the image side is movable for focusing, and the following condition is satisfied. 0.1 <n ₁ −n ₂ <0.4 (6) ν ₁ <35 (7) In the range of 1.65 ≦ n _R <1.80, −2.0 <(r _{R1 + r R2) / (r} R1 -r R2) <- at 0.75 · (range of _{3'-1) 1.55 ≦ n R} <1.65, -0.95 <(r R1 + r R2) / (R _R1 −r _R2 ) <− 0.25 · (3′−2) 1.45 ≦ n _R <1.55 −0.45 <(r _R1 + r _R2 ) / (r _R1 −r _R2 ) <0 ··· (3′-3) where n _R is the medium refractive index of the _one positive lens closest to the image side, n ₁ is the medium refractive index of the negative lens positive lens, and n ₂
Is the medium refractive index of the negative lens of the negative group, and ν ₁ is the medium Abbe number of the positive lens of the negative group.