JP2012173435A - Fixed-focus lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small sized, light-weight and wide angle fixed-focus lens of an inner focus method provided with excellent imaging performance.SOLUTION: The fixed-focus lens is composed by arranging a first lens group Ghaving negative refractive power, a second lens group Ghaving negative refractive power and a third lens group Ghaving positive refractive power in this order from an object side. The second lens group Gis composed of a single lens element. In addition, when focusing, the second lens group Gis moved along an optical axis and the first lens group Gand the third lens group Gare fixed to an image formation surface IMG.

Description

  The present invention relates to a fixed focus lens suitable for a 35 mm camera, a video camera, an electronic still camera, and the like.

  In order to make a captured image and a viewfinder image coincide with each other, a single lens reflex camera has a mechanism for reflecting light passing through a photographing lens by a mirror placed in front of the film and guiding the light to an optical viewfinder. For this reason, a fixed focus lens used in a single-lens reflex camera requires a long back focus, and the degree of design freedom is limited. On the other hand, a digital camera can realize the same as a conventional single-lens reflex camera simply by displaying an image captured by an image sensor on an electronic viewfinder. For this reason, a so-called “mirrorless single-lens camera” has emerged that realizes downsizing of the apparatus by omitting an optical viewfinder and a mirror for guiding a photographed image thereto. In the mirrorless single-lens camera, the back focus of the photographing lens can be shortened, so that there is an advantage that the degree of freedom in designing the fixed focus lens used for this is improved. Therefore, there are an increasing number of fixed focus lenses that can be mounted on mirrorless single-lens cameras (for example, see Patent Documents 1 and 2).

JP 2009-271354 A Japanese Patent No. 3445554

  Although the optical system disclosed in Patent Document 1 has a simple configuration in that the focus group is configured by one negative lens, the lens group other than the focus group has a simple configuration with a large number of lenses. It is not planned. In addition, since the angle of view is as narrow as about 6 degrees and the first lens group has a positive refractive power, the total length of the optical system tends to be long, and it cannot be said that sufficient miniaturization is achieved. For the above reasons, the optical system disclosed in Patent Document 1 is not suitable for a mirrorless single-lens camera that is required to be further downsized and widened in recent years.

  In addition, the optical system disclosed in Patent Document 2 is configured by a single negative lens to form a focus group, and a lens group other than the focus group is also formed from a small number of lenses, thereby simplifying the configuration. It can be said that However, the angle of view is as narrow as about 25 degrees. Further, since the first lens group has a positive refractive power, the total length of the optical system tends to be long. From the above, the optical system disclosed in Patent Document 2 is also unsuitable for a mirrorless single-lens camera that is required to be further reduced in size and widened in recent years.

  As described above, none of the conventional fixed focus lenses including the techniques described in the above-mentioned patent documents has achieved a sufficiently small size and wide angle.

  An object of the present invention is to provide an inner focus type fixed focus lens that is compact, lightweight, wide-angle, and has excellent imaging performance, in order to eliminate the above-described problems caused by the prior art.

  In order to solve the above-described problems and achieve the object, a fixed focus lens according to the present invention includes a first lens group having negative refractive power and a second lens having negative refractive power, which are arranged in order from the object side. A lens group and a third lens group having a positive refractive power, and the second lens group is composed of a single lens element, and during focusing, the second lens group moves along the optical axis, The first lens group and the third lens group are fixed with respect to the image plane.

  According to the present invention, it is possible to provide an inner focus type fixed focus lens that is compact, lightweight, wide-angle, and excellent in imaging performance.

  Furthermore, the fixed focus lens according to the present invention is characterized in that, in the above invention, an aperture stop is disposed between the second lens group and the third lens group.

  According to the present invention, it is possible to provide an inner focus type fixed focus lens that is lightweight and excellent in imaging performance.

Furthermore, the fixed focus lens according to the present invention is characterized in that, in the above invention, the following conditional expression is satisfied.
(1) −3.5 ≦ (R1 + R2) / (R1−R2) ≦ 1
Here, R1 represents the radius of curvature of the second lens group closest to the object side, and R2 represents the radius of curvature closest to the image side of the second lens group.

  According to the present invention, it is possible to improve the imaging performance without hindering the downsizing of the optical system.

Furthermore, the fixed focus lens according to the present invention is characterized in that, in the above invention, the following conditional expression is satisfied.
(2) 0.7 ≦ | (1-β2G) × β3G | ≦ 2.5
Here, β2G represents the paraxial imaging magnification of the second lens group in the infinitely focused state, and β3G represents the paraxial imaging magnification of the third lens group in the infinitely focused state.

  According to the present invention, it is possible to further improve the imaging performance without hindering the downsizing of the optical system.

Furthermore, the fixed focus lens according to the present invention is characterized in that, in the above invention, the following conditional expression is satisfied.
(3) 1.5 ≦ | F1 / F | ≦ 4.1
Here, F1 represents the focal length of the first lens group, and F represents the focal length of the entire optical system.

  According to the present invention, a simple and small fixed focus lens can be realized while maintaining high optical performance.

  According to the present invention, there is an effect that it is possible to provide an inner focus type fixed focus lens having a small size, light weight, wide angle and excellent imaging performance.

1 is a cross-sectional view along the optical axis showing the configuration of a fixed focus lens according to Example 1. FIG. FIG. 5 is a diagram illustrating various aberrations of the fixed focus lens according to Example 1 in an infinitely focused state. FIG. 6 is a diagram illustrating various aberrations of the fixed focus lens according to Example 1 in a focusing state of 0.025 times the shooting magnification. FIG. 6 is a diagram illustrating various aberrations of the fixed focus lens according to Example 1 in the closest focus state. FIG. 6 is a cross-sectional view along the optical axis showing the configuration of a fixed focus lens according to Example 2. FIG. 10 is a diagram illustrating various aberrations of the fixed focus lens according to Example 2 in an infinitely focused state. FIG. 10 is a diagram illustrating various aberrations of the fixed focus lens according to Example 2 when the imaging magnification is in the in-focus state of 0.025. FIG. 9 is a diagram illustrating various aberrations of the fixed focus lens according to Example 2 in the closest focus state. FIG. 6 is a cross-sectional view along the optical axis showing the configuration of a fixed focus lens according to Example 3; FIG. 9 is a diagram illustrating various aberrations of the fixed focus lens according to Example 3 in an infinitely focused state. FIG. 10 is a diagram illustrating all aberrations of the fixed focus lens according to Example 3 when the photographing magnification is in the focused state of 0.025. FIG. 10 is a diagram illustrating various aberrations of the fixed focus lens according to Example 3 in a state of focusing at the closest distance.

  Hereinafter, preferred embodiments of the fixed focus lens according to the present invention will be described in detail.

  The fixed focus lens according to the present invention includes a first lens group having a negative refractive power, a second lens group having a negative refractive power, and a third lens having a positive refractive power, which are arranged in order from the object side. And a group.

  The first lens group includes at least one negative lens and one positive lens. As described above, since the first lens group has a negative refractive power, the first lens group has a particularly large distortion and may occur on the under side. The distortion generated here can be corrected by the third lens group. However, if the third lens group has too much correcting action, spherical aberration is greatly generated in the third lens group on the under side, which is not preferable. Therefore, it is more preferable to form an aspherical surface in the lenses constituting the first lens group, and to correct distortion aberration generated in the first lens group by the aspherical effect. The first lens group is fixed with respect to the imaging surface.

  The second lens group is preferably composed of a single lens element. In the fixed focus lens according to the present invention, focusing is performed by moving the second lens group in a direction along the optical axis. For this reason, reducing the weight by configuring the second lens group, which is the focus group, as a single lens element can reduce the load on the autofocus mechanism, reduce power consumption, and reduce the outer diameter of the lens barrel. it can. The single lens element includes a single polished lens, an aspheric lens, a composite aspheric lens, and a cemented lens, and does not include, for example, two positive and negative lenses that have an air layer and are not bonded to each other.

  In the fixed focus lens according to the present invention, the second lens group is a focus group. The light beam is the thinnest in the optical system in the vicinity of the second lens group, and the focus group is the first group. This is because the aperture of the two lens groups can be minimized. That is, as in the present invention, when an optical system having a relatively wide angle (about 70 to 80 degrees) is intended to be realized by the negative group leading type, the axial F number light beam is narrowed by the first lens group, and the image side It gets thicker toward. On the other hand, off-axis rays pass through a portion higher than the optical axis in the first lens group, and pass through the lowest position relative to the optical axis in the vicinity of the second lens group. From the behavior of the on-axis light beam and off-axis light beam, it can be said that the portion where the second lens group is disposed is the portion where the light beam is the thinnest in the optical system. Therefore, the aperture of the second lens group disposed in the portion where the light beam becomes the thinnest in the optical system can be minimized in the optical system, and the weight reduction of the optical system can be promoted.

  The third lens group is fixed with respect to the image plane. This may be allowed to move, but is preferably fixed in order to prevent the lens holding mechanism from being destroyed by the entry of a finger or the like from the outside of the lens barrel.

  In the fixed focus lens according to the present invention, it is preferable that an aperture stop is disposed between the second lens group and the third lens group. By doing so, it becomes possible to easily correct various aberrations by appropriately dispersing positive and negative refractive powers before and after the aperture stop. If the aperture stop is disposed closer to the object side than the second lens group, the exit pupil position becomes deep (too close to the image side), and thus the aperture of the third lens group must be increased. If the lens diameter is increased, the weight of the lens also increases, which is not preferable.

  By providing the above features, a small, light, and wide-angle fixed focus lens can be realized.

  Furthermore, in the present invention, in order to realize a fixed focus lens having better imaging performance, various conditions as shown below are set in addition to the above features.

First, in the fixed focus lens according to the present invention, when the radius of curvature of the most object side surface of the second lens group is R1, and the radius of curvature of the most image side surface of the second lens group is R2, the following conditional expression is satisfied. It is preferable to do.
(1) −3.5 ≦ (R1 + R2) / (R1−R2) ≦ 1

  Conditional expression (1) defines the shape of the second lens group. The optical system of the present invention can maintain good imaging performance by satisfying conditional expression (1). If the lower limit of conditional expression (1) is not reached, the negative refractive power of the second lens group becomes too weak, and the stroke amount during focusing in the second lens group increases, resulting in an increase in the overall length of the optical system. It is not preferable. On the other hand, if the upper limit in conditional expression (1) is exceeded, the radius of curvature of the object side surface of the second lens group becomes too large, and it becomes difficult to correct distortion, and the field curvature becomes excessive on the under side. Therefore, it is not preferable.

In addition, the said conditional expression (1) can anticipate a more preferable effect, if the range shown next is satisfied.
(1) ′ − 3.0 ≦ (R1 + R2) / (R1−R2) ≦ 0.5

  By satisfying the range defined by the conditional expression (1) ′, it is possible to further improve the imaging performance while further shortening the total length of the optical system.

Furthermore, when the conditional expression (1) ′ satisfies the following range, a further preferable effect can be expected.
(1) ″ − 2.5 ≦ (R1 + R2) / (R1−R2) ≦ −0.1

  By satisfying the range defined by the conditional expression (1) ″, it is possible to further improve the imaging performance while achieving further shortening of the total length of the optical system.

Furthermore, in the fixed focus lens according to the present invention, the paraxial imaging magnification of the second lens group in the infinitely focused state is β2G, and the paraxial imaging magnification of the third lens group in the infinitely focused state is β3G. In this case, it is preferable that the following conditional expression is satisfied.
(2) 0.7 ≦ | (1-β2G) × β3G | ≦ 2.5

  Conditional expression (2) defines the focus sensitivity of the optical system and determines the stroke amount of the focus group from the infinite focus state to the closest focus state. The focus sensitivity indicates the ratio of the amount of movement of the focus position on the image plane to the amount of movement of the focus group. The value defined by the conditional expression (2) is an important factor that determines the size of the optical system and the imaging performance. If the lower limit of conditional expression (2) is not reached, when the desired closest focus state is to be secured, the stroke amount of the second lens group, which is the focus group, increases, and the total length of the optical system is inevitable. This is not preferable. On the other hand, exceeding the upper limit in conditional expression (2) is not preferable because not only the curvature of field becomes excessive on the over side, but also the spherical aberration becomes excessive on the over side, resulting in deterioration of imaging performance.

In addition, the said conditional expression (2) can anticipate a more preferable effect, if the range shown next is satisfied.
(2) ′ 0.6 ≦ | (1-β2G) × β3G | ≦ 2.3

  By satisfying the range defined by the conditional expression (2) ′, it is possible to further improve the imaging performance while further shortening the total length of the optical system.

Furthermore, when the conditional expression (2) ′ satisfies the following range, a further preferable effect can be expected.
(2) '' 0.5 ≦ | (1-β2G) × β3G | ≦ 2.1

  By satisfying the range defined by the conditional expression (2) ″, it is possible to further improve the imaging performance while achieving further shortening of the total length of the optical system.

Furthermore, in the fixed focus lens according to the present invention, it is preferable that the following conditional expression is satisfied, where F1 is the focal length of the first lens group and F is the focal length of the entire optical system.
(3) 1.5 ≦ | F1 / F | ≦ 4.1

  Conditional expression (3) defines the ratio of the focal length of the first lens group to the total length of the optical system. The focal length of the first lens group directly affects the overall length of the optical system. By satisfying conditional expression (3), the optical system of the present invention can shorten the overall length of the optical system while maintaining good imaging performance. If the lower limit of conditional expression (3) is not reached, it is necessary to increase the imaging magnification after the second lens group, and it becomes difficult to realize an optical system with good imaging performance with a small lens configuration. On the other hand, if the upper limit in conditional expression (3) is exceeded, the total length of the optical shape becomes too long, and the downsizing of the optical system required in the market cannot be achieved.

In addition, if the said conditional expression (3) satisfies the range shown next, a more preferable effect can be anticipated.
(3) ′ 1.4 ≦ | F1 / F | ≦ 4.0

  By satisfying the range defined by the conditional expression (3) ′, it is possible to obtain better imaging performance without hindering the downsizing of the optical system.

Furthermore, when the conditional expression (3) ′ satisfies the following range, a further preferable effect can be expected.
(3) '' 1.3 ≦ | F1 / F | ≦ 3.9

  By satisfying the range defined by the conditional expression (3) ″, it is possible to further improve the imaging performance without hindering the downsizing of the optical system.

  As described above, according to the present invention, a small, light, wide-angle fixed focus lens can be realized. In particular, by disposing the aperture stop between the second lens group and the third lens group, the positive and negative refractive powers can be appropriately distributed before and after the aperture stop, and various aberrations can be corrected. It becomes easy. Furthermore, by satisfying the above conditional expressions, it is possible to realize an inner focus type fixed focus lens that is smaller and has excellent imaging performance.

  Embodiments of the fixed focus lens according to the present invention will be described below in detail with reference to the drawings. The present invention is not limited to the following examples.

FIG. 1 is a cross-sectional view along the optical axis showing the configuration of the fixed focus lens according to the first example. This fixed focus lens includes a first lens group G ₁₁ having a negative refractive power, a second lens group G ₁₂ having a negative refractive power, and a third lens having a positive refractive power in order from the object side (not shown). a group G _13, is formed is disposed. Further, the second lens group G ₁₂ between the third lens group G _13, an aperture stop ST is disposed to define a predetermined diameter.

The first lens group G ₁₁ includes a negative lens L ₁₁₁ and a positive lens L ₁₁₂ arranged in order from the object side. The imaging plane IMG side of the negative lens L _111, aspheric surface is formed. The first lens group G ₁₁ is fixed and does not move during focusing.

The second lens group G ₁₂ includes, formed by a negative lens L _121. The second lens group G ₁₂ includes, by moving to the object side from the image plane IMG side along the optical axis to perform focusing from an infinity in-focus state to a closest distance in-focus state.

The third lens group G ₁₃ includes, in order from the object side, a positive lens L _131, a negative lens L _132, a positive lens L _133, a negative lens L _134, is formed are disposed. The positive lens L ₁₃₁ and the negative lens L ₁₃₂ are cemented. An aspheric surface is formed on each of the object side surface of the positive lens L ₁₃₁ and the image forming surface IMG side surface of the negative lens L ₁₃₄ . The third lens group G ₁₃ also is fixed and does not move during focusing.

  Various numerical data relating to the fixed focus lens according to Example 1 will be described below.

(Lens data)
r ₀ = ∞ (object surface)
d ₀ = D (0)
r ₁ = 42.314
d ₁ = 1.100 nd ₁ = 1.61881 νd ₁ = 63.9
r ₂ = 8.370 (aspherical surface)
d ₂ = 4.555
r ₃ = 13.531
d ₃ = 1.761 nd ₂ = 1.90366 νd ₂ = 31.3
r ₄ = 23.616
d ₄ = D (4)
r ₅ = -12.500
d ₅ = 0.800 nd ₃ = 1.49700 νd ₃ = 81.6
r ₆ = -215.596
d ₆ = D (6)
r ₇ = ∞ (aperture stop)
d ₇ = 1.100
r ₈ = 20.299 (aspherical surface)
d ₈ = 4.833 nd ₄ = 1.49700 νd ₄ = 81.6
r ₉ = -8.778
d ₉ = 0.800 nd ₅ = 1.67270 νd ₅ = 32.2
r ₁₀ = -18.840
d ₁₀ = 0.300
r ₁₁ = 13.379
d ₁₁ = 3.741 nd ₆ = 1.48749 νd ₆ = 70.4
r ₁₂ = -134.098
d ₁₂ = 5.836
r ₁₃ = -27.220
d ₁₃ = 0.800 nd ₇ = 1.77250 νd ₇ = 49.6
r ₁₄ = -38.344 (aspherical surface)
d ₁₄ = FB
r ₁₅ = ∞ (imaging plane)

(Cone coefficient (k) and aspherical coefficient (A ₄ , A ₆ , A ₈ , A ₁₀ ))
(Second side)
k = 0,
A ₄ = -8.09890 × 10 ^-5 , A ₆ = -2.92652 × 10 ^-6 ,
A ₈ = 4.36492 × 10 ^-8 , A ₁₀ = -9.92044 × 10 ^-10
(8th page)
k = 0,
A ₄ = 7.71954 × 10 ⁻⁶ , A ₆ = 2.59802 × 10 ⁻⁷ ,
A ₈ = 5.30322 × 10 ^-9 , A ₁₀ = -1.50109 × 10 ^-10
(14th page)
k = 0,
A ₄ = 1.68091 × 10 ^-4 , A ₆ = 8.18365 × 10 ^-7 ,
A ₈ = 7.01745 × 10 ^-9 , A ₁₀ = -3.48873 × 10 ^-11

(Numeric data for each in-focus state)
Infinity 0.025x Focal length of the closest system 20.0 19.7 18.5
Fno. 3.6 3.6 3.6
Half angle of view (ω) 36.2 36.5 37.6
Image height 14.2 14.2 14.2
Total length of optical system 55.4 55.4 55.4
D (0) ∞ 994 194
D (4) 4.76 4.55 3.79
D (6) 1.20 1.41 2.18
FB (back focus) 23.7 23.7 23.7
Focal length (F1) of the first lens group G ₁₁ -43.456

(Numerical values related to conditional expression (1))
Curvature of the most object side surface of the second lens group G ₁₂ radius (R1) = - 12.500
The curvature of the most image plane IMG side of the second lens group G ₁₂ radius (R2) = - 215.596
(R1 + R2) / (R1-R2) =-1.12

(Numerical value related to conditional expression (2))
Paraxial imaging magnification of the second lens group G ₁₂ in focus at infinity (β2G) = 0.33
Paraxial imaging magnification of the third lens group G ₁₃ in focus at infinity (β3G) = - 1.42
| (1-β2G) × β3G | = 1.0

(Numerical values related to conditional expression (3))
| F1 / F | = 2.29

  FIG. 2 is a diagram illustrating various aberrations of the fixed focus lens according to Example 1 in an infinitely focused state. FIG. 3 is a diagram illustrating various aberrations of the fixed focus lens according to Example 1 in a focusing state of 0.025 times the shooting magnification. FIG. 4 is a diagram of various aberrations of the fixed focus lens according to Example 1 in the closest focus state. In the figure, g represents an aberration with a wavelength corresponding to the g-line (λ = 435.83 nm), and d represents a wavelength corresponding to the d-line (λ = 587.56 nm). In the astigmatism diagrams, s and m represent aberrations with respect to the sagittal image surface and the meridional image surface, respectively.

FIG. 5 is a cross-sectional view along the optical axis showing the configuration of the fixed focus lens according to the second example. The fixed focus lens includes a first lens group G ₂₁ having a negative refractive power, a second lens group G ₂₂ having a negative refractive power, and a third lens having a positive refractive power in order from an object side (not shown). a group G _23, is formed are disposed. An aperture stop ST that defines a predetermined aperture is disposed between the second lens group G ₂₂ and the third lens group G ₂₃ .

The first lens group G ₂₁ includes a negative lens L ₂₁₁ and a positive lens L ₂₁₂ arranged in order from the object side. The imaging plane IMG side of the negative lens L _211, aspheric surface is formed. Incidentally, The first lens group G ₂₁ includes is fixed and does not move during focusing.

The second lens group G ₂₂ includes, formed by a negative lens L _221. The second lens group G ₂₂ includes, by moving to the object side from the image plane IMG side along the optical axis to perform focusing from an infinity in-focus state to a closest distance in-focus state.

The third lens group G ₂₃ includes, in order from the object side, a positive lens L _231, a negative lens L _232, a positive lens L _233, a negative lens L _234, is formed are disposed. The positive lens L ₂₃₁ and the negative lens L ₂₃₂ are cemented. An aspheric surface is formed on each of the object side surface of the positive lens L ₂₃₁ and the image forming surface IMG side surface of the negative lens L ₂₃₄ . The third lens group G ₂₃ also is fixed and does not move during focusing.

  Various numerical data relating to the fixed focus lens according to Example 2 will be described below.

(Lens data)
r ₀ = ∞ (object surface)
d ₀ = D (0)
r ₁ = 60.565
d ₁ = 1.100 nd ₁ = 1.61881 νd ₁ = 63.9
r ₂ = 8.914 (aspherical surface)
d ₂ = 5.053
r ₃ = 14.455
d ₃ = 1.803 nd ₂ = 1.90366 νd ₂ = 31.3
r ₄ = 25.530
d ₄ = D (4)
r ₅ = -13.100
d ₅ = 0.800 nd ₃ = 1.49700 νd ₃ = 81.6
r ₆ = -128.373
d ₆ = D (6)
r ₇ = ∞ (aperture stop)
d ₇ = 1.100
r ₈ = 19.077 (aspherical surface)
d ₈ = 4.468 nd ₄ = 1.49700 νd ₄ = 81.6
r ₉ = -9.794
d ₉ = 0.800 nd ₅ = 1.67270 νd ₅ = 32.2
r ₁₀ = -22.731
d ₁₀ = 0.300
r ₁₁ = 13.295
d ₁₁ = 3.674 nd ₆ = 1.48749 νd ₆ = 70.4
r ₁₂ = -111.586
d ₁₂ = 5.699
r ₁₃ = −31.568
d ₁₃ = 0.800 nd ₇ = 1.77250 νd ₇ = 49.6
r ₁₄ = -52.153 (aspherical surface)
d ₁₄ = FB
r ₁₅ = ∞ (imaging plane)

(Cone coefficient (k) and aspherical coefficient (A ₄ , A ₆ , A ₈ , A ₁₀ ))
(Second side)
k = 0,
A ₄ = -7.06503 × 10 ^-5 , A ₆ = -2.39334 × 10 ^-6 ,
A ₈ = 3.10569 × 10 ^-8 , A ₁₀ = -5.80198 × 10 ^-10
(8th page)
k = 0,
A ₄ = -7.02444 × 10 ^-6 , A ₆ = 6.70051 × 10 ^-7 ,
A ₈ = -1.82054 × 10 ^-8 , A ₁₀ = 2.46248 × 10 ^-10
(14th page)
k = 0,
A ₄ = 1.75449 × 10 ^-4 , A ₆ = 1.10046 × 10 ^-6 ,
A ₈ = 1.92052 × 10 ^-9 , A ₁₀ = 3.40618 × 10 ^-11

(Numeric data for each in-focus state)
Infinity 0.025x Focal length of the closest system 20.0 19.7 18.5
Fno. 2.9 2.9 2.9
Half angle of view (ω) 36.1 36.3 37.4
Image height 14.2 14.2 14.2
Total length of optical system 57.4 57.4 57.4
D (0) ∞ 942 192
D (4) 6.05 5.83 5.01
D (6) 1.20 1.43 2.24
FB (back focus) 24.5 24.5 24.5
Focal length (F1) of first lens group G ₂₁ -45.81

(Numerical values related to conditional expression (1))
Curvature of the most object side surface of the second lens group G ₂₂ radius (R1) = - 13.100
The radius of curvature (R2) of the second lens group G ₂₂ closest to the imaging plane IMG side = −128.373
(R1 + R2) / (R1-R2) =-1.23

(Numerical value related to conditional expression (2))
Paraxial imaging magnification of the second lens group G ₂₂ in focus at infinity (β2G) = 0.33
Paraxial imaging magnification of the third lens group G ₂₃ in focus at infinity (β3G) = - 1.42
| (1-β2G) × β3G | = 0.96

(Numerical values related to conditional expression (3))
| F1 / F | = 2.17

  FIG. 6 is a diagram illustrating various aberrations of the fixed focus lens according to Example 2 in an infinitely focused state. FIG. 7 is a diagram illustrating various aberrations of the fixed focus lens according to Example 2 when the photographing magnification is 0.025. FIG. 8 is a diagram of various types of aberration when the fixed focus lens according to Example 2 is in the closest focus state. In the figure, g represents an aberration with a wavelength corresponding to the g-line (λ = 435.83 nm), and d represents a wavelength corresponding to the d-line (λ = 587.56 nm). In the astigmatism diagrams, s and m represent aberrations with respect to the sagittal image surface and the meridional image surface, respectively.

FIG. 9 is a cross-sectional view along the optical axis showing the configuration of the fixed focus lens according to the third example. The fixed focus lens includes a first lens group G ₃₁ having a negative refractive power, a second lens group G ₃₂ having a negative refractive power, and a third lens having a positive refractive power in order from the object side (not shown). a group G _33, is formed are disposed. An aperture stop ST that defines a predetermined aperture is disposed between the second lens group G ₃₂ and the third lens group G ₃₃ .

The first lens group G ₃₁ includes a negative lens L ₃₁₁ and a positive lens L ₃₁₂ arranged in order from the object side. An aspherical surface is formed on the side of the imaging surface IMG of the negative lens L ₃₁₁ . The first lens group G ₃₁ is fixed and does not move during focusing.

The second lens group G ₃₂ is constituted by a negative lens L _321. The second lens group G ₃₂ is, by moving to the object side from the image plane IMG side along the optical axis to perform focusing from an infinity in-focus state to a closest distance in-focus state.

The third lens group G ₃₃ includes a positive lens L ₃₃₁ , a negative lens L ₃₃₂ , a positive lens L _333, and a negative lens L ₃₃₄ arranged in this order from the object side. The positive lens L ₃₃₁ and the negative lens L ₃₃₂ are cemented. An aspheric surface is formed on each of the object side surface of the positive lens L ₃₃₁ and the imaging surface IMG side surface of the negative lens L ₃₃₄ . The third lens group G ₃₃ also is fixed and does not move during focusing.

  Various numerical data relating to the fixed focus lens according to Example 3 will be described below.

(Lens data)
r ₀ = ∞ (object surface)
d ₀ = D (0)
r ₁ = 50.739
d ₁ = 1.100 nd ₁ = 1.72916 νd ₁ = 54.7
r ₂ = 8.504 (aspherical surface)
d ₂ = 6.121
r ₃ = 29.528
d ₃ = 1.991 nd ₂ = 1.84666 νd ₂ = 23.8
r ₄ = -126.700
d ₄ = D (4)
r ₅ = -17.281
d ₅ = 0.800 nd ₃ = 1.61800 νd ₃ = 63.4
r ₆ = -1116.807
d ₆ = D (6)
r ₇ = ∞ (aperture stop)
d ₇ = 1.100
r ₈ = 22.865 (aspherical surface)
d ₈ = 3.844 nd ₄ = 1.49700 νd ₄ = 81.6
r ₉ = -8.627
d ₉ = 0.800 nd ₅ = 1.64769 νd ₅ = 33.8
r ₁₀ = -31.391
d ₁₀ = 1.160
r ₁₁ = 20.082
d ₁₁ = 3.308 nd ₆ = 1.49700 νd ₆ = 81.6
r ₁₂ = -17.997
d ₁₂ = 8.061
r ₁₃ = -16.701
d ₁₃ = 1.000 nd ₇ = 1.77250 νd ₇ = 49.6
r ₁₄ = -18.424 (aspherical surface)
d ₁₄ = FB
r ₁₅ = ∞ (imaging plane)

(Cone coefficient (k) and aspherical coefficient (A ₄ , A ₆ , A ₈ , A ₁₀ ))
(Second side)
k = 0,
A ₄ = -7.50069 × 10 ^-5 , A ₆ = -4.43622 × 10 ^-6 ,
A ₈ = 7.95093 × 10 ^-8 , A ₁₀ = -1.17518 × 10 ^-9
(8th page)
k = 0,
A ₄ = -3.90349 × 10 ⁻⁵ , A ₆ = 4.97007 × 10 ⁻⁷ ,
A ₈ = -2.87913 × 10 ^-8 , A ₁₀ = 2.46882 × 10 ^-10
(14th page)
k = 0,
A ₄ = 1.34544 × 10 ⁻⁴ , A ₆ = 7.70023 × 10 ⁻⁷ ,
A ₈ = 5.67370 × 10 ^-9 , A ₁₀ = −2.02389 × 10 ^-11

(Numeric data for each in-focus state)
Infinity 0.025x Focal length of the closest lens system 16.0 15.8 15.2
Fno. 3.0 3.0 3.0
Half angle of view (ω) 42.4 42.7 43.7
Image height 14.2 14.2 14.2
Total length of optical system 54.7 54.7 54.7
D (0) ∞ 944 195
D (4) 3.46 3.33 2.62
D (6) 1.20 1.38 2.04
FB (back focus) 20.8 20.8 20.8
Focal length (F1) of first lens group G ₃₁ -49.828

(Numerical values related to conditional expression (1))
Curvature of the most object side surface of the second lens group G ₃₂ radius (R1) = - 17.281
The curvature of the most image plane IMG side of the second lens group G ₃₂ radius (R2) = - 1116.807
(R1 + R2) / (R1-R2) =-1.03

(Numerical value related to conditional expression (2))
Paraxial imaging magnification of the second lens group G ₃₂ in focus at infinity (β2G) = 0.27
Paraxial imaging magnification of the third lens group G ₃₃ in focus at infinity (β3G) = - 1.18
| (1-β2G) × β3G | = 0.86

(Numerical values related to conditional expression (3))
| F1 / F | = 3.11

  FIG. 10 is a diagram of various types of aberration when the fixed focus lens according to Example 3 is in focus at infinity. FIG. 11 is a diagram illustrating various aberrations of the fixed focus lens according to Example 3 when the photographing magnification is in the focused state of 0.025. FIG. 12 is a diagram illustrating various aberrations of the fixed focus lens according to Example 3 in the closest focus state. In the figure, g represents an aberration with a wavelength corresponding to the g-line (λ = 435.83 nm), and d represents a wavelength corresponding to the d-line (λ = 587.56 nm). In the astigmatism diagrams, s and m represent aberrations with respect to the sagittal image surface and the meridional image surface, respectively.

In the numerical data in each of the above embodiments, r ₁ , r ₂ ,... Are the curvature radii of the respective lenses and diaphragm surfaces, and d ₁ , d ₂ ,. thickness or their surface separations, nd _1, nd _2, ···· the d-line of each lens (lambda = 587.56 nm) refractive index in, νd _1, νd _2, ···· are each lens d The Abbe number in the line (λ = 587.56 nm) is shown. The unit of length is all “mm”, and the unit of angle is “°”.

  In each of the above aspheric shapes, the depth of the aspheric surface is Z, the curvature is c (= 1 / r: r is the radius of curvature), the height from the optical axis is h, and the light traveling direction is positive. Is represented by the following equation.

Here, k is a conical coefficient, and A ₄ , A ₆ , A ₈ , and A ₁₀ are fourth-order, sixth-order, eighth-order, and tenth-order aspheric coefficients, respectively.

  As described above, the fixed focus lens of each of the above embodiments is configured with a small number of lenses, so that it is small and lightweight, and a wide angle can be realized. In particular, by disposing the aperture stop between the second lens group and the third lens group, the positive and negative refractive powers can be appropriately distributed before and after the aperture stop, and various aberrations can be corrected. It becomes easy. Furthermore, by satisfying the above conditional expressions, it is possible to realize an inner focus type fixed focus lens that is smaller and has excellent imaging performance. In addition, since the fixed focus lens of each of the above-described embodiments uses a lens or a cemented lens in which an aspheric surface is appropriately formed, good optical performance can be maintained with a small number of lenses.

  As described above, the fixed focus lens of the present invention is useful for a 35 mm camera, a video camera, an electronic still camera, and the like, and particularly suitable for a mirrorless single-lens camera with a short back focus.

G ₁₁ , G ₂₁ , G ₃₁ 1st lens group G ₁₂ , G ₂₂ , G ₃₂ 2nd lens group G ₁₃ , G ₂₃ , G ₃₃ 3rd lens group L ₁₁₁ , L ₁₂₁ , L ₁₃₂ , L ₁₃₄ , L ₂₁₁ , L ₂₂₁ , L ₂₃₂ , L ₂₃₄ , L ₃₁₁ , L ₃₂₁ , L ₃₃₂ , L ₃₃₄ negative lens L ₁₁₂ , L ₁₃₁ , L ₁₃₃ , L ₂₁₂ , L ₂₃₁ , L ₂₃₃ , L ₃₁₂ , L ₃₃₁ , L ₃₃₃ positive Lens IMG Imaging surface ST Aperture stop

Claims (5)

A first lens group having a negative refractive power, a second lens group having a negative refractive power, and a third lens group having a positive refractive power, which are arranged in order from the object side,
The second lens group is composed of a single lens element,
The fixed focus lens, wherein the second lens group moves along an optical axis during focusing, and the first lens group and the third lens group are fixed with respect to an image plane.   The fixed focus lens according to claim 1, wherein an aperture stop is disposed between the second lens group and the third lens group. The fixed focus lens according to claim 1, wherein the following conditional expression is satisfied.
(1) −3.5 ≦ (R1 + R2) / (R1−R2) ≦ 1
Here, R1 represents the radius of curvature of the second lens group closest to the object side, and R2 represents the radius of curvature closest to the image side of the second lens group. The fixed focus lens according to claim 1, wherein the following conditional expression is satisfied.
(2) 0.7 ≦ | (1-β2G) × β3G | ≦ 2.5
Here, β2G represents the paraxial imaging magnification of the second lens group in the infinitely focused state, and β3G represents the paraxial imaging magnification of the third lens group in the infinitely focused state. The fixed focus lens according to claim 1, wherein the following conditional expression is satisfied.
(3) 1.5 ≦ | F1 / F | ≦ 4.1
Here, F1 represents the focal length of the first lens group, and F represents the focal length of the entire optical system.

Images