Optical system and imaging device having the same

Abstract

To provide an optical system which constitutes a large-aperture and a compact lens, and yet reduces ghost images in particular in a desired wavelength range.SOLUTION: An optical system OL provided herein consists of a front lens group L and a negative lens Gn disposed on the image side of the front lens group L. The lens Gn has a first lens surface that satisfies a conditional expression 45°<|Θ|<65°, where Θ represents an open angle at an effective diameter. The first lens surface has an anti-reflection film formed thereon and satisfies a given conditional expression concerning reflectance.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to optical systems and is suitable for imaging devices such as digital video cameras, digital still cameras, broadcast cameras, surveillance cameras, cameras for wearable devices, and cameras for mobile devices. [Background technology]

[0002] There is a demand for high-performance optical systems that reduce the occurrence of ghosting and flare. One known method for reducing ghosting and flare is to apply an anti-reflective coating to the lens surface.

[0003] In recent years, optical systems have become larger in aperture and smaller in size, leading to a trend towards larger lens angles. However, due to manufacturing issues with anti-reflective coatings, lenses with large angles may exhibit inferior anti-reflective properties at the periphery compared to those along the optical axis.

[0004] Patent Document 1 discloses an anti-reflective coating in which the reflectance of light at an incident angle of 0 degrees within the visible region is 0.4% or less, achieved by increasing the number of layers of the anti-reflective coating. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2012-141594 [Overview of the project] [Problems that the invention aims to solve]

[0006] Patent Document 1 attempts to reduce reflectivity in the visible range by increasing the number of anti-reflective coatings or by forming a specially constructed coating. However, when the lens opening angle is large (for example, when the opening angle exceeds 45 degrees), the anti-reflective effect may not be sufficient, especially in the peripheral areas of the lens. Furthermore, if imaging is to be performed in a wavelength range that includes not only the visible range but also the near-infrared, there may be insufficient wavelength bands for which reflection can be prevented.

[0007] Therefore, the present invention aims to provide an optical system that is large-aperture and compact, yet capable of particularly reducing ghosting in a desired wavelength range. [Means for solving the problem]

[0008] The optical system of the present invention comprises a front lens group and a negative lens Gn positioned on the image side of the front lens group, wherein the negative lens Gn has an opening angle Θ at the effective diameter, 45°<|Θ|<65° The lens has a first lens surface that satisfies the following condition, and an anti-reflective coating is formed on the first lens surface, and the reflectance when a light ray with a wavelength of 700 nm is incident perpendicularly on the first lens surface at a position on the optical axis is R_R0, the reflectance when a light ray with a wavelength of 700 nm is incident perpendicularly on the first lens surface at a position where the opening angle is 45 degrees is R_R45, and the reflectance when a light ray with a wavelength of 530 nm is incident perpendicularly on the first lens surface at a position where the opening angle is 45 degrees is R_G45 LGn is the distance from the vertex of the object-side surface of the negative lens Gn to the image plane, and TL is the total optical length of the optical system. In that case, R_R45 < 1.5% R_G45 < 1.5% R_R0 < 1.0% 0.05 <LGn / TL<0.25 It is characterized by satisfying the following conditional expression. [Effects of the Invention]

[0009] According to the present invention, it is possible to realize an optical system that is large-aperture and compact, yet can particularly reduce ghosting in a desired wavelength range. [Brief explanation of the drawing]

[0010] [Figure 1] This is a cross-sectional view of the lens of the optical system of Example 1. [Figure 2] This is an aberration diagram of the optical system of Example 1. [Figure 3] This is a cross-sectional view of the lens of the optical system of Example 2. [Figure 4] This is an aberration diagram of the optical system of Example 2. [Figure 5] This is a diagram illustrating the optical path of unwanted light. [Figure 6] This is a diagram showing the film structure of the first example of the antireflection film. [Figure 7] This is a diagram showing the film structure of the comparative example of the antireflection film. [Figure 8] This is a diagram showing the reflectance characteristics in the first example and the comparative example of the antireflection film. [Figure 9] This is a diagram showing the film structure of the second example of the antireflection film. [Figure 10] This is a diagram showing the reflectance characteristics in the second example of the antireflection film. [Figure 11] This is a diagram showing the film structure of the third example of the antireflection film. [Figure 12] This is a diagram showing the reflectance characteristics in the third example of the antireflection film. [Figure 13] This is a diagram for explaining the opening angle. [Figure 14] This is a schematic diagram showing the imaging device.

Embodiments for Carrying Out the Invention

[0011] Hereinafter, examples of the optical system of the present invention and an imaging device having the same will be described based on the accompanying drawings.

[0012] FIGS. 1 and 3 are cross-sectional views of the optical systems OL of Examples 1 and 2, respectively. The optical system OL of each example is an optical system used in an imaging device such as a digital video camera, a digital still camera, a broadcast camera, a surveillance camera, an in-vehicle camera, a camera for a wearable device, a camera for a mobile device, etc.

[0013] In each lens cross-section, the left side is the object side and the right side is the image side.

[0014] The optical system OL in each embodiment consists of a front lens group L and a negative lens Gn positioned on the image side of the front lens group L. The front lens group L is provided with an aperture diaphragm SP.

[0015] In each lens cross-sectional view, IP is the image plane, and when the optical system OL of each embodiment is used in a digital camera, the imaging surface of a solid-state image sensor (photoelectric conversion element) such as a CCD sensor or CMOS sensor is placed on it. When the optical system OL of each embodiment is used as the photographic optical system of a silver halide film camera, a photosensitive surface corresponding to the film surface is placed on the image plane IP. FL is an optical block corresponding to an optical filter, faceplate, low-pass filter, infrared cut filter, sensor protective glass, etc.

[0016] In the optical system OL of each embodiment, focusing may be performed by moving the entire optical system OL, or a part of the lenses of the optical system OL, in the optical axis direction.

[0017] Figures 2 and 4 show the aberration diagrams of the optical system OL in Examples 1 and 2, respectively. They illustrate the cases where the object distance is infinity and very close.

[0018] In the spherical aberration diagram, Fno is the F-number and indicates the amount of spherical aberration for the d-line (wavelength 587.6 nm) and the g-line (wavelength 435.8 nm). In the astigmatism diagram, ΔS indicates the amount of aberration at the sagittal image plane, and ΔM indicates the amount of aberration at the meridional image plane. In the distortion diagram, the amount of distortion for the d-line is shown. In the chromatic aberration diagram, the amount of lateral chromatic aberration at the g-line is shown. ω is the half-angle of view (°).

[0019] Next, we will describe the characteristic configurations of the optical systems in each embodiment.

[0020] As mentioned above, the optical system OL consists of a front lens group L and a negative lens Gn. If the refractive power of the front lens group L is increased for miniaturization, the positive Petzval sum increases, and a large underexposure occurs due to field curvature. To counteract this, by placing a negative refractive power lens Gn on the image side, it is possible to correct the positive Petzval sum generated by the lens group L. In this case, it is preferable to position the negative lens Gn with its concave surface facing the object.

[0021] Furthermore, when the opening angle at the effective diameter is Θ, the negative lens Gn has a first lens surface that satisfies the following condition. In the optical system OL of each embodiment, the object-side surface of the negative lens Gn is the first lens surface. 45° < |Θ| ​​< 65° (1)

[0022] Condition (1) specifies the absolute value of the opening angle of the negative lens Gn. Exceeding the upper limit of condition (1) is undesirable because it makes lens molding difficult. Exceeding the lower limit of condition (1) is also undesirable because it may make it difficult to suppress field curvature and astigmatism.

[0023] Here, we will explain the definition of the opening angle Θ at the effective diameter using Figure 13. The opening angle Θ at the effective diameter is defined as the intersection point O of a line passing through the vertex A of the lens surface of the lens with effective diameter EA and parallel to the optical axis, and the normal to the tangent to the lens surface passing through position B on the lens surface of the lens with effective diameter EA, where the length of OB is the radius of curvature R of the reference sphere. Θ=∠BOA=sin -1 {(EA / 2) / R} It is calculated by the following. Here, the radius of curvature R of the reference sphere means the radius of curvature of the sphere passing through the vertex A and position B on the lens surface. In this specification, the effective diameter of the lens is defined as the diameter of the circle whose radius is the height from the optical axis of the ray passing through the position furthest from the optical axis among the ray rays passing through the lens surface.

[0024] In the calculation of Θ, the opening angle was calculated using the tangent and normal at the effective diameter position. In the following explanation, "the position where the opening angle is 45 degrees" refers to the position where the opening angle is 45 degrees when calculated using the tangent and normal at any position on the lens surface.

[0025] Furthermore, in the optical system OL of each embodiment, an anti-reflective coating made of a multilayer film is formed on the first lens surface of the negative lens Gn to prevent light reflected from the first lens surface of the negative lens Gn from becoming ghost light.

[0026] Generally, the reflectivity characteristics of an anti-reflective coating are expressed by the reflectivity for each wavelength of light incident perpendicularly on the optical axis of the lens. However, the thickness of the anti-reflective coating tends to decrease as the lens opening angle increases. Therefore, even if an anti-reflective coating provides the desired reflectivity characteristics on the optical axis, it may not necessarily provide the desired reflectivity characteristics for lens surfaces with an opening angle of 45 degrees or more. In other words, for lenses with large opening angles, reflected light at the periphery can become a problem.

[0027] Figure 5 is an illustrative diagram showing the optical path of ghosting in an optical system with such a large-angle lens. When light rays incident from the object side are reflected from the lens surface and reach the image plane, they become ghost light (light that causes ghosting), which can degrade the image quality. Furthermore, if the reflected light on the red, long-wavelength side is generated at a high intensity, the visibility of the ghost light increases, making it more noticeable.

[0028] Therefore, the anti-reflective coating used in the optical system OL of each embodiment is configured to reduce the visibility of ghosting, particularly caused by lens surfaces with large opening angles (first lens surfaces), by appropriately designing the reflectance for each wavelength. Specifically, the anti-reflective coating used in the optical system OL of each embodiment satisfies the following conditional equation. R_R45 < 1.5% (2) R_G45 < 1.5% (3) R_R0 < 1.0% (4)

[0029] Here, let R_R0 be the reflectance when a light ray with a wavelength of 700 nm is incident perpendicularly on the optical axis of the first lens surface. Let R_R45 be the reflectance when a light ray with a wavelength of 700 nm is incident perpendicularly on the first lens surface at a position where the opening angle is 45 degrees. Let R_G45 be the reflectance when a light ray with a wavelength of 530 nm is incident perpendicularly on the first lens surface at a position where the opening angle is 45 degrees.

[0030] If the upper limit of condition (2) is exceeded, the reflectance on the longer wavelength side increases, resulting in highly visible ghosting, which is undesirable because the ghosting becomes conspicuous.

[0031] If the upper limit of condition (3) is exceeded, the reflectance in the visible range becomes high, making ghosting more noticeable, which is undesirable.

[0032] Exceeding the upper limit of condition (5) is undesirable because it reduces the amount of transmitted light.

[0033] By satisfying conditions (2) and (3), ghosting can be reduced in the wavelength range from the visible region to the near-infrared region. Furthermore, by simultaneously satisfying conditions (2) and (4), a substantially equivalent anti-reflective effect can be obtained from the center to the periphery of the lens, which is preferable from the viewpoint of ghosting reduction.

[0034] With the above configuration, it is possible to realize an optical system that is large-aperture and compact, yet can reduce ghosting particularly in the desired wavelength range.

[0035] Preferably, at least one of the upper or lower limits of the numerical range in conditional expressions (1), (2), (3), and (4) should be set to one of the following: 48°<|Θ|<63° (1a) R_R45 < 1.2% (2a) R_G45 < 1.2% (3a) R_R0 < 0.8% (4a)

[0036] Even more preferably, at least one of the upper or lower limits of the numerical range in conditional expressions (1), (2), (3), and (4) is set to one of the following: 50° < |Θ| ​​< 60° (1b) R_R45 < 1.1% (2b) R_G45 < 0.9% (3b) R_R0 < 0.5% (4b)

[0037] Next, specific examples of anti-reflective coatings used in the optical system OL of each embodiment will be described. In the optical system OL of Examples 1 and 2, one of the anti-reflective coatings described below, from the first to the third example, is formed on the first lens surface.

[0038] Figure 6 shows the film structure of an anti-reflective coating 61 as a first example of an anti-reflective coating. In Figure 6, the substrate 62 is a lens having the anti-reflective coating 61. The anti-reflective coating 61 is designed so that, starting from the substrate 62 side and moving towards the air side, the first, third, fifth, and seventh layers are low refractive index layers L, the second, fourth, sixth, and eighth layers are high refractive index layers H, and the ninth layer, closest to the air, is an even lower refractive index layer LL. By combining the high refractive index layers H and the low refractive index layers L, the reflected light generated at the interface of each layer and the light rays incident on each layer cancel each other out through interference, making it possible to suppress reflectivity. Furthermore, a high anti-reflective effect is obtained by using a nine-layer structure.

[0039] Figure 7 is an explanatory diagram of the film structure of an anti-reflective coating 71 as a comparative example. In Figure 7, the substrate 72 is a lens having the anti-reflective coating 71. The anti-reflective coating 71 is designed so that, from the substrate 72 side toward the air side, the first, third, fifth, and seventh layers are low refractive index layers L, the second, fourth, and sixth layers are high refractive index layers H, and the ninth layer, closest to the air, is an even lower refractive index layer LL.

[0040] Table 1 shows the configuration of the anti-reflective coating in the first example. Table 2 shows the configuration of the anti-reflective coating in the comparative example.

[0041] [Table 1]

[0042] [Table 2]

[0043] Table 1 shows the anti-reflective coating using a substrate 62 made of a resin material (K22R / ZEON) with a refractive index of 1.545 at room temperature of 25 degrees Celsius. Table 1 shows the refractive index of this anti-reflective coating 61 with respect to the d line and the optical film thickness (optical film thickness) (refractive index × geometric film thickness) of each layer. The optical film thickness is expressed for both the position on the optical axis and the position where the opening angle is 45 degrees. The difference in optical film thickness depending on the position is due to the deposition angle of the film, etc. The 1st, 3rd, 5th, and 7th layers are low refractive index layers L with a refractive index of 1.610, and the film material contains Al2O3. The 2nd, 4th, 6th, and 8th layers are refractive index layers H with a refractive index of 2.017, and the film material contains ZrO2. The 9th layer, closest to the air, is a low refractive index layer LL with a refractive index of 1.486, and the film material contains MgF2. The anti-reflective coating 61 is colorless and transparent.

[0044] Table 2 shows that the anti-reflective coating uses a resin substrate (K22R / ZEON) with a refractive index of 1.545 at room temperature of 25 degrees Celsius as the substrate 72. Table 1(b) shows the refractive index of this anti-reflective coating 61 for the d line and the optical film thickness (refractive index × geometric film thickness) of each layer. The first, third, fifth, and seventh layers are low refractive index layers L with a refractive index of 1.610, and the film material contains Al2O3. The second, fourth, sixth, and eighth layers are refractive index layers H with a refractive index of 2.017, and the film material contains ZrO2. The ninth layer, closest to the air, is a low refractive index layer LL with a refractive index of 1.486, and the film material contains MgF2.

[0045] Figure 8 shows the reflectance characteristics of the anti-reflective coating of the first example and the anti-reflective coating of the comparative example. In Figure 8, the horizontal axis represents wavelength (unit: nm), and the vertical axis represents reflectance (unit: %). 81 is the reflectance when a light ray is perpendicularly incident on the lens surface on the optical axis where the anti-reflective coating of the first example is provided. 82 is the reflectance when a light ray is perpendicularly incident on the lens surface on the lens surface where the opening angle is 45 degrees. 83 is the reflectance when a light ray is perpendicularly incident on the lens surface on the optical axis where the anti-reflective coating of the comparative example is provided. 84 is the reflectance when a light ray is perpendicularly incident on the lens surface on the lens surface where the opening angle is 45 degrees.

[0046] Comparing the anti-reflective coating of the first example with that of the comparative example, the reflectivity characteristics of the first example show that the low-reflectivity band is shifted to the longer wavelength side. By designing the anti-reflective coating so that the low-reflectivity band shifts to the longer wavelength side when light rays are incident perpendicularly to a position on the optical axis of the lens surface, the reflectivity when light rays are incident perpendicularly to a position where the lens surface has an opening angle of 45 degrees can be suppressed from the visible range to the long-wavelength range. Comparing the reflectivity characteristics when light rays are incident perpendicularly to a position where the lens surface has an opening angle of 45 degrees, the first example suppresses the reflectivity to 1.0% or less up to the long-wavelength range (~780nm). Compared to an anti-reflective coating that uniformly suppresses reflectivity in the visible light range, reducing the reflectivity on the long-wavelength side makes it possible to suppress highly visible ghosting and obtain a high-quality optical image. Furthermore, even when using image sensors that are highly sensitive to the long-wavelength side, such as in surveillance cameras, suppressing the reflectivity on the long-wavelength side further suppresses ghosting and flare, resulting in high-resolution performance.

[0047] Figure 9 is an explanatory diagram of the film structure of an anti-reflective coating 91 as a second example of an anti-reflective coating. In Figure 9, the substrate 92 is a lens having the anti-reflective coating 91. The anti-reflective coating 91 is designed so that, from the substrate 92 side to the air side, the first, third, and fifth layers are low refractive index layers L, the second, fourth, and sixth layers are high refractive index layers H, and the seventh layer closest to the air is a layer with an even lower refractive index LL. A high anti-reflective effect is obtained by using a seven-layer structure.

[0048] Table 3 shows the configuration of the anti-reflective coating in the second example.

[0049] [Table 3]

[0050] The anti-reflective coating in Table 3 uses a resin substrate (K22R / ZEON) with a refractive index of 1.545 at room temperature of 25 degrees Celsius as the substrate 92. The first, third, and fifth layers are low refractive index layers L with a refractive index of 1.610, and the film material contains Al2O3. The second, fourth, and sixth layers are refractive index layers H with a refractive index of 2.017, and the film material contains ZrO2. The seventh layer, closest to the air, is a low refractive index layer LL with a refractive index of 1.486, and the film material contains MgF2. The anti-reflective coating 91 is colorless and transparent.

[0051] Figure 10 shows the reflectance characteristics of the anti-reflective coating of the second example. In Figure 10, the horizontal axis represents wavelength (unit: nm), and the vertical axis represents reflectance (unit: %). 101 shows the reflectance when a light ray is perpendicularly incident on the lens surface on the optical axis where the anti-reflective coating of the second example is applied. 102 shows the reflectance when a light ray is perpendicularly incident on the lens surface where the anti-reflective coating of the second example is applied at an opening angle of 45.

[0052] Figure 11 is an explanatory diagram of the film structure of an anti-reflective coating 111 as a third example of an anti-reflective coating. In Figure 11, the substrate 112 is a lens having the anti-reflective coating 111. The anti-reflective coating 111 is designed so that, from the substrate 112 side to the air side, the first, third, fifth, seventh, and ninth layers are low refractive index layers L, the second, fourth, sixth, eighth, and tenth layers are high refractive index layers H, and the eleventh layer closest to the air is a layer with an even lower refractive index LL. By having an eleven-layer structure, a high anti-reflective effect is obtained.

[0053] Table 4 shows the configuration of the anti-reflective coating in the third example.

[0054] [Table 4]

[0055] The anti-reflective coating in Table 3 uses a resin substrate (E48R / ZEON) with a refractive index of 1.5311 at room temperature of 25 degrees Celsius as the substrate 112. The first, third, fifth, seventh, and ninth layers are low refractive index layers L with a refractive index of 1.610, and the film material contains Al2O3. The second, fourth, sixth, and eighth layers are refractive index layers H with a refractive index of 2.017, and the film material contains ZrO2. The eleventh layer, closest to the air, is a low refractive index layer LL with a refractive index of 1.486, and the film material contains MgF2. The anti-reflective coating 111 is colorless and transparent.

[0056] Figure 12 shows the reflectance characteristics of the anti-reflective coating of the third example. In Figure 12, the horizontal axis represents wavelength (unit: nm), and the vertical axis represents reflectance (unit: %). 121 shows the reflectance when a light ray is incident perpendicularly on the optical axis of the lens surface on which the anti-reflective coating of Example 5 is provided. 122 shows the reflectance when a light ray is incident perpendicularly on the lens surface on which the anti-reflective coating of Example 3 is provided at an opening angle of 45.

[0057] Next, we will describe the conditions that are preferable to satisfy in the optical system OL of each embodiment. It is preferable that the optical system OL of each embodiment satisfies one or more of the following conditions. 0.0 <Rmax_R0 / Rmax_G0<0.1 (5) 0.0 <Rmax_R45 / Rmax_G45<4.0 (6) 0.5 <D_45 / D_0<0.9 (7) 0.0 <Rmin_R0 / Rmin_G0<3.0 (8) 0.0 <R_R0 / R_G0<0.9 (9) 1.0 < |fGn / f| < 1.8 (10) 0.05 <LGn / TL<0.25 (11) 0.4 <SL / TL<0.8 (12) 1.45 <NdGn<1.65 (13) 0.3 <R_R45 / R_R0<3.0 (14) 0.27λ <dn<0.40λ (15)

[0058] Here, let Rmax_G0 be the maximum reflectance in the 450-550 nm wavelength range when a light ray is perpendicularly incident on the optical axis of the first lens surface. Let Rmax_R0 be the maximum reflectance in the 650-750 nm wavelength range when a light ray is perpendicularly incident on the optical axis of the first lens surface. Let Rmax_G45 be the maximum reflectance in the 450-550 nm wavelength range when a light ray is perpendicularly incident on the first lens surface at a position where the opening angle is 45 degrees. Let Rmax_R45 be the maximum reflectance in the 650-750 nm wavelength range when a light ray is perpendicularly incident on the first lens surface at a position where the opening angle is 45 degrees. Let D_45 be the optical thickness of the anti-reflective coating at the position where the opening angle of the first lens surface is 45 degrees, and D_0 be the optical thickness of the anti-reflective coating at the position on the optical axis of the first lens surface.

[0059] Let Rmin_G0 be the minimum reflectance in the 480-550 nm wavelength range when a light ray is incident perpendicularly on the optical axis of the first lens surface. Let Rmin_R0 be the minimum reflectance in the 650-850 nm wavelength range when a light ray is incident perpendicularly on the optical axis of the first lens surface. Let R_G0 be the reflectance when a light ray with a wavelength of 530 nm is incident perpendicularly on the optical axis of the first lens surface.

[0060] Let fGn be the focal length of the negative lens Gn. Let f be the total focal length of the optical system OL. Also, let LGn be the distance from the vertex of the object-side surface of the negative lens Gn to the image plane. Let TL be the total optical length of the optical system OL.

[0061] Furthermore, let SL be the distance from the aperture diaphragm SP to the vertex of the object-side surface of the negative lens Gn. In addition, let NdGn be the refractive index of the negative lens Gn. And let dn(nm) be the optical thickness of the uppermost layer, which is the air-side layer of the anti-reflective coating, for light rays with a wavelength of λ = 587.56 nm.

[0062] Next, I will explain the technical meaning of the aforementioned conditional expression.

[0063] Conditional equation (5) defines the maximum reflectance Rmax_R0 in the 650-750 nm wavelength range relative to the reflectance Rmax_G0 in the 450-550 nm wavelength range when a light ray is perpendicularly incident on the optical axis of the first lens surface. Conditional equation (5) indicates that Rmax_G0 is greater than Rmax_R0. In other words, it indicates that the low reflectance wavelength band is shifted to the longer wavelength side. This makes it possible to suppress the generation of highly visible ghosting.

[0064] If the upper limit of condition (5) is exceeded, the reflectance on the longer wavelength side becomes high, making red ghosting more likely and more noticeable, which is undesirable. Also, if an image sensor with high sensitivity on the longer wavelength side is used, it becomes difficult to suppress ghosting and flare, and the resolution deteriorates, which is undesirable. Setting the lower limit of condition (5) to 0 or greater is also a good idea. In other words, by not setting Rmax_R0 too small relative to Rmax_G0, the difficulty of manufacturing the anti-reflective coating can be reduced.

[0065] If the upper limit of condition (6) is exceeded, the reflectance on the longer wavelength side becomes high, making red ghosting more likely and more noticeable, which is undesirable. Also, if an image sensor with high sensitivity on the longer wavelength side is used, it becomes difficult to suppress ghosting and flare, and the resolution deteriorates, which is undesirable. Setting the lower limit of condition (6) to 0 or greater is also a good idea. In other words, by not setting Rmax_R45 too small relative to Rmax_G45, the difficulty of manufacturing the anti-reflective coating can be reduced.

[0066] If the upper limit of condition (7) is exceeded, the difficulty of manufacturing the anti-reflective coating for each embodiment increases, which is undesirable. In this case, for example, a special optical thin-film forming apparatus equipped with a planetary rotation mechanism would be required. If the lower limit of condition (7) is fallen below, the reflectivity when light is incident perpendicularly on the first lens surface at a position where the opening angle of the first lens surface is 45 degrees deteriorates too much compared to the reflectivity when light is incident perpendicularly on the optical axis of the first lens surface, which is also undesirable.

[0067] Conditional expression (8) defines Rmin_G0 for Rmin_R0.

[0068] If the upper limit of condition (8) is exceeded, the reflectance on the longer wavelength side becomes high, making red ghosting more likely and more noticeable, which is undesirable. If the lower limit of condition (8) is exceeded, the reflectance in the visible region becomes high, which is also undesirable.

[0069] Furthermore, shifting the reflectivity characteristics to the longer wavelength side so that there is a minimum value in the 480-550 nm wavelength range makes it possible to suppress the reflectivity on the longer wavelength side, which is preferable because it makes it possible to suppress the occurrence of highly visible ghosting.

[0070] If the upper limit of condition (9) is exceeded, the reflectance on the longer wavelength side becomes high, making red ghosting more likely and more noticeable, which is undesirable. If the lower limit of condition (9) is fallen below, the reflectance in the visible region becomes high, which is also undesirable.

[0071] If the upper limit of condition (10) is exceeded, the correction of field curvature becomes excessive, which is undesirable. If the lower limit of condition (10) is fallen below, the correction of field curvature becomes insufficient, which is also undesirable.

[0072] If the upper limit of condition (11) is exceeded, the off-axis rays incident on lens Gn become too low. As a result, the on-axis and off-axis light beams passing through lens Gn are not sufficiently separated in the direction perpendicular to the optical axis, making image field curvature correction difficult. If the lower limit of condition (11) is exceeded, the above-mentioned correction effect increases, but the arrangement of optical blocks FL, etc. becomes difficult, which is undesirable from a functional standpoint for the optical system OL.

[0073] If the upper limit of condition (12) is exceeded, the off-axis rays incident on lens Gn become too low. As a result, the on-axis and off-axis light beams passing through lens Gn are not sufficiently separated in the direction perpendicular to the optical axis, making image field curvature correction difficult. If the lower limit of condition (12) is exceeded, the distance from the aperture diaphragm SP to lens Gn becomes longer, and the entire optical system OL becomes larger, which is undesirable.

[0074] Conditional equation (13) specifies the refractive index NdGn of lens Gn under conditions of 25°C (room temperature). Exceeding the upper limit of conditional equation (13) is undesirable because it makes it difficult to mold lens Gn. Exceeding the lower limit of conditional equation (13) is also undesirable because the opening angle required to impart the desired refractive power to lens Gn becomes too large.

[0075] If the upper limit of condition (14) is exceeded, the reflectivity at positions with large opening angles increases, resulting in an undesirable anti-reflective effect at positions with large opening angles being excessively inferior to that at positions on the optical axis (center of the lens surface). If the lower limit of condition (14) is exceeded, the reflectivity at the center of the lens increases, reducing the amount of transmitted light, which is also undesirable.

[0076] If the upper limit of condition (15) is exceeded, the reflectance on the short wavelength side increases, reducing the anti-reflective effect, which is undesirable. If the value falls below the lower limit of condition (15), the reflectance on the long wavelength side increases, reducing the anti-reflective effect, which is also undesirable.

[0077] Furthermore, it is preferable that at least one of the upper or lower limits of the numerical ranges of conditional expressions (5) to (15) be within the range of the following conditional expressions (5a) to (15a). 0.01 <Rmax_R0 / Rmax_G0<0.04 (5a) 0.5 <Rmax_R45 / Rmax_G45<3.5 (6a) 0.6 <D_45 / D_0<0.8 (7a) 0.1 <Rmin_R0 / Rmin_G0<2.5 (8a) 0.3 <R_R0 / R_G0<0.8 (9a) 1.1 < |fGn / f| < 1.7 (10a) 0.08 <LGn / TL<0.20 (11a) 0.50 <SL / TL<0.75 (12a) 1.49 <NdGn<1.60 (13a) 0.4 <R_R45 / R_R0<2.8 (14a) 0.30λ <dn<0.35λ (15a)

[0078] More preferably, at least one of the upper or lower limits of the numerical range of conditional expressions (5) to (15) is within the range of the following conditional expressions (5b) to (15b). 0.020 <Rmax_R0 / Rmax_G0<0.035 (5b) 0.6 <Rmax_R45 / Rmax_G45<3.0 (6b) 0.70 <D_45 / D_0<0.75 (7b) 0.15 <Rmin_R0 / Rmin_G0<2.3 (8b) 0.4 <R_R0 / R_G0<0.7 (9b) 1.2 < |fGn / f| < 1.6 (10b) 0.10 <LGn / TL<0.15 (11b) 0.6 <SL / TL<0.7 (12b) 1.51 <NdGn<1.58 (13b) 1.1 <R_R45 / R_R0<2.7 (14b) 0.31λ <dn<0.34λ (15b)

[0079] Next, we will describe the preferred configurations that the optical system OL of each embodiment should satisfy.

[0080] Preferably, the anti-reflective coating provided on the first lens surface consists of at least seven layers. By creating multiple layers of anti-reflective coating, reflection of incident light can be suppressed over a wide wavelength range. Furthermore, by combining high-refractive-index layers and low-refractive-index layers when creating multiple layers of anti-reflective coating, the reflected light generated at the interface of each layer and the light rays incident on each layer cancel each other out through interference, thereby suppressing reflectivity.

[0081] Furthermore, it is preferable that the negative lens Gn has a concave surface on the object side, and that an anti-reflective coating be provided on this concave surface, which serves as the first lens surface. In this case, the concave surface may be aspherical.

[0082] Furthermore, it is preferable that the image-side lens surface of the negative lens Gn be an aspherical surface having at least one inflection point. An inflection point is defined as the point where, when x is the displacement from the vertex of the surface in the optical axis direction and h is the height from the direction perpendicular to the optical axis (radial direction), and the aspherical shape is x(h), the second derivative of x(h) obtained by differentiating x(h) twice with respect to h is zero, and the sign of the second derivative changes before and after that point. In other words, it is the point where the surface shape switches from a concave shape to a convex shape, or from a convex shape to a concave shape. Having an inflection point makes it easier to correct image field curvature because the peripheral refractive power can be determined independently of the paraxial refractive power. Furthermore, it is possible to suppress the increase in the angle of incidence of light rays passing through the optical system OL onto the imaging surface (image sensor). The position of the inflection point can be placed at any position radially outward from the optical axis as long as it is within the effective diameter of the image-side surface of the negative lens Gn, and it is preferable to place it in the peripheral part.

[0083] Furthermore, the negative lens Gn is preferably a resin lens. Using resin as the material makes it easier to process the lens shape with an inflection point.

[0084] Furthermore, when designing an anti-reflective coating and extending the wavelength range with low reflectivity to the near-infrared region, it is preferable to trade off the reflectivity on the shorter wavelength side to obtain the desired reflection characteristics with fewer coatings. Therefore, it is preferable that the maximum reflectivity when light in the wavelength range of 450 nm to 550 nm is perpendicularly incident on the optical axis of the first surface is 3% (preferably 4%) or more. This means that the reflectivity in the short wavelength range of the anti-reflective coating used in this embodiment is higher than that of an anti-reflective coating used in the short wavelength range in a typical anti-reflective coating used in the visible range. By deliberately increasing the reflectivity in the short wavelength range in this way, it becomes easier to reduce the reflectivity on the longer wavelength side.

[0085] The numerical values ​​corresponding to Examples 1 and 2 are shown below.

[0086] In the surface data of each numerical example, r represents the radius of curvature of each optical surface, and d (mm) represents the axial distance (distance on the optical axis) between the m-th surface and the (m + 1)-th surface. Here, m is the surface number counted from the light incident side. Also, nd represents the refractive index with respect to the d-line of each optical member, and νd represents the Abbe number of the optical member. The Abbe number νd of a certain material, when the refractive indices at the d-line (587.6 nm), F-line (486.1 nm), C-line (656.3 nm), and g-line (wavelength 435.8 nm) of the Fraunhofer lines are Nd, NF, NC, and Ng respectively, νd = (Nd - 1) / (NF - NC) is expressed as.

[0087] Also, when the optical surface is an aspherical surface, an asterisk (*) is attached to the right side of the surface number. The aspherical shape, when X is the displacement amount from the surface vertex in the optical axis direction, h is the height from the optical axis in the direction perpendicular to the optical axis, R is the paraxial radius of curvature, k is the conic constant, and A4, A6, A8, A10, A12, ··· are the aspherical coefficients of each order, x=(h 2 / R) / [1 + {1 - (1 + k)(h / R) 2} 1 / 2 +A4×h 4 +A6×h 6 +A8×h 8 +A10×h 10 ··· is expressed as. Note that "e±XX" in each aspherical coefficient means "×10± XX ".

[0088] [Numerical Example 1] Unit: mm Surface Data Surface Number r d nd νd 1 -13.230 0.65 1.56732 42.8 2 -556.432 0.10 3 12.115 1.63 2.00100 29.1 4 23.466 1.60 5 (Aperture Stop) ∞ -0.50 6* 14.904 2.40 1.76802 49.2 7* -23.793 0.10 8 91.141 3.69 1.83481 42.7 9 -9.345 0.46 1.95906 17.5 10 26.563 0.86 11 -51.000 0.49 1.51742 52.4 12 11.704 3.89 2.00100 29.1 13 -30.167 3.37 14* -19.774 1.20 1.53500 55.7 15* 15.005 0.88 16 ∞ 0.50 1.51633 64.1 17 ∞ 0.44 Image plane ∞ Aspherical data Side 6 K = 0.00000e+00 A 4=-1.13433e-04 A 6=-1.86886e-07 A 8=-1.33697e-08 Side 7 K = 0.00000e+00 A 4= 1.63863e-04 A 6=-1.05546e-06 A 8=-2.97841e-09 Page 14 K = 0.00000e+00 A 4=-3.79721e-03 A 6= 8.71603e-05 A 8=-7.64087e-07 A10=-1.93734e-08 A12= 4.17224e-10 Page 15 K = 0.00000e+00 A 4=-2.85274e-03 A 6= 7.79552e-05 A 8=-1.39826e-06 A10= 1.42733e-08 A12=-6.07417e-11 Focal length 12.40 F-number 1.30 Half-angle (°): 32.82 Image height 8.00 Lens length: 21.76 BF 0.44

[0089] [Numerical Example 2] Unit: mm Surface data Face number rd nd νd 1 -34.075 2.70 1.65412 39.7 2 113.073 1.07 3* 36.667 9.04 1.85135 40.1 4* -47.289 6.97 5 (aperture) ∞ -0.44 6 44.314 7.13 1.77250 49.6 7 -43.910 1.24 1.95906 17.5 8 50.551 3.25 9 986.796 1.33 1.51742 52.4 10 30.657 10.58 1.95375 32.3 11 -91.045 12.06 12* -35.618 3.25 1.53110 55.9 13* 62.751 2.14 14 ∞ 1.35 1.51633 64.1 15 ∞ 0.29 Image plane ∞ Aspherical data 3rd page K = 0.00000e+00 A 4=-6.83768e-06 A 6=-5.74184e-09 A 8= 1.37624e-11 Side 4 K = 0.00000e+00 A 4= 3.56356e-06 A 6=-8.14309e-09 A 8= 1.80140e-11 Side 12 K = 0.00000e+00 A 4=-1.77536e-04 A 6= 7.52910e-07 A 8=-2.20433e-09 A10= 3.38511e-12 A12=-1.40735e-15 Page 13 K = 0.00000e+00 A 4=-1.29198e-04 A 6= 4.72083e-07 A 8=-9.75986e-10 A10= 1.04486e-12 A12=-4.30655e-16 Focal length 33.53 F-number 1.30 Half-angle (°): 32.83 Image height 21.64 Lens length: 61.97 BF 0.29

[0090] Table 5 shows the various values ​​of the optical system OL for each embodiment, and Table 6 shows the various values ​​of the anti-reflective coating for the first to third examples.

[0091] [Table 5]

[0092] [Table 6]

[0093] [Imaging device] Next, an embodiment of a digital still camera (imaging device) using the optical system of the present invention will be described with reference to Figure 14. In Figure 14, 10 is the camera body, and 11 is a lens device including one of the optical systems OL described in Examples 1 and 2.

[0094] 12 is a solid-state image sensor (photoelectric conversion element) such as a CCD sensor or CMOS sensor, which is built into the camera body and receives the optical image formed by the lens device 11 and converts it into photoelectric energy. The camera body 10 may be a so-called single-lens reflex camera with a quick-turn mirror, or a so-called mirrorless camera without a quick-turn mirror.

[0095] Thus, by applying the optical system OL of the present invention to an imaging device such as a digital camera, it becomes possible to obtain high-quality images with reduced ghosting, particularly in the desired wavelength range, while maintaining a large aperture and compact size.

[0096] Although preferred embodiments and examples of the present invention have been described above, the present invention is not limited to these embodiments and examples, and various combinations, modifications, and changes are possible within the scope of its gist. [Explanation of Symbols]

[0097] OL optical system L Front lens group Gn Negative Lens Gn

Claims

1. An optical system comprising a front lens group and a negative lens Gn positioned on the image side of the front lens group, The negative lens Gn, when the opening angle at the effective diameter is Θ, 45°<|Θ|<65° It has a first lens surface which is a lens surface that satisfies the following condition: An anti-reflective coating is formed on the first lens surface. When a light ray with a wavelength of 700 nm is incident perpendicularly on the first lens surface at a position on the optical axis, R_R0 is the reflectance when the light ray with a wavelength of 700 nm is incident perpendicularly on the first lens surface at a position where the opening angle is 45 degrees, R_R45 is the reflectance when the light ray with a wavelength of 530 nm is incident perpendicularly on the first lens surface at a position where the opening angle is 45 degrees, LGn is the distance from the vertex of the object-side surface of the negative lens Gn to the image plane, and TL is the total optical length of the optical system, R_R45<1.5% R_G45<1.5% R_R0 < 1.0% 0.05<LGn / TL<0.25 An optical system characterized by satisfying the following conditional equation.

2. When a light ray is incident perpendicularly on the optical axis of the first lens surface, Rmax_G0 is the maximum value of the reflectance in the wavelength range of 450 to 550 nm, and when a light ray is incident perpendicularly on the optical axis of the first lens surface, Rmax_R0 is the maximum value of the reflectance in the wavelength range of 650 to 750 nm. 0.0<Rmax_R0 / Rmax_G0<0.1 The optical system according to claim 1, characterized in that it satisfies the following condition.

3. When the light ray is perpendicularly incident on the first lens surface at a position where the opening angle is 45 degrees, and the maximum reflectance in the wavelength range of 450 to 550 nm is Rmax_G45, and when the light ray is perpendicularly incident on the first lens surface at a position where the opening angle is 45 degrees, and the maximum reflectance in the wavelength range of 650 to 750 nm is Rmax_R45, 0.0<Rmax_R45 / Rmax_G45<4.0 The optical system according to claim 1 or 2, characterized in that it satisfies the following conditional equation.

4. When the optical thickness of the anti-reflective coating at the position where the opening angle of the first lens surface is 45 degrees is D_45, and the optical thickness of the anti-reflective coating at the position on the optical axis of the first lens surface is D_0, 0.5<D_45 / D_0<0.9 The optical system according to any one of claims 1 to 3, characterized in that it satisfies the following conditional expression.

5. When a light ray is incident perpendicularly on the optical axis of the first lens surface, Rmin_G0 is the minimum value of the reflectance in the wavelength range of 480 to 550 nm, and when a light ray is incident perpendicularly on the optical axis of the first lens surface, Rmin_R0 is the minimum value of the reflectance in the wavelength range of 650 to 850 nm. 0.0<Rmin_R0 / Rmin_G0<3.0 The optical system according to any one of claims 1 to 4, characterized in that it satisfies the following conditional expression.

6. When a light ray with a wavelength of 530 nm is incident perpendicularly on the optical axis of the first lens surface, let R_G0 be the reflectance. 0.0<R_R0 / R_G0<0.9 The optical system according to any one of claims 1 to 5, characterized in that it satisfies the following conditional expression.

7. When the focal length of the negative lens Gn is fGn and the focal length of the entire optical system is f, 1.0<|fGn / f|<1.8 The optical system according to any one of claims 1 to 6, characterized in that it satisfies the following conditional expression.

8. It further has an aperture diaphragm, When the total optical length of the optical system is TL, and the distance from the aperture diaphragm to the vertex of the object-side surface of the negative lens Gn is SL, 0.4<SL / TL<0.8 The optical system according to any one of claims 1 to 7, characterized in that it satisfies the following conditional expression.

9. When the refractive index of the negative lens Gn is NdGn, 1.45<NdGn<1.65 The optical system according to any one of claims 1 to 8, characterized in that it satisfies the following conditional expression.

10. 0.3<R_R45 / R_R0<3.0 The optical system according to any one of claims 1 to 9, characterized in that it satisfies the following conditional expression.

11. When the optical thickness of the uppermost layer, which is the layer closest to the air in the anti-reflective coating, is dn (nm) for light rays with a wavelength λ = 587.56 nm, 0.27λ<dn<0.40λ The optical system according to any one of claims 1 to 10, characterized in that it satisfies the following conditional expression.

12. The optical system according to any one of claims 1 to 11, characterized in that the anti-reflective coating has at least seven layers.

13. The optical system according to any one of claims 1 to 12, characterized in that the negative lens Gn is a resin lens.

14. The negative lens Gn has a concave surface on the object side, The optical system according to any one of claims 1 to 13, characterized in that the first lens surface is the concave surface.

15. The optical system according to claim 14, characterized in that the first lens surface is aspherical.

16. The optical system according to any one of claims 1 to 15, characterized in that the image-side lens surface of the negative lens Gn is an aspherical surface having an inflection point.

17. An optical system comprising a front lens group and a negative lens Gn disposed on the image side of the front lens group, The negative lens Gn, when the opening angle at the effective diameter is Θ, 45°<|Θ|<65° It has a first lens surface which is a lens surface that satisfies the following condition: An anti-reflective coating is formed on the first lens surface. When a light ray with a wavelength of 700 nm is incident perpendicularly on the first lens surface at a position on the optical axis, the reflectance is R_R0; when a light ray with a wavelength of 700 nm is incident perpendicularly on the first lens surface at a position where the opening angle is 45 degrees, the reflectance is R_R45; when a light ray with a wavelength of 530 nm is incident perpendicularly on the first lens surface at a position where the opening angle is 45 degrees, the reflectance is R_G45; the focal length of the negative lens Gn is fGn; and the focal length of the entire optical system is f, R_R45<1.5% R_G45<1.5% R_R0 < 1.0% 1.0<|fGn / f|<1.8 An optical system characterized by satisfying the following conditional equation.

18. An imaging device characterized by having an optical system according to any one of claims 1 to 17 and an image sensor that receives an image formed by the optical system.

Images