JP2009244531A - Optical element - Google Patents

Abstract

Provided is an optical element that suppresses generation of unnecessary light so that ghosts and flares can be suppressed even when a high-luminance object is imaged while ensuring high resolution, and that can be mass-produced at low cost. To do.
Since each of the ND filter films Fn1 to Fn3 has a function of reducing the light transmittance, the light quantity of the light beam passing through the inside of the ND filter film Fn1 is the highest, and passes through the inside of the ND filter film Fn2. The light amount decreases in the order of the light amount of the light beam that passes through, the light amount of the light beam that passes near the inside of the ND filter film Fn3, and the light amount of the light beam that passes near the inside of the diaphragm film Ms. The outermost diaphragm film Ms can exhibit the function as the aperture diaphragm S because the light transmittance is substantially zero.
[Selection] Figure 4

Description

  The present invention relates to an optical element for imaging, and more particularly to an optical element suitable for use in an optical system for forming an optical image on a solid-state imaging element such as a CCD image sensor or a CMOS image sensor.

  In recent years, surveillance cameras including solid-state imaging devices such as CCD (Charge Coupled Devices) type image sensors or CMOS (Complementary Metal-Oxide Semiconductor) type image sensors have been developed.

By the way, when the subject image is formed on the light receiving surface of the solid-state imaging device using the imaging lens, the aperture diameter is adjusted in accordance with the subject light amount in order to ensure proper exposure. However, depending on the shooting scene, it may be unavoidable that high-luminance subjects such as headlights and the sun enter the shooting angle of view, but if the aperture diameter is automatically adjusted, the aperture will become extremely small, and light diffraction will occur. There is a risk that the resolution may be reduced due to the wraparound. Such a problem also arises when intentionally shooting a high-luminance subject such as sunset. On the other hand, in Patent Document 1, the ND filter is used to reduce the amount of subject light and suppress the extremely small aperture to improve the resolution.
Japanese Patent Laid-Open No. 06-167738 JP 05-281590 A JP 2002-229095 A JP 2007-178823 A

  On the other hand, since the diaphragm and ND filter are made of a plate material, there is a certain amount of thickness. Therefore, when light from the object side hits the side edge on the optical axis side and reflects, it enters the image side from the diaphragm or ND filter. As a result, stray light may be generated, causing ghosts and flares, resulting in a deterioration in image quality. In particular, surveillance cameras and the like have a high risk of generating a ghost that cannot be ignored because high-intensity subjects such as headlights and the sun cannot enter the shooting angle of view because of their nature.

  On the other hand, in patent document 2, by providing micro unevenness | corrugation in the inner edge of an aperture blade, the reflected light of an inner edge is diffused and generation | occurrence | production of a ghost etc. is suppressed. In Patent Document 3, the inner edge of the diaphragm blade is inclined so as to spread toward the object side, and a fine step is provided to diffuse the reflected light of the inner edge and suppress the occurrence of ghosts and the like. is doing. Furthermore, in Patent Document 4, non-periodic irregularities are formed on the edge line of the edge of the diaphragm or ND filter, and the reflected light at the edge is diffused to suppress the occurrence of ghosts and the like.

  However, the techniques of Patent Documents 2 to 4 all form minute irregularities on the edge of a diaphragm or ND filter, which is a resin plate material with a thickness of about several hundreds of micrometers, and it takes time to process the irregularities. There is a problem that the cost increases. Further, if the processing accuracy of the unevenness is suppressed within a predetermined tolerance, the cost further increases. In addition, even if minute irregularities are processed on the edge of the aperture or the ND filter, it is necessary to allow a certain amount of reflected light to enter the inside. In the case of a surveillance camera that cannot be avoided, it becomes a big problem.

  The present invention has been made in view of the above problems, and suppresses the generation of unnecessary light so that ghosts and flares can be suppressed even when a high-luminance subject is imaged while ensuring high resolution. An object is to provide an optical element that can be mass-produced at low cost.

The optical element according to claim 1 is an imaging optical element in which a film is formed on at least a part of an optical surface.
The coating film is formed so that light transmittance increases toward the optical axis.

  According to the present invention, since the coating is formed on at least a part of the optical surface so that the light transmittance increases toward the optical axis, the coating can be provided with functions such as an ND filter and a diaphragm. This eliminates the need for a small aperture that causes a reduction in resolution due to the diffraction phenomenon. Furthermore, since the film thickness is very thin compared to conventional ND filters and apertures, the area of the inner edge is reduced, resulting in the generation of reflected light. Therefore, it is possible to suppress the occurrence of unnecessary light and to prevent ghost and flare defects.

  According to a second aspect of the present invention, there is provided the optical element according to the first aspect of the invention, wherein the coating is formed by overlapping a plurality of layers.

  According to a third aspect of the present invention, there is provided the optical element according to the second aspect of the invention, wherein the number of the plurality of layers is reduced as it goes toward the optical axis.

  An optical element according to a fourth aspect is characterized in that, in the invention according to the second or third aspect, the plurality of layers have different transmittances.

  The optical element according to claim 5 is the invention according to any one of claims 1 to 4, wherein the coating has an opening including an optical axis.

  An optical element according to a sixth aspect of the invention is characterized in that, in the invention according to any one of the first to fifth aspects, the coating is formed by multilayer coating, ink jetting, or vapor deposition.

  An optical element according to a seventh aspect is characterized in that, in the invention according to any one of the first to sixth aspects, the thickness of the coating film is 50 μm or less.

The optical element according to claim 8 is characterized in that, in the invention according to any one of claims 1 to 7, when the aperture stop diameter is φS and the film thickness of the coating is Ts, the following expression is satisfied. And In the present specification, φS means the diameter S.
Ts / φS ≦ 0.1 (1)

  Here, if the value Ts / φS exceeds the upper limit of the equation (1), unnecessary light reflection and diffraction phenomenon on the edge surface of the film become remarkable, and the image quality may be deteriorated.

The optical element according to claim 9 satisfies the following expression when the aperture stop diameter is φS and the minimum aperture diameter of the coating is φ1 in the invention according to any one of claims 1 to 8. Features.
φS / φ1 ≧ 1.1 (2)

  Here, when the value Ts / φ1 falls below the lower limit of the expression (2), the diffraction intensity increases as in the conventional diaphragm member, and there is a possibility that a radial bright line is generated around a strong light source.

  An optical element according to a tenth aspect is the invention according to any one of the first to ninth aspects, wherein the optical element is arranged in the vicinity of an aperture stop.

  An optical element according to an eleventh aspect is the optical element according to any one of the first to tenth aspects, wherein an antireflection means is formed on the optical surface, and the coating is formed on the antireflection means. It is characterized by. As the antireflection means, there are an AR coating, a nano antireflection structure ARS, and the like.

  With reference to FIG.1 and FIG.2, the imaging device 100 carrying the optical system concerning embodiment of this invention is demonstrated. The imaging device 100 is preferably used for a mobile phone, but can also be applied to a surveillance camera or a vehicle-mounted camera. FIG. 1 is a block diagram of the imaging apparatus 100.

  As illustrated in FIG. 1, the imaging apparatus 100 includes an optical system 101, a solid-state imaging device 102, an A / D conversion unit 103, a control unit 104, an optical system driving unit 105, a timing generation unit 106, and imaging. An element driving unit 107, an image memory 108, an image processing unit 109, an image compression unit 110, an image recording unit 111, a display unit 112, and an operation unit 113 are configured.

  The optical system 101 has a function of forming a subject image on the imaging surface of the solid-state imaging device 102. The solid-state image sensor 102 is an image sensor such as a CCD or CMOS, and photoelectrically converts incident light for each of R, G, and B and outputs an analog signal thereof. The A / D conversion unit 103 converts an analog signal into digital image data.

  The control unit 104 controls each unit of the imaging device 100. The control unit 104 includes a CPU (Central Processing Unit), a RAM (Random Access Memory), and a ROM (Read Only Memory), and various programs read out from the ROM and expanded in the RAM, and various in cooperation with the CPU. Execute the process.

  The optical system driving unit 105 controls the optical system 101 in zooming, focusing (movement of a second lens group G2, a fourth lens group G4, and a fifth lens group G5 described later), exposure, and the like under the control of the control unit 104. Is controlled. The timing generator 106 outputs a timing signal for analog signal output. The image sensor drive unit 107 performs scanning drive control of the solid-state image sensor 102.

  The image memory 108 stores image data so as to be readable and writable. The image processing unit 109 performs various image processes on the image data. The image compression unit 110 compresses captured image data by a compression method such as JPEG (Joint Photographic Experts Group). The image recording unit 111 records image data on a recording medium such as a memory card set in a slot (not shown).

  The display unit 112 is a color liquid crystal panel or the like, and displays image data after shooting, a through image before shooting, various operation screens, and the like. The operation unit 113 includes a release button, various modes, and various operation keys for setting values, and outputs information input by the user to the control unit 104.

  Here, the operation of the imaging apparatus 100 will be described. In subject photographing, subject monitoring (through image display) and image photographing execution are performed. In the monitoring, an image of the subject obtained through the optical system 101 is formed on the light receiving surface of the solid-state image sensor 102. A solid-state imaging device 102 disposed behind the imaging optical axis of the optical system 101 is scanned and driven by a timing generation unit 106 and an imaging device driving unit 107, and serves as a photoelectric conversion output corresponding to an optical image formed at regular intervals. Output analog signal for one screen.

  The analog signal is appropriately gain-adjusted for each primary color component of RGB, and then converted into digital data by the A / D conversion unit 103. The digital data is subjected to color process processing including pixel interpolation processing and γ correction processing by the image processing unit 109 to generate a luminance signal Y and color difference signals Cb, Cr (image data) as digital values, and the image memory. 108, the signal is periodically read out and the video signal is generated and output to the display unit 112.

  The display unit 112 functions as an electronic viewfinder in monitoring, and displays captured images in real time. In this state, zooming, focusing, exposure, and the like of the optical system 101 are set by driving the optical system driving unit 105 based on an operation input through the operation unit 113 by the user.

  In such a monitoring state, still image data is photographed when the user operates the release button of the operation unit 113 at the timing at which still image photographing is desired. In response to the operation of the release button, one frame of image data stored in the image memory 108 is read out and compressed by the image compression unit 110. The compressed image data is recorded on a recording medium by the image recording unit 111.

  Note that the description in each of the above embodiments and examples is an example of a suitable optical system and imaging apparatus according to the present invention, and the present invention is not limited to this.

  For example, in each of the above embodiments and examples, an example of a digital still camera has been described as an imaging apparatus equipped with an optical system. However, the present invention is not limited to this, and a video camera or a mobile phone with an imaging function is described. It may be a device such as a portable terminal having at least an imaging function, such as PHS (Personal Handyphone System) and PDA (Personal Digital Assistant).

  Further, an imaging device equipped with an optical system may be used as an imaging unit mounted on the device. Here, with reference to FIG. 2, an example of a mobile phone 300 on which the imaging apparatus 100 is mounted will be described. FIG. 2 is a block diagram showing an internal configuration of the mobile phone 300.

  As shown in FIG. 2, the mobile phone 300 has a control unit (CPU) 310 that performs overall control of each unit and executes a program corresponding to each process, and an operation unit 320 that inputs a number and the like using keys. A display unit 330 that displays captured images in addition to predetermined data, a wireless communication unit 340 for realizing various information communication with an external server or the like via the antenna 341, and the imaging apparatus 100. A storage unit (ROM) 360 that stores necessary data such as a system program, various processing programs, and a terminal ID of the mobile phone 100; various processing programs and data executed by the control unit 310; Alternatively, the imaging apparatus 100 includes a temporary storage unit (RAM) 370 that is used as a work area for temporarily storing imaging data and the like. There.

  Note that the control unit 104 of the imaging apparatus 100 and the control unit 310 of the mobile phone 300 are communicably connected. In such a case, the functions of the display unit 112 and the operation unit 113 shown in FIG. However, the operation of the imaging apparatus 100 itself is basically the same. More specifically, an external connection terminal (not shown) of the imaging device 100 is connected to the control unit 310 of the mobile phone 300, and a release signal is transmitted from the mobile phone 300 side to the imaging device 100 side and obtained by imaging. Image signals such as luminance signals and color difference signals are output from the imaging apparatus 100 side to the control unit 310 side. Such an image signal can be stored in the storage unit 360 or displayed on the display unit 330 by the control system of the mobile phone 300, and further transmitted to the outside as video information via the wireless communication unit 340. .

  In addition, an image pickup apparatus equipped with an optical system is arranged with a control unit and an image processing unit arranged on a substrate, and is coupled to a separate unit having a display unit and an operation unit by a connector or the like. You may comprise as a camera module on the assumption that it is used.

  3A and 3B are cross-sectional views of the optical system 101 used in the present embodiment, in which FIG. 3A shows a wide end state, FIG. 3B shows an intermediate position state, and FIG. 3C shows a tele end state. Show. In FIG. 2, an optical system 101 is a negative-positive two-group zoom lens, and in order from the object side, a first group G1 composed of a biconcave lens L1, a positive meniscus lens L2 convex on the object side, an aperture stop S, A second group G2 including a convex lens L2, an image-side convex negative meniscus lens L4, and an IR cut filter F are included. All lenses are made of glass and are aspherical.

  FIG. 4 is a cross-sectional view of the lens L3 of the optical system 101, but the film is exaggerated for easy understanding. In FIG. 4, an AR coat Mar is formed on both surfaces of the optical surface of the lens L3, and three layers of ND filter films Fn1, Fn2, and Fn3 are formed on the AR coat Mar on the object side surface. In addition, a diaphragm film Ms for forming a fixed aperture diaphragm S is formed. The ND filter films Fn1, Fn2, Fn3 and the diaphragm film Ms as the coating have openings with different diameters so as to include the optical axis.

  More specifically, the ND filter films Fn1, Fn2, and Fn3 have an opening diameter that increases toward the object side (φ1 <φ2 <φ3). Further, the opening diameter φS of the diaphragm film Ms is equal to the ND filter film. It is larger than the opening diameter φ3 of Fn3 (φ3 <φS). That is, the number of stacked ND filter films decreases toward the optical axis, thereby obtaining the effect of shifting the phase of the diffracted light and weakening the intensity. As a result, the light beam that passes through the inside of the diaphragm film Ms passes through all of the ND filter films Fn1, Fn2, and Fn3 and the AR coat Mar, and the light beam that passes through the inside of the ND filter film Fn3 passes through the ND filter film Fn1, A light beam that passes through Fn2 and the AR coat Mar and passes through the vicinity of the inside of the ND filter film Fn2 passes through the ND filter film Fn1 and the AR coat Mar, and a light flux that passes through the vicinity of the inside of the ND filter film Fn1 Only to pass through.

Further, the total film thickness of the ND filter films Fn1, Fn2, Fn3 and the diaphragm film Ms as the coating film is Ts, the aperture diaphragm diameter is φS, and the aperture diameter of the ND filter film Fn1 having the minimum opening of the film is φ1. Sometimes the following equation is satisfied:
Ts / φS ≦ 0.1 (1)
φS / φ1 ≧ 1.1 (2)

  When the ND filter film is formed on the AR coating Mar as in the present embodiment, the AR coating is not easily cracked or peeled off, and the ink material does not contaminate the chamber during the AR coating. There is an advantage that a lens can be manufactured by a very simple process by forming a diaphragm by an ink jet method or the like.

  As is apparent from the above, each of the ND filter films Fn1 to Fn3 has a function of reducing the light transmittance, so that the light quantity of the light beam passing through the inside of the ND filter film Fn1 is the highest, and the inner side of the ND filter film Fn2 The light quantity decreases in the order of the light quantity of the light beam passing through the vicinity, the light quantity of the light flux passing through the inner vicinity of the ND filter film Fn3, and the light quantity of the light flux passing through the inner vicinity of the diaphragm film Ms. The outermost diaphragm film Ms can exhibit the function as the aperture diaphragm S because the light transmittance is substantially zero. As a result, the Fno. Because it is possible to reduce the amount of peripheral light while maintaining a bright image, it is not necessary to use a small aperture even when shooting high-luminance subjects such as sunsets, even though it is advantageous for correcting various aberrations. The resolution can be secured. Further, the thickness of the inner peripheral edge of the diaphragm film Ms and the ND filter films Fn1 to Fn3 that can be formed by coating or printing on the optical surface of the lens is very thin, 50 μm or less as compared with the conventional plate material. It is possible to effectively suppress the reflected light from going to the image side and becoming unnecessary light. Even if the ND filter film is used, unnecessary light having a low frequency component may not be removed. However, this can be removed from the image signal by post-processing such as image processing.

  However, the ND filter films Fn1 to Fn3 are not limited to three layers, and two layers or less, or four layers or more may be received, or the transmittance may be different for each layer. Further, at least the ND filter film Fn1 may not have an opening.

  When forming the diaphragm film Ms as a film, it is preferably provided on the object side. The reason is that, from the viewpoint of the ray angle, placing the stop on the object side has a lower probability of unnecessary light reaching the image plane due to reflection from the inner periphery of the stop. However, it may be formed on the image side or on both sides. In the optical system for the portable terminal camera and the optical system for the surveillance camera described later, it is preferable to form a diaphragm film Ms as a film on the object side. On the other hand, in an on-vehicle camera optical system to be described later, it is preferable to form a diaphragm film Ms as a film on either the image side of the third lens or the object side of the fourth lens. It is the object side of the lens.

  The material of the optical element for forming the film may be glass or plastic, but glass is more desirable in consideration of the adhesion (reliability) of ink painting and AR coating.

  FIG. 5 is a cross-sectional view of a positive lens L3 according to a modification. In the present modification example, the ND filter films Fn1, Fn2, Fn3 and the diaphragm film Ms are formed on the AR coat Mar formed on the image side surface of the positive lens L3 as a flat surface with respect to the embodiment of FIG. The points formed are different. Since other points are common, description is omitted.

  FIG. 6 is a cross-sectional view of a positive lens L3 according to another modification. In the present modification, ND filter films Fn1, Fn2, Fn3 and a diaphragm film Ms are formed on the object side surface of the positive lens L3, and an AR coat Mar is further formed thereon, as compared with the embodiment of FIG. The film is different. Since other points are common, description is omitted.

  When the AR coat Mar is formed on the ND filter films Fn1, Fn2, Fn3 and the diaphragm film Ms as in the present embodiment, the reflection of the coat is suppressed, and the effect of sanitizing unnecessary light is increased. There is an advantage that there are few restrictions. The position where the AR coat Mar is provided is not limited to the above, and is arbitrary.

  FIG. 7 is a cross-sectional view of a negative lens L4 according to a modification. In this modified example, as shown in FIG. 4, instead of forming the ND filter films Fn1, Fn2, Fn3 and the diaphragm film Ms on the positive lens L3, the object side surface of the negative lens L4 is placed on the AR coat Mar. The difference is that the ND filter films Fn1, Fn2, Fn3 and the diaphragm film Ms are formed. Since other points are common, description is omitted.

  In the ND filter of the present invention, a laminated film of a niobium film and a dielectric film is suitably used as the optical absorption film. The niobium film has a refractive index smaller than that of a conventionally used metal film, can be formed thicker than the conventional metal film at a low concentration portion in the optical absorption film, and a stable thin film can be formed. Thereby, the removal part in the front-end | tip of a low concentration site | part can be made small.

FIG. 8 shows a schematic cross-sectional view of the ND filter film Fn. On the lens L, a total of eight layers of Al ₂ O ₃ films 41 that are antireflection films for reducing the reflectance and TiO _x films 42 that are light absorption films for reducing the transmittance are alternately laminated. Yes. Further, an MgF ₂ film 43, which is a low refractive material, is formed on the ninth outermost layer by 1 / 4λ (λ = 500 to 600 nm) with an optical film thickness n × d (n: refractive index, d: physical film thickness). It is possible to further enhance the antireflection effect. In the present embodiment uses the MgF ₂ film 43 as the outermost layer, it is also possible to use a SiO ₂ film in place of the MgF ₂ film 43.

The transmittance of the ND filter film Fn varies depending on the total film thickness of the TiOx film 42 that is the second, fourth, sixth, and eighth light absorption films, and the transmittance decreases as the film thickness increases. Further, the neutrality of the transmittance within the wavelength range of 400 to 700 nm changes depending on the x of the composition of the TiO _x film 42 described above, and the transmittance distribution becomes neutral by appropriate selection. A preferable numerical value of x is in a range of 0.5 to 2.0, and when x = 1.2 or less, a phenomenon in which the transmittance of light having a short wavelength is inclined with a wavelength of about 550 nm as a boundary. Will occur. On the other hand, when x = 1.2 or more, the transmittance of light having a short wavelength is increased. Therefore, it is preferable that the transmittance is neutral by monitoring the transmittance at the time of vapor deposition. However, the ND filter film is not limited to the above configuration. The diaphragm film can be obtained by evaporating metal or the like.

  Such an ND filter film and a diaphragm film can be accurately formed at low cost by multilayer film coating, ink jetting, or vapor deposition.

  FIG. 9 is a cross-sectional view of a wide-angle optical system 101 ′ suitable for an in-vehicle camera. The optical system 101 ′ includes, in order from the object side, an object side convex negative meniscus lens L1, a biconcave negative lens L2, a biconvex lens L3, an aperture stop S, a cemented lens of a biconvex lens L4 and a biconcave lens L5, and a biconvex lens L6. The lenses L2 and L3 are made of a plastic material, and the other lenses are made of a glass material. The object side of the lens L2, the object side of the lens L3, and both surfaces of the lens L6 are aspheric. In the present embodiment, the ND filter film as shown in FIG. 4 is formed on the object side surface of the lens L4 closest to the aperture stop S. However, instead of providing the aperture stop S, a stop film may be provided. .

  FIG. 10 is a cross-sectional view of an optical system 101 ″ suitable for a surveillance camera. The optical system 101 ″ is, in order from the object side, an aperture stop S made of a diaphragm film, a biconvex lens L1, and a negative meniscus lens 2 convex on the object side. , A biconvex lens L3, a negative meniscus lens L4 convex on the object side, and an IR cut filter F. The lenses L3 and L4 are made of a plastic material, the other lenses are made of a glass material, and the lenses L2, L3, and L4 Both sides are aspherical. In the present embodiment, the ND filter film and the diaphragm film as shown in FIG. 4 are formed on the object side surface of the lens L1, but an aperture diaphragm may be provided instead of the diaphragm film.

  However, the lens provided with the diaphragm or the ND filter film as the coating of the present invention is preferably a vehicle-mounted camera or a surveillance camera that is highly likely to receive a high-luminance subject, but it is not limited to the above applications. Needless to say.

1 is a block diagram of an imaging apparatus 100. FIG. 4 is a block diagram showing an internal configuration of a mobile phone 300. FIG. It is sectional drawing of the optical system 101 used for this Embodiment. 3 is a cross-sectional view according to an example of a lens L3 of the optical system 101. FIG. 3 is a cross-sectional view according to an example of a lens L3 of the optical system 101. FIG. 3 is a cross-sectional view according to an example of a lens L3 of the optical system 101. FIG. 3 is a cross-sectional view according to an example of a lens L4 of the optical system 101. FIG. It is a schematic cross section of ND filter film Fn. It is sectional drawing of the wide angle optical system 101 'suitable for a vehicle-mounted camera. It is sectional drawing of the optical system 101 '' suitable for a surveillance camera.

Explanation of symbols

100 imaging device 101 optical element 102 solid-state imaging element 103 conversion unit 104 control unit 105 optical system driving unit 106 timing generation unit 107 imaging element driving unit 108 image memory 109 image processing unit 110 image compression unit 111 image recording unit 112 display unit 113 Unit 113 operation unit 300 mobile phone 310 control unit 320 operation unit 330 display unit 340 wireless communication unit 341 antenna 360 storage unit F IR cut filter L1 to L6 lens Fn1 to Fn3 ND filter film Mar diaphragm film S aperture diaphragm CCD solid-state imaging device

Claims (11)

In an imaging optical element in which a film is formed on at least a part of the optical surface,
The optical element is characterized in that the coating is formed so that light transmittance increases toward the optical axis.   The optical element according to claim 1, wherein the coating is formed by stacking a plurality of layers.   3. The optical element according to claim 2, wherein the number of the plurality of layers is reduced as it goes toward the optical axis.   The optical element according to claim 2, wherein the plurality of layers have different transmittances.   The optical element according to claim 1, wherein the coating has an opening including an optical axis.   The optical element according to claim 1, wherein the coating is formed by multilayer coating, ink jetting, or vapor deposition.   The optical element according to claim 1, wherein the film has a thickness of 50 μm or less. The optical element according to claim 1, wherein an aperture stop diameter is φS, and a film thickness of the coating is Ts.
Ts / φS ≦ 0.1 (1) The optical element according to claim 1, wherein the following expression is satisfied when an aperture stop diameter is φS and a minimum aperture diameter of the coating is φ1.
φS / φ1 ≧ 1.1 (2)   The optical element according to claim 1, wherein the optical element is disposed in the vicinity of an aperture stop.   The optical element according to any one of claims 1 to 10, wherein antireflection means is formed on the optical surface, and the coating is formed on the antireflection means.