JP2010256476A - Imaging optical system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a diffraction optical element capable of reducing flares which may be generated when strong light such as solar light is made incident on the diffraction optical element. <P>SOLUTION: An adhesion type blaze diffraction optical element configured by sticking an optical material having a first refractive index and an optical material having a second refractive index to each other and forming a diffraction grating on a boundary between both the optical materials is arranged on a front group lens. Parallel light flux made incident from a direction with an angle of 20° further from a maximum view angle to the outside on the entire front lens area of an imaging optical system arrives at the diffraction optical element. About the light flux made incident on the imaging system at the angle, when a cross section formed by light intersecting with the optical axis of the imaging optical system and the optical axis is defined as a meridional section, a mirror barrel or a product hood is configured so that the light flux made incident on the diffraction optical element crossing the optical axis becomes 90% and more of the entire light flux in the width of the meridional section of the light flux arriving at the diffraction optical element. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an imaging optical system used in an optical system, and relates to a diffractive optical element that can handle a plurality of wavelengths.

  Conventionally, as a method of reducing chromatic aberration of an optical system, a method of providing a diffractive optical element having a diffractive action in a part of the optical system is known (Non-Patent Document 1).

The shape of the diffractive optical element is a shape obtained by adding a phase term defined by an optical path difference function to the shape of the base. First, the shape of the base is the shape of the surface of the lens, and may be a spherical shape, an aspherical shape, or a planar shape. Further, the additional amount of the optical path length by the diffractive lens structure is obtained by using the height h from the optical axis, the optical path difference function coefficient Cn of the nth order (even order), and the wavelength λ.
φ (h) = (C1h ² + C2h ⁴ + C3h ⁶ + ...) × 2π / λ
It is represented by an optical path difference function φ (h) defined by As an example, when a grating shape is added to the surface of a lens having a curvature R by the above optical path difference function φ (h), the position in the optical axis direction is X, h is an annular number counted from the center, and d is the grating thickness. When

It is possible to create a diffractive lens to which a diffraction effect is added. That is, in the above formula, the first two terms represent the base shape, and the third term represents the shape with the phase term added by the optical path difference function. For the second term, the position of x is discontinuous at the part where the zone number changes, and this causes a lattice shape.

  When a diffractive optical element is used in an optical system, it is necessary that the diffraction efficiency of the light beam of the designed order is sufficiently high over the entire wavelength range used.

  When the diffraction efficiency is low, that is, when there are many light beams having diffraction orders other than the design order, these light beams form flare light because they are imaged at different locations from the light beams of the design order.

  19 and 20 are explanatory views of a conventional diffractive optical element. In FIG. 19, reference numeral 192 denotes a ring zone of the diffraction grating, and optical power can be given by changing the pitch between the grating and the grating. Further, in FIG. 20, the first diffraction grating 205 and the second diffraction grating 206 are arranged with air interposed therebetween, and this configuration makes it possible to obtain high diffraction efficiency over a wide wavelength range. ing.

  FIG. 18 is an explanatory diagram of a laminated diffraction grating. 211 is a first diffraction grating, 212 is a second diffraction grating, and 213 is an air layer. The first diffraction grating and the second diffraction grating are made of materials having different dispersions. In the diffraction grating of this embodiment, the first diffraction grating 211 includes the ultraviolet curable resin 1 (nd = 1.635, νd = 23.0), the first An ultraviolet curable resin 2 (nd = 1.524, νd = 50.8) is used for the second diffraction grating 212, the grating thickness d 1 of the first diffraction grating 211 is 7.8 μm, the grating thickness d 2 of the second diffraction grating 212 is 10.7 μm, The thickness d3 of the air layer between the two lattices is 1.0 μm. The lattice pitch is 140 μm, and the design order is primary. At this time, the diffraction efficiency is almost 100% over the entire visible range. As shown in FIG. 18, the grating wall surface 215 of the first diffraction grating 211 and the grating wall surface 216 of the second diffraction grating 212 are diffracted when the incident light beam 215 is substantially parallel to the light beam direction in the resin of the first grating. Efficiency is best. From this point of view, proposals have been made to optimize the light flux within the effective angle of view by setting the angle of the wall surface of the diffraction grating (Patent Documents 1 and 2).

  Further, a patent has been proposed in which the reflection flare is prevented from reaching the image plane by tilting the wall surface in an obtuse angle direction (Patent Document 3).

  Further, a patent has been proposed in which light shielding means is provided on the grating wall surface of the close contact type diffractive optical element (Patent Document 4).

  Further, a configuration has been proposed in which the diffractive optical element is kept away from the front lens of the imaging optical system so that strong light incident from outside the screen does not strike the diffractive optical element (Patent Document 5).

JP 2003-294924 A JP 10-186118 A JP 2005-292571 A JP 2004-126394 A JP 2007-112440 A SPIE Vol.1354 International Lens Design Conference (1990)

  However, the methods of Patent Document 1 and Patent Document 2 are effective in increasing the diffraction efficiency of light in the effective screen and suppressing the grating wall surface with respect to the light in the effective screen, but there is sunlight outside the screen. There was a problem in the live-action shooting of the backlight scene.

  That is, when this element is used in an imaging optical system, if the incident angle of light deviates from the angle of the wall surface, the light beam is reflected by the wall surface and reaches the image surface as a flare.

  As a countermeasure, Patent Document 3 proposes a patent in which the reflection flare is prevented from reaching the image plane by inclining the angle of the wall surface in an obtuse angle direction. However, recent studies have revealed that the influence of unnecessary light cannot be greatly reduced by changing the position of transmitted light by tilting it at an obtuse angle. This is because the generation of unnecessary light is not geometric optical behavior, but is caused by the fact that light reaches a position that cannot be reached by geometric optical ray tracing by causing a diffraction phenomenon.

  Further, as for the method of providing the light shielding means on the grating wall surface of the close-contact type diffractive optical element as in Patent Document 4, since the light shielding member is provided on the wall surface, there are many problems in processing, which may cause a significant cost increase. Strong nature. In particular, if the light shielding means is provided in the entire grating of the diffractive optical element, it is expected that the manufacturing tact will be greatly increased.

  When a diffractive optical element is used in the imaging optical system, light from outside the effective field angle is incident on the element in addition to the light beam within the effective field angle used for image formation. When a diffractive optical element is used on the image side from the stop, the light beam entering at a large angle is less likely to be a problem because it is blocked by the lens barrel, but the diffractive optical element is used before the stop of the optical system. In this case, light from outside the effective angle of view easily enters the diffractive optical element, and the occurrence of flare becomes a problem.

  Even when a normal light source is outside the effective angle of view, the amount of flare is limited because the amount of light is low, and it is not a big problem. On the other hand, since sunlight in daytime shooting reaches 100,000 lx at the maximum, attention is required. In particular, when taking a picture during backlighting, measures such as attaching a light shielding hood are often taken, but flare may be caused depending on the incident angle of sunlight to the imaging optical system.

  In particular, strong light such as sunlight entering from a relatively low angle of 20 degrees or less outside the maximum angle of view easily hits the diffractive optical element directly, resulting in flare using the diffractive optical element as a secondary light source. To do. When the flare light reaches the image plane, problems such as image fogging and ghosting occur.

  In addition, when the diffractive optical element is lowered from the front lens as in Patent Document 5, it is difficult to say that it is the best position for correcting aberrations such as magnification color, and the design that maximizes the aberration correction effect of the diffractive optical element. Cannot be done.

  The present invention prevents flare that occurs when a strong light source such as sunlight outside the screen is incident on the diffractive optical element of the photographing optical system, and makes good use of the aberration correction capability of the diffractive optical element while fully utilizing it. Is provided.

  In order to solve the above-described problems, the present invention provides an imaging optical system including a plurality of lenses, and includes an optical material having a first refractive index and a second optical material including a front group lens, a diaphragm, and a rear group lens from the object side. A blazed diffractive optical element of close contact type, in which a diffraction grating is formed at the boundary by adhering an optical material having a refractive index of 20 mm, is arranged in the front group lens, and the direction with an angle of 20 degrees further outward from the maximum field angle A parallel light beam incident on the entire front lens of the imaging optical system reaches the diffractive optical element and crosses the optical axis of the imaging optical system in the light beam incident on the imaging optical system with the angle. When the cross section formed by the light beam and the optical axis is a meridional cross section, the width of the meridional cross section of the light beam reaching the diffractive optical element is the width of the light beam incident on the diffractive optical element across the optical axis with respect to the total light flux. Over 90% Characterized in that to constitute a lens barrel or product hood as.

  According to the present invention, it is possible to suppress the influence of strong light obtained from the outside of the effective angle of view while making the most of the effect of correcting chromatic aberration when a diffractive optical element is used in the imaging optical system. It is possible to provide an image pickup optical system that can obtain a good picked-up image with less flare and fog even when intense light is incident from the outside of the screen.

Explanatory drawing of the optical system of the 1st Example of this invention. 1 is a device cross-sectional view of a first embodiment of the present invention. The flare generation | occurrence | production condition explanatory drawing of the element upper side of 1st Example of this invention. Light incident angle on the diffractive optical element. Explanatory drawing of the diffraction efficiency change with respect to the incident angle of the element upper side of 1st Example. The flare generation | occurrence | production condition explanatory drawing of the element lower side of 1st Example of this invention. Explanatory drawing of the diffraction efficiency change with respect to the incident angle of the element lower side of 1st Example. FIG. 5 is another element cross-sectional view of the first embodiment of the present invention. The flare generation | occurrence | production state explanatory drawing of the upper incident light ray of the other diffractive optical element of this invention. Explanatory drawing of the diffraction efficiency change with respect to the incident angle of the other element upper side of a 1st Example. The flare generation | occurrence | production explanatory drawing of the lower side incident light ray of the other diffractive optical element of this invention. Explanatory drawing of the diffraction efficiency change with respect to the incident angle of the other element lower side of a 1st Example. The flare suppression countermeasure area description top view of the 2nd Example of this invention. The flare suppression countermeasure area explanatory sectional drawing of 2nd Example of this invention. Explanatory drawing of the optical system of 2nd Example of this invention. Explanatory drawing of the grating | lattice wall light shielding of a diffractive optical element. Explanatory drawing of the grating wall inclination countermeasure of a diffractive optical element. The enlarged view of the conventional diffractive optical element. The top view of the conventional diffractive optical element. Sectional drawing of the conventional diffractive optical element.

Example 1
FIG. 1 is an explanatory diagram when the first embodiment of the present invention is applied to a 400 mm telephoto lens. In the figure, 1 is a diffractive optical element, 2 is an aperture, 3 is an image plane such as a CCD, and 4 is a maximum. A light beam having an angle of view, 5 is an optical axis of the imaging optical system, 6 is a part of parallel light such as sunlight from outside the effective field angle, 7 is a parallel light beam that is incident on the diffractive optical element 1 and is 20 degrees outside the screen, Reference numeral 8 denotes a lens hood.

  If the first lens group has a positive power, such as a 400mm telephoto lens, the diffractive optical element should be used on the object point side as much as possible to correct the lateral chromatic aberration. It is desirable to place the element on the side. In this embodiment, the first lens unit has a positive refractive power, the second lens unit has a negative refractive index, and the third lens unit has a positive refractive index. It is effective for the first lens.

  As the diffractive optical element, the chromatic aberration is improved as an optical system by using a diffraction grating having a grating pitch narrowed from the center of the grating to the outside so as to add a phase having a positive power.

  In this embodiment, a diffractive optical element is provided between the first lens and the second lens. For this reason, when shooting a backlight scene, light beams such as sunlight entering from an angle of about 20 degrees outside the screen cannot be blocked by the lens hood or the optical axis barrel. Therefore, a part of the light beam enters the diffractive optical element. If you want to block all the light around 20 degrees off the screen with a lens hood, a very long lens hood is required. This very long lens hood is problematic in terms of an increase in the weight of the imaging optical system, operability, and storage. In reality, it is unlikely that such a long lens hood is included in the product.

  In this embodiment, a light beam that is harmful to the contact-type diffractive optical element can be almost cut with light of around 20 degrees. Specifically, an imaging optical system that suppresses the occurrence of flare without increasing the weight of the imaging optical system including the lens hood by cutting most of the light beam incident on the upper side of the diffractive optical element in FIG. provide.

  FIG. 2 is an explanatory diagram of the diffractive optical element of the present invention. In the figure, 21 is an optical axis, 22 is a first lens, 23 is a second lens, 24 is a first low refractive index high dispersion material, and 25 is a high refractive index low dispersion material. The reason for combining the low refractive index high dispersion material and the high refractive index low dispersion material in this way is to obtain high diffraction efficiency in a wide wavelength range.

  In addition, 26 is a light beam outside the effective screen incident on the grating wall surface of the annular zone of the diffraction grating above the optical axis, and 27 is a light beam outside the effective screen incident on the grating wall surface of the annular zone of the diffraction grating below the optical axis. Is shown. Although the light beam 26 and the light beam 27 indicate a part of the parallel light flux, the light beam is incident from above and hits the upper side of the diffractive optical element. In the light incident on the element, the light beam is incident on the grating wall surface and reflected.

  FIG. 3 is an explanatory diagram of the vicinity of the diffraction grating of the present invention. In the figure, 31 is a first material having a low refractive index and high dispersion, 32 is a second material having a high refractive index and low dispersion, and 33 is an average angle between the maximum angle and the minimum angle within the effective field angle of a light beam forming an image. The auxiliary line 35 shown indicates light rays incident on the diffraction grating wall surface from outside the effective angle of view.

  FIG. 5 is a graph showing the state of flare generation at this time, where the incident angle to the element is 10 °, the horizontal axis represents the diffraction angle, and the vertical axis represents the amount of light when the total amount of light is 100%. In addition, the diffraction grating used in the calculation is calculated as an optimized grating so that the first-order diffraction efficiency is the highest. The angle of the horizontal axis of the graph is an angle with respect to the surface normal of the element, and takes the sign of the angle as shown in FIG.

  The wall surface of the diffractive optical element is configured to be inclined by the average angle of the light beam incident on the grating of the diffractive optical element so that light within the effective angle of view is not kicked by the wall surface as much as possible. As a method of determining the average angle, as shown in FIG. 4, the angle of light incident on the diffractive optical element is obtained and determined. FIG. 4 shows the incident angle with respect to the surface on which the diffractive optical element is formed with the horizontal axis representing the incident angle when the center of the diffractive optical element is 0, and the vertical axis representing the ray angle. 41 indicates the maximum angle within the effective field angle, 42 indicates the minimum angle within the effective field angle, and 43 indicates the average angle. In order to maximize the diffraction efficiency with respect to the average angle of light rays within the effective angle of view, it is better to incline the wall surface of the diffraction grating in a direction corresponding to the average angle.

  The direction of the grating wall surface in FIG. 3 is parallel to the direction of the auxiliary line 33 that is the average value of the maximum angle and the minimum angle within the effective field angle of the incident light to the diffraction grating. At this time, the light beam 35 incident on the diffractive optical element passes through the low refractive index material and then enters the high refractive index material. When the light beam 35 enters the diffraction grating at a low angle as shown in FIG. To do.

  In FIG. 5, there is a saturated peak around 10 degrees, which is the position where the first-order diffracted light is generated. In addition, there is a light amount peak in the vicinity of −10 degrees, which is a flare generated by the total reflected light on the grating wall surface. It can be seen that flare due to diffraction occurs around this peak. In the graph of FIG. 5, the angle at which light reaches the image plane is a flare near 0 ° ± 1 ° to ± 2 °, and the total reflection peak near −10 ° does not reach the image plane. In addition, the diffracted light from the totally reflected light has a shape with a skirt extending to around 0 degrees. Accordingly, the light 26 incident on the upper side of the diffractive optical element in FIG. 2 generates flare and degrades the image.

  FIG. 6 is an explanatory diagram showing a situation when a light beam incident on the lower side of the diffractive optical element enters the wall surface. In the figure, 61 is a first material having a low refractive index and high dispersion, 62 is a second material having a high refractive index and low dispersion, and 63 is an average angle between the maximum angle and the minimum angle within the effective field angle of light rays forming an image. The auxiliary line 65 shown indicates light rays incident on the diffraction grating wall surface from outside the effective angle of view. As shown in FIG. 6, the light beam that has passed through the first material having a low refractive index is separated into two light beams by Fresnel reflection and transmission at the boundary with the second material having a high refractive index. In a contact-type diffractive optical element with improved diffraction efficiency with respect to light in the visible range, the refractive index difference between the first material and the second material is usually 0.1 or less. This light is transmitted light. For example, when incident on the wall surface of the diffraction grating at 80 degrees with respect to the normal direction of the wall surface, about 94% is transmitted light and 6% is reflected light. The flare light that actually reaches the CCD surface is flare that travels in the direction of the reflected light, and is not the reflected light itself but the diffracted light generated around the transmitted light. Therefore, the occurrence of flare due to the light incident on the lower side of the element across the optical axis is very small and does not cause a problem with respect to image degradation.

  FIG. 7 is a graph showing the state at this time with the diffraction angle on the horizontal axis and the light amount when the total light amount is 100% on the vertical axis when the incident angle to the element is 10 °. In addition, the diffraction grating used in the calculation is calculated as an optimized grating so that the first-order diffraction efficiency is the highest. The angle of the horizontal axis of the graph is an angle with respect to the surface normal of the element, and takes the + side as shown in FIG.

  In FIG. 7, there is a saturated peak around −10 degrees, which is the position where the first-order diffracted light is generated. Further, there is a light amount peak near +10 degrees, which is a flare generated by Fresnel reflected light on the grating wall surface. Flares due to diffraction occur around this peak, but in any case, the amount of flare is weak light. In addition, in the graph of FIG. 7, the angle at which light reaches the image plane is light in the vicinity of 0 ° ± 1 ° to ± 2 °, and the amount of flare is weak and is not at a level that degrades the image. The characteristics of this flare generation have been clarified by performing a strict electromagnetic field analysis, but such a phenomenon cannot be grasped in the conventional scalar calculation. Further, in the diffractive optical element as shown in FIG. 20 in which air is sandwiched between two gratings, both the light beam incident on the upper side of the diffractive optical element and the light beam incident on the lower side generate unnecessary diffracted light. Generation of unnecessary diffracted light cannot be suppressed even if the light beam incident on the upper side of the element having the above structure is shielded.

  FIG. 8 is an explanatory view of another type of diffractive optical element of the present invention. In the figure, 81 is an optical axis, 82 is a first lens, 83 is a second lens, 84 is a first high refractive index / low dispersion material, and 25 is a low refractive index / high dispersion material. The reason for combining the low refractive index high dispersion material and the high refractive index low dispersion material in this way is to obtain high diffraction efficiency in a wide wavelength range.

  In addition, 86 is a light beam outside the effective screen which is incident on the grating wall surface of the annular zone of the diffraction grating above the optical axis, and 87 is a light beam outside the effective screen which is incident on the grating wall surface of the annular zone of the diffraction grating below the optical axis. Is shown. Although the light ray 86 and the light ray 87 indicate a part of the parallel light flux, the case where the light ray enters from above and hits the upper side of the diffractive optical element, and the case where the light ray enters from above and passes the optical axis 81, the lower diffractive optics. In the light incident on the element, the light beam is incident on the grating wall surface and reflected.

  FIG. 9 is an explanatory view of the angle of the diffraction grating wall surface before the implementation of the present invention, and FIG. 11 is an explanatory view of the wall surface angle of the diffraction grating of the present invention. In the figure, 111 and 121 are first materials having a high refractive index and low dispersion, 112 and 122 are second materials having a low refractive index and high dispersion, and 113 and 123 are maximum angles within the effective field angle of light rays forming an image. Auxiliary lines 115 and 125 indicating the average angle of the minimum angle indicate light rays incident on the diffraction grating wall surface from outside the effective angle of view.

  In FIG. 9, the light ray 95 outside the effective screen passes through the high refractive index material 91 and is totally reflected on the wall surfaces of the diffraction gratings of the high refractive index material 91 and the low refractive index material 92. Due to total reflection, the amount of flare at this time is strong, but it does not reach the CCD surface angularly.

  FIG. 13 shows a graph of the state at this time with the diffraction angle on the horizontal axis and the light amount when the total light amount is 100% on the vertical axis when the incident angle to the element is 10 °. In addition, the diffraction grating used in the calculation is calculated as an optimized grating so that the first-order diffraction efficiency is the highest. The angle of the horizontal axis of the graph is an angle with respect to the surface normal of the element, and takes the + side as shown in FIG.

  In FIG. 10, there is a saturated peak around −10 degrees, which is the position where the first-order diffracted light is generated. In addition, there is a light amount peak near +10 degrees, which is a flare generated by the total reflected light on the grating wall surface. It can be seen that flare due to diffraction occurs around this peak. In the graph of FIG. 10, the angle at which light reaches the image plane is a flare near 0 ° ± 1 ° to ± 2 °, and the total reflection peak near + 10 ° does not reach the image plane. In addition, the diffracted light from the totally reflected light has a shape with a skirt extending to around 0 degrees.

  FIG. 11 is an explanatory diagram showing a situation when a light beam incident on the lower side of the diffractive optical element enters the wall surface. In the figure, 111 is a first material having a high refractive index and low dispersion, 112 is a second material having a low refractive index and high dispersion, and 113 is an average angle between the maximum angle and the minimum angle within the effective field angle of a light beam forming an image. The auxiliary line 115 shown indicates light rays that enter the diffraction grating wall surface from outside the effective angle of view. As shown in FIG. 11, the light beam that has passed through the first material having a high refractive index passes through the boundary with the second material having a low refractive index, and then again reaches the diffraction grating wall surface at the boundary with the first material having a high refractive index. , The light beam is separated into two light beams by Fresnel reflection and transmission. In a contact-type diffractive optical element with improved diffraction efficiency with respect to light in the visible range, the refractive index difference between the first material and the second material is usually 0.1 or less. This light is transmitted light. For example, when incident on the wall surface of the diffraction grating at 80 degrees with respect to the normal direction of the wall surface, about 94% is transmitted light and 6% is reflected light. The flare light that actually reaches the CCD surface is flare that travels in the direction of the reflected light, and is not the reflected light itself but the diffracted light generated around the transmitted light. Therefore, the occurrence of flare due to the light incident on the lower side of the element across the optical axis is very small and does not cause a problem with respect to image degradation.

  FIG. 12 is a graph showing the state at this time with the diffraction angle on the horizontal axis and the light quantity when the total light quantity is 100% on the vertical axis when the incident angle to the element is 10 °. In addition, the diffraction grating used in the calculation is calculated as an optimized grating so that the first-order diffraction efficiency is the highest. The angle of the horizontal axis of the graph is an angle with respect to the surface normal of the element, and takes the minus side as shown in FIG.

  In FIG. 12, there is a saturated peak near +10 degrees, which is the position where the first-order diffracted light is generated. Further, there is a light amount peak in the vicinity of −10 degrees, which is flare generated by Fresnel reflected light on the grating wall surface. Flares due to diffraction occur around this peak, but in any case, the amount of flare is weak light. In the graph of FIG. 12, the angle at which the light reaches the image plane is light in the vicinity of 0 ° ± 1 ° to ± 2 °, and the amount of flare is very weak and is not at a level that degrades the image.

  As described above, in the diffractive optical element of the type shown in FIG. 8 as well, for light near 20 degrees outside the effective field angle, the occurrence of flare is suppressed for the light beam incident on the lower side of the element. Therefore, in the close contact type diffractive optical element having the configuration of the present invention, the parallel light flux incident on the entire front lens of the imaging optical system from the direction having an angle of 20 degrees further outward from the maximum field angle is generated. When the light beam that reaches the diffractive optical element and is incident on the imaging optical system with the angle is a meridional cross section formed by a light beam intersecting the optical axis of the imaging optical system and the optical axis, By configuring the lens barrel or the product hood so that the light flux entering the diffractive optical element across the optical axis is 90% or more of the total light flux in the width in the meridional section of the light flux reaching the diffractive optical element. It is possible to suppress flare.

In the present invention, the diffractive optical element is created between the first lens and the second lens of the optical system. However, by placing the diffractive optical element on the image plane side, it is possible to block off-screen light by the lens barrel. Therefore, the lens hood can be shortened. On the other hand, lowering the diffractive optical element to the image plane side reduces the effect of improving lateral chromatic aberration. Therefore, in order to obtain the effect of the present invention, when the effective diameter of the lens closest to the object side is φf and the effective diameter of the diffractive optical element is φdo,
φdo / φf ≧ 0.7
Is desirable. By adopting this configuration, the magnification color correction effect can be maximized, and a remarkable effect can be obtained with respect to shortening the overall length of the optical system and high performance.

  FIG. 13 is an explanatory view of a second embodiment of the present invention. FIG. 13 is an explanatory view of the diffraction grating as viewed from the front. FIG. 14 is a sectional view including the optical axis. In the figure, 131 is a range in which the wall surface of the diffraction grating is tilted with respect to the average angle between the maximum angle and the minimum angle within the effective field angle of the light beam forming the image, and 132 is the angle of the light beam forming the image. The range of the average angle between the maximum angle and the minimum angle within the effective field angle is shown.

  FIG. 15 is an explanatory diagram when the second embodiment of the present invention is applied to a 400 mm telephoto lens. In the figure, 151 is a diffractive optical element, 152 is a stop, 153 is an image surface such as a CCD, and 154 is a maximum. 155 is an optical axis of the imaging optical system, 156 is a part of parallel light such as sunlight from outside the effective field angle, 157 is a parallel light beam of 20 degrees outside the screen incident on the diffractive optical element 151, Reference numeral 158 denotes a lens hood.

  In FIG. 15, a part of the parallel light beam incident from 20 degrees outside the screen is shielded by the lens hood or the lens barrel, but the width of the region where the parallel light beam hits the diffractive optical element in the meridional section is W0, W1 / W0 <0.9, where W1 is the width of the region below the optical axis, that is, the region where the off-screen 20 degree light beam enters across the optical axis. At this time, the region 131 is a problem that unnecessary diffracted light is easily generated as described above. For this reason, in the present invention, a countermeasure against flare is applied to the region 131.

  As a specific countermeasure against flare, the occurrence of unnecessary flare can be suppressed by applying a light shielding member to the wall surface as shown in FIG. In FIG. 16, in the conventional configuration, the light shielding member is applied to the entire area of the element. However, since the cost is significantly increased, it is difficult to actually implement the light shielding member. According to the present invention, it is possible to suppress an increase in cost by shielding part of the vicinity of the optical axis having a large ring zone pitch. On the other hand, it is possible to obtain a large flare suppression effect by shielding the area where strong flare occurs. As a result, it is possible to provide a diffractive optical element with less flare while minimizing the cost increase.

  As another flare countermeasure, flare can be suppressed by tilting the wall surface of the grating from an average angle that is the angle at which the best diffraction efficiency is obtained with respect to the light in the screen as shown in FIG. is there. On the other hand, there is a concern about tilting efficiency from tilting the wall surface of the vest, but since the ring pitch near the optical axis of the diffraction grating is wide, the degradation of diffraction efficiency due to tilting the wall surface is slight and acceptable. It is.

DESCRIPTION OF SYMBOLS 1 Diffractive optical element 2 Diaphragm 3 Image surface of CCD etc. 4 Light beam of maximum field angle 5 Optical axis of imaging optical system 6 Part of parallel light such as sunlight from outside effective field angle 7 Enters diffractive optical element 1 Parallel light beam 20 degrees outside the screen 8 Lens hood

Claims (10)

  In an imaging optical system composed of a plurality of lenses, an optical material having a first refractive index and an optical material having a second refractive index, which are composed of a front group lens, a diaphragm, and a rear group lens from the object side, are brought into close contact with each other. A close-contact type blaze diffractive optical element with a diffraction grating formed at the boundary is placed on the front lens group, and the entire front lens of the imaging optical system is directed from a direction with an angle of 20 degrees outward from the maximum field angle. The parallel light beam incident on the diffractive optical element reaches the diffractive optical element, and in the light beam incident on the imaging optical system with the angle, the cross section formed by the light beam intersecting the optical axis of the imaging optical system and the optical axis When the meridional cross section is used, the width of the light beam reaching the diffractive optical element in the width of the meridional cross section is such that the light beam incident on the diffractive optical element across the optical axis is 90% or more of the total light flux. Product hood An imaging optical system, characterized in that the configuration was. In the imaging optical system according to claim 1, when the effective beam diameter on the most object side of the front lens group is φf, and the effective beam diameter of the diffractive optical element is φdo,
φdo / φf ≧ 0.7
The imaging optical system according to claim 1, wherein a diffractive optical element is disposed at a position where   The imaging optical system according to claim 1 has a first lens group having a positive refractive power, a second lens group having a negative refractive index, and a positive or negative refractive index in order from the object side. The imaging optical system according to claim 1, wherein the three-lens group has a three-group configuration.   4. The imaging optical system according to claim 1, wherein the diffractive optical element is formed in close contact with the first lens arranged closest to the object side. An imaging optical system according to claim 1. In an imaging optical system composed of a plurality of lenses, an optical material having a first refractive index and an optical material having a second refractive index, which are composed of a front group lens, a diaphragm, and a rear group lens from the object side, are brought into close contact with each other. A blazed diffractive optical element of close contact type with a diffraction grating formed at the boundary is placed on the front lens group, from the direction with an angle of 20 degrees outward from the maximum field angle toward the entire front lens of the imaging optical system. An incident parallel light beam reaches the diffractive optical element and a cross section formed by a light beam intersecting the optical axis of the imaging optical system and the optical axis is meridional in the light beam incident at an angle to the imaging optical system. When the cross section is taken, the lens barrel or the product is such that the light beam incident on the diffractive optical element across the optical axis is less than 90% of the total light flux in the width in the meridional cross section of the light beam reaching the diffractive optical element. Food Form, the area where the collimated light impinges on the upper side of the diffractive optical element,
In the luminous flux incident from the direction having an angle of 20 degrees outward from the maximum angle of view, a flare such as a light shielding means on the grating wall surface is not present in the diffraction grating existing in the range reaching the diffractive optical element without crossing the optical axis. An imaging optical system characterized by taking measures. In the imaging optical system according to claim 5, when the effective beam diameter on the most object side of the front lens group is φf and the effective beam diameter of the diffractive optical element is φdo,
φdo / φf ≧ 0.7
The imaging optical system according to claim 5, wherein a diffractive optical element is disposed at a position where   The imaging optical system according to claim 5 has a first lens group having a positive refractive power, a second lens group having a negative refractive index, and a positive or negative refractive index in order from the object side. The imaging optical system according to claim 5 or 6, wherein the three-lens group has a three-group configuration.   8. The imaging optical system according to claim 5, wherein the diffractive optical element is formed in close contact with the first lens disposed closest to the object side. An imaging optical system according to claim 1.   6. The imaging optical system according to claim 5, wherein the flare countermeasure is such that the angle of the grating wall surface is inclined in a direction to suppress flare with respect to the angle at which the diffraction efficiency is the best. The imaging optical system according to claim 8.   6. The imaging optical system according to claim 5, wherein the countermeasure against flare has a diffraction order of a diffraction grating different from a use order of other diffraction gratings.