IMAGE-PICKUP OPTICAL SYSTEM

Abstract

An image-pickup optical system includes, from the object side to the image side, a first lens unit having positive optical power, an aperture stop, and a second lens unit. A diffractive optical element and a lens composed of extraordinary partial dispersion material are appropriately disposed in the optical system. The image-pickup optical system is capable of reducing the generation of flare and ghosts at the diffractive optical element and effectively correcting chromatic aberration.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image-pickup optical system, such as for example a shooting or photographing system.

2. Description of the Related Art

A known image-pickup optical system having a long focal length includes, from the object side to the image side, a front lens component having positive refractive power and a rear lens component having negative refractive power. Such an optical system is known as a telephoto type optical system or, simply, a telephoto lens.

In a typical telephoto lens, as the focal length increases, longitudinal chromatic aberration and lateral chromatic aberration worsens in particular among the various types of chromatic aberration. In general, these types of chromatic aberration are corrected by combining a positive lens and a negative lens, where the positive lens is constructed of low-dispersion material, such as fluorite, having extraordinary partial dispersion and the negative lens is constructed of a high-dispersion material.

Extraordinary partial dispersion material efficiently corrects chromatic aberration but is difficult to process. The relative density of extraordinary partial dispersion material is greater than other low-dispersion materials not having an extraordinary partial dispersion characteristic. Thus, by using extraordinary partial dispersion material, the weight of the entire lens system increases.

For example, the relative density of fluorite is 3.18, and a commercial product FK01 known to be an extraordinary partial dispersion material similar to fluorite has a relative density of 3.63. In contrast, a commercial product FK5 that is a material whose extraordinary partial dispersion is small has a relative density of 2.46, and a commercial product BK7 has a relative density of 2.52.

Another known method for correcting chromatic aberration of an optical system is a method using a diffractive optical element (refer to “SPIE” (Vol. 1354, International Lens Design Conference (1990)) and Japanese Patent Laid-Open Nos. 6-324262 (corresponding to U.S. Pat. No. 5,790,321), 6-331887 (corresponding to U.S. Pat. No. 5,796,520), and 2000-258685 (corresponding to U.S. Pat. No. 6,381,079)).

The method uses the physical phenomenon in which chromatic aberration caused by a light beam having a specific wavelength is generated in opposite directions on the refracting surface and the diffracting surface in an optical system.

When a diffractive optical element is used as the lens closest to the object in a first lens unit of a telephoto lens, longitudinal chromatic aberration and lateral chromatic aberration of the optical system is very effectively corrected. However, when very intense light, such as sun light from the sun outside the angle of view, is incident on the diffractive optical element, flare and ghosts are generated. As a result, the entire image may be covered with a whitish haze.

SUMMARY OF THE INVENTION

The present invention provides an image-pickup optical system capable of efficiently correcting chromatic aberration while suppressing, as much as possible, the generation of flare and ghosts at a diffractive optical element.

The image-pickup optical system includes, from the object side to the image side, a first lens unit having positive optical power, an aperture stop, and a second lens unit. A diffractive optical element and a lens composed of extraordinary partial dispersion material are appropriately disposed in the optical system.

Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of lenses included in an image-pickup system according to a first numerical example.

FIG. 2 illustrates the aberration of the image-pickup system according to the first numerical example.

FIG. 3 is a cross-sectional view of lenses included in an image-pickup system according to a second numerical example.

FIG. 4 illustrates the aberration of the image-pickup system according to the second numerical example.

FIG. 5 is a cross-sectional view of lenses included in an image-pickup system according to a third numerical example.

FIG. 6 illustrates the aberration of the image-pickup system according to the third numerical example.

FIG. 7 is a schematic view illustrating an example of a diffractive optical element.

FIG. 8 illustrates the wavelength dependency of the diffractive optical element illustrated in FIG. 7.

FIG. 9 is a schematic view illustrating another exemplary diffractive optical element.

FIG. 10 illustrates the wavelength dependency of the diffractive optical element illustrated in FIG. 9.

FIG. 11 is a schematic view illustrating another example of a diffractive optical element.

FIG. 12 illustrates the wavelength dependency of the diffractive optical element illustrated in FIG. 11.

FIG. 13 is a schematic view of the main components of an image-pickup apparatus.

DESCRIPTION OF THE EMBODIMENTS

Image-pickup optical systems according to embodiments of the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a cross-sectional view of lenses included in an image-pickup system according to a first embodiment. FIG. 2 illustrates the aberration of the image-pickup system according to the first embodiment when the system is focused at an object at an infinite distance.

FIG. 3 is a cross-sectional view of lenses included in an image-pickup system according to a second embodiment. FIG. 4 illustrates the aberration of the image-pickup system according to the second embodiment when the system is focused at an object at an infinite distance.

FIG. 5 is a cross-sectional view of lenses included in an image-pickup system according to a third embodiment. FIG. 6 illustrates the aberration of the image-pickup system according to the third embodiment when the system is focused at an object at an infinite distance.

The image-pickup systems illustrated in the cross-sectional views of FIGS. 1, 3, and 5 each include a first lens unit L1 having positive refractive power (optical power=inverse of focal length), a second lens unit L2 having positive or negative refractive power, and an aperture stop S. The aperture stop S is interposed between the first lens unit L1 and the second lens unit L2.

Each image-pickup system also includes a glass block G that is provided on the basis of an optical design to correspond to optical filters and face plates.

An image plane IP is also provided. When the image-pickup optical system according to this embodiment is used in a video camera or a digital still camera, the imaging plane of a solid-state image-pickup element (photoelectric transducer), such as a charge-coupled device (CCD) sensor or a complementary metal-oxide semiconductor (CMOS) sensor is provided at the position of the image plane IP. When the image-pickup optical system according to this embodiment is used in a silver halide film camera, the film surface is provided at the position of the image plane IP.

The Abbe number νd and the partial dispersion ratio θgF are determined by the following expressions: νd=(Nd−1)/(NF−NC) and θgF=(Ng−NF)/(NF−NC) where the indices of refraction of a material for the g line, d line, F line, and C line are Ng, Nd, NF, and NC, respectively.

At this time, the material that satisfies the following conditions is referred to as an extraordinary partial dispersion material: 75<νd and 0.5<θgF<0.6

According to the first embodiment shown in the cross-sectional view of FIG. 1, a compound lens L1do of the first lens unit L1 is the second compound lens when counted from the image side to the object side of the first lens unit L1. A diffractive optical element L1doa is provided on the bonding surface of the compound lens L1do.

A positive lens L1a is the second positive lens when counted from the image side to the object side of the first lens unit L1. In the first embodiment, extraordinary partial dispersion material (here, fluorite) is used for constructing the positive lens L1a.

The focus is moved from an object at an infinite distance to an object at a close distance by moving a lens subunit L1F to the image side. The lens subunit L1F is composed of a compound lens. The lens subunit L1F has negative refractive power and is disposed closest to the image in the first lens unit L1.

An image stabilizing lens subunit L2IS is included in the second lens unit L2. By moving the image stabilizing lens subunit L2IS in such a manner that it has a vector component orthogonal to the optical axis, the imaging position of the image-pickup optical system can be changed.

By moving the image stabilizing lens subunit L2IS in this way, blurriness of an image caused by hand movement is corrected. To move the image stabilizing lens subunit L2IS in such a manner that it has a vector component orthogonal to the optical axis, the image stabilizing lens subunit L2IS may be moved in the direction orthogonal to the optical axis or may be rotated around a point on the optical axis.

In FIG. 2, the d line, g line, C line, and F line of spherical aberration represent the aberration at the lines.

For astigmatism, ds and gs represent the sagittal image planes of the d line and the g line and dM and gM represent the meridional image planes. For lateral chromatic aberration, the g line and F line represent aberration.

According to this embodiment, lateral chromatic aberration is corrected by providing the lens L1a constructed of extraordinary partial dispersion material at a position where the paraxial chief ray h is relatively high.

Longitudinal chromatic aberration is corrected by providing the diffractive optical element L1doa at a position where the paraxial marginal ray h is relatively low.

According to the second embodiment shown in the cross-sectional view of FIG. 3, a compound lens L1do of a first lens unit L1 is the second compound lens when counted from the image side to the object side of the first lens unit L1. A diffractive optical element L1doa is provided on the bonding surface of the compound lens L1do.

A positive lens L1a is the second positive lens when counted from the image side to the object side of the first lens unit L1. A positive lens L1a is a positive meniscus lens having a convex surface on the object side and is constructed with extraordinary partial dispersion material (which, in this case, is fluorite).

The focus is moved from an object at an infinite distance to an object at a closer distance by moving the lens subunit L1F to the image side. The lens subunit L1F is composed of a compound lens. The lens subunit L1F has a negative refractive power and is disposed closest to the image in the first lens unit L1.

An image stabilizing lens subunit L2IS is included in a second lens unit L2. By moving the image stabilizing lens subunit L2IS in such a manner that it has a vector component orthogonal to the optical axis, the imaging position of the image-pickup optical system can be changed.

Descriptions of the aberration illustrated in FIG. 4 is the same as that illustrated in FIG. 2.

According to this embodiment, lateral chromatic aberration is corrected by providing the diffractive optical element L1doa at a position where the paraxial chief ray h is relatively high.

Longitudinal chromatic aberration is corrected by providing the lens L1a constructed of extraordinary partial dispersion material at a position where the paraxial marginal ray h is relatively low.

According to the third embodiment shown in the cross-sectional view of FIG. 5, a diffractive optical element L1doFa is provided on the bonding surface of a compound lens L1doF of a first lens unit L1. The compound lens L1doF has negative refractive power and is disposed closest to the image in a first lens unit L1.

The focus can be moved from an object at an infinite distance to an object at a close distance by moving the compound lens L1doF towards the image. That is that the compound lens L1doF is the lens subunit for focusing.

According to the third embodiment, among the positive lens included in a first lens unit L1, the second and third positive lenses L1a and L2a when counted from the object side to the image side are constructed of extraordinary partial dispersion material (in this case, fluorite).

An image stabilizing lens subunit L2IS is included in a second lens unit L2. By moving the image stabilizing lens subunit L2IS in such a manner that it has a vector component orthogonal to the optical axis, the imaging position of the image-pickup optical system can be changed.

Descriptions of the aberration illustrated in FIG. 6 is the same as that illustrated in FIGS. 2 and 4.

According to this embodiment, lateral chromatic aberration is corrected by providing the lenses L1a and L2a constructed of extraordinary partial dispersion material at positions where the paraxial chief ray h is relatively high.

Longitudinal chromatic aberration is corrected by providing the diffractive optical element L1doFa at a position near the aperture stop S where the paraxial marginal ray h is relatively low.

In each embodiment, a diffractive optical element is provided at a position that is closer to the object than the aperture stop S and that satisfies the following expression: 0.1<φdo/φf<0.7  (1) where φf represents the effective diameter of a lens (positive lens G11 in each embodiment) disposed closest to the object in the image-pickup optical system and φdo represents the effective diameter of the diffractive optical element.

Furthermore, a lens constructed of extraordinary partial dispersion material, i.e., a material that satisfies the following conditions: 0.5<θgF<0.6  (2) 75<νd  (3) is disposed at a position that satisfies the following condition: 0.2<φdo/φa<1.5  (4) where φa represents the effective diameter of the lens constructed of extraordinary partial dispersion material.

Expressions 1 to 4 define the positions where the diffractive optical element and the lenses constructed of extraordinary partial dispersion material are to be provided in the optical system.

The expressions 2 and 3 define the partial dispersion ratio θgF of the extraordinary partial dispersion material and the Abbe number νd.

Expression 1 defines the position where the diffractive optical element is to be provided so as to prevent highly intense light, such as sun light from the sun outside the angle of view, from being incident on the diffractive optical element and causing flare.

When the lower limit of Expression 1 is surpassed, less flare is generated by the highly intense direct light outside the angle of view incident on the diffractive optical element. However, it becomes difficult to correct aberration, in particular, chromatic aberration, of the image-pickup optical system.

When the higher limit of Expression 1 is exceeded, chromatic aberration can be effectively corrected. However, it becomes difficult to reduce flare generated by highly intense direct light outside the angle of view being incident on the diffractive optical element.

Expressions 2 and 3 define the range of dispersion of the extraordinary partial dispersion material. If a material with dispersion not included in the range defined by the expressions 2 and 3 is used, it becomes difficult to correct chromatic aberration of the image-pickup optical system.

Expression 4 defines the position where to provide a lens constructed of extraordinary partial dispersion material.

When the lower limit of Expression 4 is surpassed, the diameter of the lens constructed of extraordinary partial dispersion material becomes too great. This is undesirable since the weight of the entire optical system also becomes great.

When the higher limit of Expression 4 is exceeded, it becomes difficult to correct chromatic aberration.

By satisfying the conditions defined by the Expressions above, an image-pickup optical system capable of reducing flare generated by highly intense direct light outside the angle of view being incident on the diffractive optical element and capable of effectively correcting chromatic aberration can be provided.

By setting the numerical range of Expressions 1 to 4 as indicated below, flare can be reduced and chromatic aberration can be corrected even more effectively: 0.20<θdo/φf<0.68  (1a) 0.5<θgF<0.55  (2a) 85<νd  (3a) 0.30<do/φa<1.25  (4a)

More desirably, the partial dispersion ratio θgF and the Abbe number νd may be set as below: 0.52<θgF<0.54  (2b) 90<νd  (3b)

In each embodiment, the image-pickup optical system includes only one diffractive optical element.

However, as in the third embodiment, a plurality of lenses constructed of a material satisfying Expressions 2 and 3 may be provided in the image-pickup optical system.

The image-pickup optical system according to an embodiment satisfies the following condition: 0.10<|fa/f|<0.80  (5) where fa represents the focal length of the lens constructed of a material satisfying Expressions 2 and 3 and f represents the focal length of the entire system when focused at an object at infinite distance.

Expression 5 defines the refractive power of a lens constructed of extraordinary partial dispersion material. When the lower limit of Expression 5 is surpassed, the refractive power of the lens constructed of extraordinary partial dispersion material becomes too great. This is undesirable because chromatic aberration will be over-corrected.

When the higher limit of Expression 5 is exceeded, the refractive power of the lens constructed of extraordinary partial dispersion material becomes too small, and it becomes difficult to correct chromatic aberration.

In the third embodiment, a plurality of lenses constructed of extraordinary partial dispersion material is provided. Each of these lenses satisfies Expression 5.

It is desirable for correcting chromatic aberration to set the numerical range of Expression 5 as below: 0.20<|fa/f|<0.65  (5a)

The image-pickup optical system according to an embodiment satisfies the following condition: 0.10<|Rdo/Ldo−img|<1.00  (6) where Rdo represents the radius of curvature of the surface where the diffractive optical element is provided and Ldo-img represents the length from the diffractive optical element to the image plane.

Expression 6 defines the ratio of the radius of curvature of the surface provided with the diffractive optical element with respect to the length from the diffractive optical element to the image plane. When the lower limit of Expression 6 is surpassed, it is undesirable since flare generated by highly intense direct light outside the angle of view being incident on the diffractive optical element is increased.

When the higher limit of Expression 6 is exceeded, the distance between the diffractive optical element and the image plane becomes too small.

As a result, the spot diameter of unwanted diffracted light near the designed order (for example, if the designed order is first order, “near the designed order” refers to the 0 order and second order) becomes too small. This is undesirable since flare caused by the unwanted diffracted light becomes highly visible.

More desirably, Expression 6 may be set as below: 0.11<|Rdo/Ldo−img|<0.98  (6a)

The image-pickup optical system according to an embodiment satisfies the following condition: 0.05<|fnf/f|<0.2  (7) where fnf represents the focal length of a lens sub unit that is disposed closest to the image in the first lens unit L1 and that has a negative refractive power.

Expression 7 relates to the refractive power of a lens sub unit for focusing included the image-pickup optical system. When the lower limit of Expression 7 is surpassed, the refractive power of the focusing lens becomes too strong. This is undesirable because the difference in chromatic aberration correction becomes great.

When the higher limit of Expression 7 is exceeded, the refractive power of the focusing lens becomes too small. This is undesirable because the amount of movement while focusing is carried out becomes great.

More desirably, Expression 7 may be set as below: 0.07<|fnf/f|<0.18  (7a)

The image-pickup optical system according to an embodiment satisfies the condition below: 0.4≦TL/f≦0.85  (8) where TL represents the entire length of the lens system.

Here, “the entire length of the lens system” is the distance from the first lens surface closest to the object (i.e., the lens surface closest to the object in the image-pickup optical system) to the image plane.

Expression 8 defines a value known as a telephoto ratio. In the image-pickup optical system according to an embodiment, the refractive powers of elements, such as the first and second lens units, and the distance between main points of elements are set to satisfy Expression 8.

When the telephoto ratio becomes smaller than the lower limit of Expression 8, it becomes difficult to correct chromatic aberration by using a diffractive optical element and a lens constructed of extraordinary partial dispersion material.

When the telephoto ratio exceeds the higher limit, the technological advantage of using a diffractive optical element and a lens constructed of extraordinary partial dispersion material becomes small.

When an image-pickup apparatus is constituted of an image-pickup optical system according to an embodiment and a solid-state image-pickup element, the following conditions are satisfied: 0.03<f/fdo<0.30  (9) 0.02≦himg/f≦0.06  (10) where fdo represents the focal length for only a diffractive optical element and 2·himg represents the effective length (effective diagonal length) of the solid-state image-pickup element.

Expression 9 relates to the optical power of the diffractive optical element in the image-pickup optical system.

When the lower limit of Expression 9 is surpassed, the optical power of the diffractive optical element becomes too small. As a result, it is difficult to correct chromatic aberration. When the higher limit of Expression 9 is exceeded, the optical power of the diffractive optical element becomes too great. This is undesirable because the diffraction efficiency is worsened.

Expression 10 relates to a ratio of the image-pickup range of the image-pickup optical system to the focal length. An image-pickup optical system according to an embodiment is suitable for a telephoto lens system having a long focal length within the range represented by Expression 10.

According to an embodiment, it is significantly advantageous to use a diffractive optical element and a lens constructed of extraordinary partial dispersion material that satisfy the above-identified conditions in an image-pickup optical system according to an embodiment that has a focal length satisfying Expression 10.

It is preferable for correcting chromatic aberration if Expression 9 defines the following numerical range: 0.05<f/fdo<0.25  (9a)

As described above in the embodiments, a diffractive optical element and a lens constructed of extraordinary partial dispersion material are suitably provided in an optical system. Accordingly, a small and light weight image-pickup optical system that has excellent optical characteristics and that is capable of reducing flare caused by highly intense direct light, such as sun light from the sun outside the angle of view, being incident on the diffractive optical element and sufficiently correcting chromatic aberration can be provided.

Next, the structure of a diffractive optical element according to an embodiment will be described.

The diffractive optical element has an optical characteristic that is different from that of typical glass. In other words, the diffractive optical element has negative dispersion and extraordinary partial dispersion.

More specifically, the Abbe number νd equals −3.45 and the partial dispersion ratio θgF equals 0.296.

By applying these characteristics to a refractive optical system, chromatic aberration can be effectively corrected.

The diffractive optical element according to an embodiment may have an aspherical effect by changing the pitch of the grating.

The diffractive optical element may have a two-layer structure in which an air gap 101 is interposed between diffractive gratings 106 and 107, as shown in FIG. 7. Moreover, the diffractive optical element may have a three-layer structure in which an air gap 101 is interposed between diffractive gratings 106 to 108, as shown in FIG. 9. Moreover, the diffractive optical element may have a two-layer structure including diffractive gratings 104 and 105 having different (or the same) grating thickness, as shown in FIG. 11.

A diffractive optical element shown in FIG. 7 is constituted by a first diffractive grating 106 that is constructed of ultraviolet curable resin on a substrate 102′ and a second diffractive grating 107 that is constructed of ultraviolet curable resin on a substrate 102.

FIG. 8 illustrates the wavelength dependency of diffraction efficiency of a first order diffracted light in the diffractive optical element that has a two-layer structure, as illustrated in FIG. 7. As shown in FIG. 8, the diffraction efficiency of the designed order (e.g., first order) represents high one of 95% or more within the entire wavelength range being used.

A diffractive optical element shown in FIG. 9 is constituted of a first diffractive grating 106 that is constructed of ultraviolet curable resin on a substrate 102′, a second diffractive grating 107 that is constructed of ultraviolet curable resin on a substrate 102, and a layer 108 constructed by filling the second diffractive grating 107.

FIG. 10 illustrates the wavelength dependency of diffraction efficiency of a first order diffracted light in the diffractive optical element that has a three-layer structure, as illustrated in FIG. 9. As shown in FIG. 10, in 95% or more of the entire wavelength range being used, the diffraction efficiency of the designed order (e.g., first order) is high.

A diffractive optical element shown in FIG. 11 is constituted of a first diffractive grating 104 that is constructed of ultraviolet curable resin on a substrate 102 and a second diffractive grating 105 that is constructed of ultraviolet curable resin on the first diffractive grating 104.

FIG. 12 illustrates the wavelength dependency of diffraction efficiency of a first order diffracted light in the diffractive optical element that has a two-layer structure, as illustrated in FIG. 11.

As shown in FIG. 12, in 95% or more of the entire wavelength range being used, the diffraction efficiency of the designed order (first order) is high.

The illustrated diffractive gratings in FIGS. 7, 9, and 11, are enlarged in the height direction so as to show the structure of the diffractive grating in a visually apparent manner. The actual height (depth) of a diffractive grating is on the order of several micrometers to several tens of micrometers, which is negligibly small from a geometric point of view. Therefore, a diffractive optical element having a multiple-layer structure, such as that shown in FIG. 7, 9, or 11, can be presumed to have a geometrically flat structure.

Next, numerical examples will be described.

In each numerical example, ri represents the radius of curvature of the ith plane when counted from the object side; di represents the axial distance between the ith plane and the (i+1)th plane when counted from the object side; ndi represents the index of refraction at the d-line of the material of the ith optical member; and νdi represent the Abbe number at the d-line.

Moreover, f represents the focal length; Fno represents the F number; ω represents the half field angle; AIR represents air; and himg presents a value equal to half the effective image-pickup range.

The phase shape φ of the grating of an diffractive optical element according to an embodiment is represented by the following expression: φ(h,m)=(2π/mλ0)·(C1·h²+C2·h⁴+C3·h⁶+ . . . ) where m represents the diffraction order of the diffracted light, λ0 represents the designed wavelength, h represents the height in a direction orthogonal to the optical axis, and Ci (i=1, 2, 3 . . . ) is the phase coefficient.

Even when the diffractive optical element is a relief grating, the grating height is negligibly small from a geometric point of view. Therefore, in the numerical examples, the lens surfaces where the diffractive optical elements are provided have the same radius of curvature and the surface spacing is set to zero.

The relationship between the above-described expressions and the various numerical values according to the numerical examples are shown in Table 1.

First Numerical Example
	himg/f		Fno		2ω
	0.028		1:56		3.2°
	Radius of		Surface		Index of		Abbe number
	curvature ri		spacing di		refraction nd		νd
(Diffraction	r1	183.709	d1	19.772	nd1	1.48749	νd1	70.24
plane→)	r2	−1914.310	d2	76.479	nd2	AIR	νd2	AIR
	r3	117.091	d3	15.496	nd3	1.43387	νd3	95.1
	r4	−880.797	d4	5.203	nd4	AIR	νd4	AIR
	r5	−370.906	d5	4.000	nd5	1.83481	νd5	42.71
	r6	168.693	d6	4.406	nd6	AIR	νd6	AIR
	r7	86.311	d7	14.614	nd7	1.48749	νd7	70.24
	r8	3790.412	d8	0.426	nd8	AIR	νd8	AIR
	r9	47.560	d9	5.500	nd9	1.48749	νd9	70.24
	r10	42.322	d10	22.977	nd10	AIR	νd10	AIR
	r11	219.209	d11	3.600	nd11	1.88300	νd11	40.77
	r12	51.566	d12	10.000	nd12	1.59270	νd12	35.31
	r13	331.125	d13	53.030	nd13		νd13
	r14	214.624	d14	3.200	nd14	1.84666	νd14	23.78
	r15	341.843	d15	6.961	nd15	1.88300	νd15	40.77
	r16	59.457	d16	48.596	nd16		νd16
	r17	∞ (aperture stop)	d17	12.601	nd17	AIR	νd17	AIR
	r18	49.695	d18	1.300	nd18	1.84666	νd18	23.78
	r19	23.534	d19	5.000	nd19	1.62004	νd19	36.26
	r20	−269.332	d20	2.279	nd20	AIR	νd20	AIR
	r21	125.381	d21	5.500	nd21	1.71736	νd21	29.52
	r22	−54.221	d22	1.300	nd22	1.75500	νd22	52.32
	r23	35.909	d23	2.720	nd23	AIR	νd23	AIR
	r24	−50.755	d24	1.300	nd24	1.77250	νd24	49.60
	r25	311.894	d25	17.033	nd25	AIR	νd25	AIR
	r26	61.596	d26	5.500	nd26	1.88300	νd26	40.77
	r27	45.622	d27	6.869	nd27	1.62004	νd27	36.26
	r28	−184.650	d28	2.972	nd28	AIR	νd28	AIR
	r29	248.756	d29	3.500	nd29	1.84666	νd29	23.78
	r30	37.320	d30	10.692	nd30	1.67270	νd30	32.10
	r31	−79.415	d31	29.174	nd31	AIR	νd31	AIR
	r32	∞	d32	2.000	nd32	1.51633	νd32	64.14
	r33	∞	d33	60.000	nd33	AIR	νd33	AIR
C1 −6.00886E−05
C2 −1.33138E−09
C3 −7.51923E−12

Second Numerical Example
	himg/f		Fno		2ω
	0.028		1:56		3.2°
	Radius of		Surface		Index of		Abbe number
	curvature ri		spacing di		refraction nd		νd
(Diffraction	r1	166.160	d1	19.499	nd1	1.48749	νd1	70.24
plane→)	r2	−1163.016	d2	72.229	nd2	AIR	νd2	AIR
	r3	177.935	d3	8.274	nd3	1.48749	νd3	70.24
	r4	988.698	d4	4.361	nd4	AIR	νd4	AIR
	r5	−358.277	d5	4.000	nd5	1.83481	νd5	42.71
	r6	256.779	d6	0.150	nd6	AIR	νd6	AIR
	r7	94.821	d7	16.037	nd7	1.48749	νd7	70.24
	r8	−324.716	d8	10.000	nd8	1.88300	νd8	40.77
	r9	−1293.341	d9	10.793	nd9	AIR	νd9	AIR
	r10	68.212	d10	5.300	nd10	1.43387	νd10	95.10
	r11	50.302	d11	73.745	nd11	AIR	νd11	AIR
	r12	318.221	d12	3.200	nd12	1.83400	νd12	37.16
	r13	55.715	d13	6.047	nd13	1.84666	νd13	23.78
	r14	73.930	d14	50.366	nd14		νd14
	r15	∞ (aperture stop)	d15	26.733	nd15	AIR	νd15	AIR
	r16	47.616	d16	1.300	nd16	1.84666	νd16	23.78
	r17	24.996	d17	4.198	nd17	1.57501	νd17	41.51
	r18	−295.809	d18	0.150	nd18	AIR	νd18	AIR
	r19	80.950	d19	3.231	nd19	1.72151	νd19	29.23
	r20	−123.356	d20	1.300	nd20	1.72916	νd20	54.68
	r21	33.183	d21	3.500	nd21	AIR	νd21	AIR
	r22	−54.791	d22	1.300	nd22	1.72916	νd22	54.68
	r23	167.423	d23	17.016	nd23	AIR	νd23	AIR
	r24	57.530	d24	9.151	nd24	1.61293	νd24	37.01
	r25	−47.174	d25	1.400	nd25	1.69680	νd25	55.53
	r26	−143.031	d26	3.311	nd26	AIR	νd26	AIR
	r27	375.410	d27	3.500	nd27	1.84666	νd27	23.78
	r28	32.317	d28	10.791	nd28	1.69895	νd28	301279.00
	r29	−123.212	d29	31.120	nd29	AIR	νd29	AIR
	r30	∞	d30	2.000	nd30	1.51633	νd30	64.14
	r31	∞	d31	60.000	nd31	AIR	νd31	AIR
C1 −4.82889E−05
C2 2.90800E−09
C3 −1.47575E−13

Third Numerical Example
	himg/f		Fno		2ω
	0.028		1:56		3.2°
	Radius of		Surface		Index of		Abbe number
	curvature ri		spacing di		refraction nd		νd
(Diffraction	r1	171.976	d1	20.678	nd1	1.48749	νd1	70.24
plane→)	r2	−1104.531	d2	63.678	nd2	AIR	νd2	AIR
	r3	109.519	d3	15.390	nd3	1.43387	νd3	95.10
	r4	−1124.991	d4	1.896	nd4	AIR	νd4	AIR
	r5	−429.096	d5	4.000	nd5	1.83481	νd5	42.71
	r6	171.176	d6	27.688	nd6	AIR	νd6	AIR
	r7	79.413	d7	14.795	nd7	1.43387	νd7	95.10
	r8	10421.755	d8	0.190	nd8	AIR	νd8	AIR
	r9	46.398	d9	5.300	nd9	1.69680	νd9	55.53
	r10	39.479	d10	54.311	nd10	AIR	νd10	AIR
	r11	286.973	d11	3.200	nd11	1.88300	νd11	40.77
	r12	32.697	d12	3.675	nd12	1.84666	νd12	23.78
	r13	51.152	d13	72.076	nd13		νd13
	r14	∞ (aperture stop)	d14	10.718	nd14	AIR	νd14	AIR
	r15	74.735	d15	1.300	nd15	1.84666	νd15	23.78
	r16	28.786	d16	4.042	nd16	1.62004	νd16	36.26
	r17	−100.204	d17	7.089	nd17	AIR	νd17	AIR
	r18	102.007	d18	3.616	nd18	1.72825	νd18	28.46
	r19	−43.002	d19	1.300	nd19	1.75500	νd19	52.32
	r20	35.333	d20	2.635	nd20	AIR	νd20	AIR
	r21	−61.433	d21	1.300	nd21	1.88300	νd21	40.77
	r22	432.550	d22	22.958	nd22	AIR	νd22	AIR
	r23	72.136	d23	10.000	nd23	1.69895	νd23	30.13
	r24	−40.769	d24	1.400	nd24	1.84666	νd24	23.78
	r25	−81.046	d25	1.439	nd25	AIR	νd25	AIR
	r26	307.130	d26	3.500	nd26	1.84666	νd26	23.78
	r27	32.147	d27	9.897	nd27	1.69895	νd27	30.13
	r28	−369.342	d28	33.927	nd28	AIR	νd28	AIR
	r29	∞	d29	2.000	nd29	1.51633	νd29	64.14
	r30	∞	d30	60.000	nd30	AIR	νd30	AIR
C1 −1.56689E−04
C2 −4.11408E−09
C3 −1.94261E−10

	TABLE 1
		First	Second	Third
	Conditional	Numerical	Numerical	Numerical
	expression	Example	Example	Example
	(1)	0.389	0.652	0.233
	(2)	0.537	0.537	0.537
		—	—	0.537
	(3)	95.15	95.15	95.15
		—	—	95.15
	(4)	0.574	1.231	0.341
		—	—	0.400
	(5)	0.494	0.619	0.462
		—	—	0.235
	(6)	0.177	0.957	0.129
	(7)	0.090	0.151	0.122
	(8)	0.58	0.58	0.58
	(9)	0.094	0.076	0.246
	(10)	0.028	0.028	0.028

A camera (image-pickup apparatus) including an image-pickup optical system according to an embodiment of the present invention will be described with reference to FIG. 13.

FIG. 13 is a schematic view of the main components of a single-lens reflex camera. FIG. 13 illustrates an image-pickup lens 10 including an image-pickup optical system 1 according to an embodiment. The image-pickup optical system 1 is held by a lens barrel 2, which is a holding member. A camera main body 20 includes a quick-return mirror 3, a focusing screen 4, a penta-roof prism 5, and an ocular lens 6. The quick-return mirror 3 reflects the light beam from the image-pickup lens 10 upward. The focusing screen 4 is disposed at the image formation position of the image-pickup lens 10. The penta-dach prism 5 converts an inverse image formed on the focusing screen 4 into an upright image. The ocular lens 6 is used by an observer to observe the upright image.

On a light-sensitive surface 7 is disposed a solid-state image-pickup element (photoelectric transducer), such as a charge-coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS), or a silver halide film. When capturing an image, the quick-return mirror 3 is retracted from the optical path, and an image is formed by the image-pickup lens 10 on the light-sensitive surface 7.

The image-pickup optical system according to an embodiment is suitable for such an image-pickup apparatus.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures and functions.

This application claims the priority of Japanese Application No. 2005-310118 filed Oct. 25, 2005, which is hereby incorporated by reference herein in its entirety.

Claims

1. An image-pickup optical system comprising: a first lens unit having positive optical power; an aperture stop; and a second lens unit; wherein the first lens unit, the aperture stop, and the second lens unit are disposed this in order from the object side to the image side of the image-pickup optical system, wherein the image-pickup optical system includes a the diffractive optical element which is disposed closer to the object than the aperture stop and is disposed at a position satisfying, 0.1<φdo/φf<0.7, where φf represents the beam effective diameter of the lens disposed closest to the object in the image-pickup optical system and φdo represents the beam effective diameter of the diffractive optical element, wherein the image-pickup optical system includes a lens which is provided, the lens being composed of a material satisfying, 0.5<θgF<0.6 and 75<νd, where νd represents the Abbe number and θgF represents a partial dispersion ratio, and wherein the lens is disposed at a position satisfying, 0.2<φdo/φa<1.5, where φa represents the beam effective diameter of the lens.

2. The image-pickup optical system according to claim 1, wherein only one diffractive optical element is provided.

3. The image-pickup optical system according to claim 1, wherein 0.10<|fa/f|<0.80 is satisfied, where fa represents the focal length of the lens and f represents the focal length of the entire image-pickup optical system.

4. The image-pickup optical system according to claim 1, wherein 0.10<|Rdo/Ldo−img|<1.00 is satisfied, where Rdo represents the radius of curvature of the plane provided with the diffractive optical element and Ldo-img represents the distance from the diffractive optical element to the image plane.

5. The image-pickup optical system according to claim 1, wherein the first lens unit comprises a lens sub-unit having negative optical power which is provided at a position closest to the image in the first lens unit, the lens sub-unit moving during focusing, and wherein 0.05<|fnf/f|<0.2 is satisfied, where fnf represents the focal length of the lens sub-unit and f represents the focal length of the entire image-pickup optical system.

6. The image-pickup optical system according to claim 1, wherein 0.4≦TL/f≦0.85 is satisfied, where TL represents the entire length of the image-pickup optical system and f represents the focal length of the entire image-pickup optical system.

7. A camera comprising: an image-pickup optical system according to claim 1; and a solid-state image-pickup element configured to receive an image formed by the image-pickup optical system.

8. The camera according to claim 7, wherein 0.03<f/fdo<0.30 and 0.02≦himg/f≦0.06 are satisfied, where fdo represents the focal length of the diffractive optical element, f represents the focal length of the entire image-pickup optical system, and 2·himg equals the effective diagonal length of the solid-state image-pickup element.