JP2000147379A - Zoom lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a rear focusing system zoom lens which has good optical performance over an entire varying power range from a wide angle end to a telephone end, or an entire object distance from an infinity object to a super close object and which utilizes a diffraction optical device. SOLUTION: In this zoom lens having the four lens groups of a first group L1 having positive refracting power, a second group L2 having negative refracting power, a third group L3 having the positive refracting power, and a fourth group L4 in order from an object side, and performing variable power by moving the groups L2 and L4; the groups L1 and L2 have at least one diffraction optical device which is rotation symmetry to each optical axis, and when the number of a lens provided in the group L1 is defined as N1C, the power variation ratio of an entire system is defined as Z, and the Abbe number of the material of the optical lens among the lenses possessed by the groups L1 and L2 is defined as ν, the expressions of N1C<√Z/2, and ν <72 are satisfied.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a zoom lens, and more particularly to a photographic camera, a video camera, a broadcast camera, etc., in which various aberrations, especially chromatic aberrations, are corrected well by using a diffractive optical element in a part of the lens system. The present invention relates to a zoom lens which has a large aperture ratio and a high zoom ratio and is used to reduce the size of the entire lens system.

[0002]

2. Description of the Related Art Recently, as home video cameras and the like have been reduced in size and weight, zoom lenses for imaging have been reduced in size.
In particular, in the case of zoom lenses, efforts are being made to reduce the overall length of the lens, reduce the diameter of the front lens, and simplify the configuration.

Further, there is a strong demand for a recent zoom lens for a video camera which has a high zoom ratio of 10 times or more and has a small lens system as a whole.

As one means for achieving these objects, there is known a so-called rear focus type zoom lens which performs focusing by moving a lens group other than the first group on the object side.

In general, a rear focus type zoom lens has a smaller effective diameter of the first lens group than a zoom lens which moves and focuses the first lens group, so that the entire lens system can be easily miniaturized, and close-up photography can be performed. In particular, since extremely close-up photography is facilitated and the relatively small and lightweight lens group is moved, the driving force of the lens group is small and quick focusing can be performed.

As such a rear focus type zoom lens, for example, JP-A-62-215225, JP-A-62-220616, and JP-A-62-262.
No. 4213, JP-A-63-247316 and JP-A-4-43311 disclose, in order from the object side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a positive lens having a positive refractive power. The zoom lens has four lens groups, a third lens group and a fourth lens group having a positive refractive power. The second lens group is moved to perform zooming, and the fourth lens group is moved to change the image plane due to zooming and focus. There has been proposed a four-group type rear focus type zoom lens which performs the above.

In Japanese Patent Application Laid-Open No. Hei 4-301612, a first group of positive refractive power and a second group of negative refractive power are sequentially arranged from the object side.
Group, a third group having a positive refractive power, a fourth group having a positive refractive power, and a fifth lens group having a negative refractive power. A five-group type zoom lens has been proposed in which the fourth group is moved to correct and focus the image plane fluctuation caused by zooming, and the entire lens system is made closer to a telephoto type to shorten the overall length of the lens. .

On the other hand, in many zoom lenses, high optical performance is obtained while satisfactorily correcting various aberrations by providing an aspherical surface in the lens system and reducing the size of the entire lens system.

In addition to the method of correcting chromatic aberration among various aberrations by combining glass materials having different dispersions, an optical system corrected by providing a diffractive optical element having a diffractive action on a lens surface or a part of the optical system is used. For example, JP-A-4-213421
And Japanese Patent Application Laid-Open No. 6-324262, and US Pat. No. 5,268,790. this house,
U.S. Pat. No. 5,268,790 proposes a zoom lens using diffractive optical elements in the second and third units.

[0010]

Generally, in order to correct chromatic aberration in a zoom lens having a high zoom ratio, a high dispersion negative lens and a low dispersion positive lens are frequently used in the first lens unit. It is necessary to use a low dispersion negative lens and a high dispersion positive lens also in the second lens unit.

In particular, in a zoom lens having a zoom ratio of about 10 times or more and about 50 times, it is important to correct chromatic aberration, and it is necessary to use two or three or more low-dispersion positive lenses in the first lens unit. As a result, the number of lenses in the first group tends to increase, and the entire lens system tends to increase in size.

On the other hand, when the rear focus system is adopted in the zoom lens, the whole lens system can be reduced in size, quick focus can be achieved, and further advantages such as easy close-up photographing can be obtained.

[0013] On the other hand, however, there is a problem that aberration variation at the time of focusing increases, and it becomes very difficult to obtain high optical performance over the entire object distance from an object at infinity to an object at a short distance.

For example, in a zoom lens having a large aperture ratio and a high zoom ratio, the fluctuation of chromatic aberration due to zooming becomes large, and it becomes very difficult to obtain high optical performance over the entire zoom range and over the entire object distance. The problem arises.

In particular, in a zoom lens composed of four or five groups having a high zoom ratio of 10 times or more, for example, about 50 times, a lens made of low dispersion glass to correct chromatic aberration generated in each lens group. And a laminated lens are often used. Then, a method has been adopted in which the number of lenses in the lens group is reduced by using an aspherical surface for the lens group, and the overall length of the lens is shortened.

However, when the number of lenses is reduced, elements for correcting chromatic aberration become insufficient, and it becomes difficult to satisfactorily correct fluctuations in chromatic aberration due to zooming.

Generally, if low dispersion glass is used for the positive lens, chromatic aberration can be reduced. However, low-dispersion glass generally has a low refractive index and tends to have a lens shape that is difficult to process. Therefore, for example, if the refractive power of the first group or the second group is weakened, the refractive power of the other lens group must also be weakened accordingly. As a result, the diameter of the first group becomes larger, and as a result, the first lens group becomes larger. It is necessary to increase the lens thickness of one group, and the overall length of the lens increases.

According to the present invention, in a four-group or five-group rear-focus type zoom lens, by appropriately setting the lens configuration of each lens group, over the entire zoom range from the wide-angle end to the telephoto end, It is another object of the present invention to provide a large aperture ratio and high zoom ratio zoom lens having excellent optical performance over the entire object distance from an object at infinity to a very close object.

In particular, in a four-group or five-group type rear focus zoom lens, diffractive optical elements are introduced into the first and second groups, respectively, and the first and second groups are formed by utilizing the diffractive optical action. While reducing the chromatic aberration generated in the second group, the number of lenses in the first and second groups is reduced, the overall length of the lens is reduced, and the first group is reduced in weight. It is an object of the present invention to provide a zoom lens having good optical performance over the entire zoom range.

[0020]

According to the present invention, there is provided a zoom lens system comprising: (1-1) a first unit having a positive refractive power, a second unit having a negative refractive power, and a third unit having a positive refractive power in order from the object side. In a zoom lens having four lens groups, a first lens group and a fourth lens group, wherein the second lens group and the fourth lens group are moved to perform zooming, the first lens group and the second lens group are each relative to the optical axis. At least one rotationally symmetric diffractive optical element
N1C, the number of lenses of the first group
When the zoom ratio of the entire system is Z and the Abbe number of an arbitrary lens material of the lenses of the first and second groups is ν,

[0021]

(Equation 3) Is satisfied.

(1-2) First positive refractive power from the object side
Group, second group of negative refractive power, third group of positive refractive power, fourth group
A first lens unit and a fifth lens unit having a positive refractive power and a fifth lens unit having a positive refractive power. The first group has at least one diffractive optical element rotationally symmetric with respect to the optical axis, the number of lenses of the first group is N1C, the zoom ratio of the entire system is Z, and the first and second groups are When the Abbe number of an arbitrary lens material is ν,

[0023]

(Equation 4) Is satisfied.

[0024]

1 to 3 are a sectional view of a lens at a wide angle end, an aberration diagram at a wide angle end, and an aberration diagram at a telephoto end according to Numerical Embodiment 1 of the present invention. 4 to 6 show Numerical Embodiment 2 of the present invention.
3A to 3C are a lens cross-sectional view at the wide-angle end, an aberration diagram at the wide-angle end, and an aberration diagram at the telephoto end. 7 to 9 are a lens cross-sectional view at the wide-angle end, an aberration diagram at the wide-angle end, and an aberration diagram at the telephoto end according to Numerical Example 3 of the present invention.

First, the features of the lens configuration of Numerical Examples 1 and 2 shown in FIGS. 1 and 4 will be described. 1 and 4, L1 is a first group having a positive refractive power, L2 is a second group having a negative refractive power, L3 is a third group having a positive refractive power, and L4 is a fourth group having a positive refractive power. It is. SP denotes an aperture stop, which is arranged in front of the third lens unit L3. G is a glass block such as a color separation optical system, a face plate, and a filter. IP
Is the image plane.

In this embodiment, when zooming from the wide-angle end to the telephoto end, the second lens unit is moved to the image plane side as indicated by the arrow, and the image plane fluctuation due to the zooming is changed by projecting the fourth lens unit to the object side. The movement is corrected while having the trajectory.

Also, a rear focus system is adopted in which the fourth unit is moved on the optical axis to perform focusing. A solid line curve 4a and a dotted line curve 4b of the fourth lens group shown in the same figure show the image plane fluctuation caused by zooming from the wide-angle end to the telephoto end when focusing on an object at infinity and an object at a short distance, respectively. The movement locus for correction is shown. The first and third units are fixed during zooming and focusing. Note that the first unit may be moved at the time of zooming in order to reduce the sharing of zooming of the second unit.

In the present embodiment, the fourth unit is moved to correct the image plane fluctuation caused by zooming, and the fourth unit is moved for focusing. In particular, as shown by curves 4a and 4b in the same figure, the zoom lens is moved so as to have a convex locus toward the object side when zooming from the wide-angle end to the telephoto end. Thereby, the space between the third and fourth units is effectively used, and the overall length of the lens is effectively reduced.

In this embodiment, for example, when focusing from an object at infinity to an object at a short distance at the telephoto end, the fourth unit is moved forward as indicated by a straight line 4c in FIG.

In the present embodiment, each of the first and second units is provided with a diffraction optical element rotationally symmetric with respect to at least one optical axis, and the phases thereof are appropriately set.
Chromatic aberration generated in the group is reduced, and chromatic aberration is favorably corrected over the entire zoom range.

The diffractive optical element according to the present embodiment is binary-manufactured by a lithographic technique which is a technique for producing a holographic optical element (HOE). The diffractive optical element is a binary optics (BINARY OPT)
ICS). In this case, in order to further increase the diffraction efficiency, a saw-like shape called a kinoform may be used. Further, it may be manufactured by molding by those manufactured by these methods.

The shape of the diffractive optical element in the present embodiment is as follows: when the reference wavelength (d-line) is λ, the distance from the optical axis is h, and the phase is φ (h), φ (h) = 2π / λ ( C ₁ · h ² + C ₂ · h ⁴ + C ₃ · h ⁶ +... C _(i) · h ²ⁱ ) (1)

Next, features of other configurations of the first and second numerical embodiments will be described.

(A-1) Each of the first and second units has at least one diffractive optical element rotationally symmetric with respect to the optical axis, and the first unit has N1C lenses. When the zoom ratio of the system is Z and the Abbe number of an arbitrary lens material of the lenses of the first and second groups is ν,

[0035]

(Equation 5) Are satisfied.

As a result, chromatic aberration can be satisfactorily corrected over the entire zoom range while reducing the number of lenses in the first and second groups. It is more preferable that the conditional expression (a2) satisfies the following condition: 23 <ν <72 (a2 ′).

(A-2) In the first embodiment, the first lens unit L
Numeral 1 is composed of two positive lenses G1 and G2, and a diffractive optical element is provided on a lens surface on the image plane side of the positive lens G1 on the object side. The second unit L2 includes two negative lenses G3 and G4, and a diffractive optical element is provided on the object-side lens surface of the negative lens G4 on the image plane side. Third group L3 and fourth group L
The most object-side lens surface of No. 4 is composed of an aspherical surface.

(A-3) In the numerical example 2, the first lens unit L
Numeral 1 comprises a negative lens G1 and a positive lens G2, and a diffractive optical element is provided on a lens surface on the image plane side of the positive lens G2 on the image plane side. The second unit L2 includes two negative lenses G3 and G4, and a diffractive optical element is provided on the object-side lens surface of the image-side negative lens G4. The lens surfaces closest to the object in the third unit L3 and the fourth unit L4 are each formed of an aspheric surface.

(A-4) In the first and second numerical embodiments, the third unit L3 is a positive lens unit having a stop SP in front and fixed during zooming, and the fourth positive unit L4 is a variable power. And the distance adjustment is also performed by the fourth lens unit L4.

(A-5) As another lens configuration of the first lens unit L1, in Numerical Embodiment 1, the first lens unit L1 is composed of two positive lenses, and at least one diffractive optical element is provided before, after, or between them. It has a plate having a surface.

Still another configuration of the first lens unit L1 includes a positive lens, a negative lens, or a negative lens and a positive lens, and has a diffractive optical element on either surface. At this time, the positive lens and the negative lens may be bonded. At that time, the chromatic aberration is corrected in cooperation with this bonding surface,
The diffractive optical element needs to increase the positive refractive power.

(A-6) As another configuration of the second unit L2, as in Numerical Examples 1 and 2, it is composed of two negative lenses, and at least one diffractive optical element is provided before, after, or between them. To have a plate.

Still another second lens unit L2 has a positive lens and a negative lens, or a negative lens and a positive lens, and has a diffractive optical element on either surface. Is also good.

Next, the features of the lens configuration of the numerical example 3 in FIG. 7 will be described. Numerical example 3 is a five-unit zoom lens having a high zoom ratio of about 50 times.

In FIG. 7, L1 is a first group (first lens group) having a positive refractive power, L2 is a second group (second lens group) having a negative refractive power, and L3 is a positive lens having a positive refractive power. The third unit (third lens unit), L4 is a fourth unit (fourth lens unit) having a negative refractive power, L4
Reference numeral 5 denotes a fifth group (fifth lens group) having a positive refractive power. SP
Denotes an aperture stop, which is arranged in front of the third lens unit L3.
IP is an image plane.

G is a glass block such as a color separation optical system, a face plate, and a filter. At the time of zooming from the wide-angle end to the telephoto end, the second lens unit L2 is moved to the image plane side as indicated by an arrow, and the image plane fluctuation due to zooming is adjusted so that the fourth lens unit has a locus convex toward the object side. I am moving it and correcting it. In addition, a rear focus type in which the fourth unit is moved on the optical axis to perform focusing is adopted.

A solid curve 4a and a dotted curve 4b of the fourth group shown in FIG. 7 are images when zooming from the wide-angle end to the telephoto end when focusing on an object at infinity and an object at a short distance, respectively. The movement locus for correcting the surface fluctuation is shown. First group,
The third and fifth units are fixed during zooming and focusing.

In the present embodiment, the fourth unit is moved to correct the image plane fluctuation caused by zooming, and the fourth unit is moved for focusing. In particular, as shown by curves 4a and 4b in the same figure, the zoom lens is moved so as to have a convex locus toward the object side when zooming from the wide-angle end to the telephoto end. As a result, the third group and the fourth group can effectively use air, and the total length of the lens can be effectively reduced.

In this embodiment, for example, when focusing from an object at infinity to an object at a short distance at the telephoto end, the fourth unit is moved forward as shown by a straight line 4c in FIG.

In the five-unit zoom lens according to the present embodiment, the first unit is composed of one negative lens and three positive lenses, and the optical axis is located on the image surface side of the cemented lens of the negative lens and the positive lens. Is provided with a rotationally symmetric diffractive optical element. The shape of the diffractive optical element is the same as that of Numerical Examples 1 and 2 represented by Expression (1).

The second lens unit L2 is composed of two negative lenses and a cemented lens composed of a negative lens and a positive lens. The second lens unit L2 diffracts the image on the image side of the second negative lens. An optical element is provided.

Next, the features of the other configuration of the third embodiment will be described.

(A-1) Each of the first and second units has at least one diffractive optical element rotationally symmetric with respect to the optical axis, and the first unit has N1C lenses. When the zoom ratio of the system is Z and the Abbe number of an arbitrary lens material of the lenses of the first and second groups is ν,

[0054]

(Equation 6) Are satisfied.

As a result, chromatic aberration can be satisfactorily corrected over the entire zoom range while reducing the number of lenses in the first and second groups. It is more preferable that the conditional expression (a2) satisfies the following condition: 23 <ν <72 (a2 ′).

(A-2) The third lens unit L3 comprises four lenses: a positive lens, a cemented lens of a positive lens and a negative lens, and a positive lens.

(A-3) The fourth unit L4 comprises a cemented lens of a positive lens and a negative lens.

(A-4) The fifth unit L5 includes three lenses: a positive lens, and a cemented lens of a negative lens and a positive lens.

Next, the features of each numerical example of the present invention will be described.

In any of the embodiments, it is better not to dispose a diffractive optical element on the lens surface closest to the object, except in special cases such as unavoidable aberration correction. This means that the diffractive optical element has a rather narrow width, for example a few μm or sub-μm.
In order to protect the lens surface from dust and the like.

By disposing a diffractive optical element in the first lens unit L1,
By appropriately selecting the phase of the diffractive optical element, the first
Chromatic aberration generated in the group L1, for example, 2 such as d line and g line
The chromatic aberration of the wavelength is kept small, and the variation due to zooming of the chromatic aberration as a whole is kept small. At this time, chromatic aberration (secondary spectrum) remains at the telephoto end.

On the other hand, by arranging the diffractive optical element in the second lens unit L2 and appropriately selecting the phase of the diffractive optical element, the chromatic aberration generated in the second lens unit L2, for example, two wavelengths such as d-line and g-line Is suppressed small, and the variation due to zooming of the chromatic aberration as a whole is suppressed small. At this time, the chromatic aberration (secondary spectrum) remaining at the telephoto end is
The direction is opposite to that of the first lens unit L1.

In the present invention, the lens unit on the object side of the stop S and the zooming unit, that is, the first unit L1 and the second unit L
A secondary spectrum is reduced by changing the magnification by using a diffractive optical element for No. 2.

That is, as described above, the first lens unit L1 and the second lens unit L2 for zooming are each provided with at least one diffractive optical surface that is rotationally symmetric with respect to the optical axis. In the second lens unit L2, the chromatic aberration at the reference wavelength (d-line and g-line) is suppressed to a small value, and the secondary spectra respectively generated in the variable power unit are generated in the opposite directions, so that the first lens unit L1 and the second lens unit L2 To achieve good chromatic aberration as a whole.

With this configuration, the first lens unit L
The lens constituting 1 has one or two or more low-dispersion two positive lenses or a high-dispersion negative lens and a low-dispersion positive lens, respectively. Achromatism is performed by sharing lenses. The number of lenses used for correcting chromatic aberration is reduced by the diffractive optical element, and the number of constituent lenses is reduced as a whole. Further, the lens constituting the second unit L2 also has two or more low-dispersion negative lenses and high-dispersion positive lenses, respectively, or one lens. Further, the negative lens and the positive lens are bonded together, or a plurality of lenses are used. I'm sharing colors. The number of lenses used for correcting chromatic aberration is reduced by the diffractive optical element, and the number of constituent lenses is reduced as a whole.

By adopting such a configuration, a zoom lens can be achieved with an inexpensive configuration without using an expensive glass material having a high dispersion or a low dispersion even if a high zoom ratio or a miniaturization is performed. Even with a zoom lens exceeding 10 times, further miniaturization can be achieved while maintaining good performance. Specifically, the Abbe number of the lenses constituting the first lens group and the second lens group can be in the range of 23 <ν <72, but can be in the range of 23 <ν <65 at a magnification of about 10 times. It is.

As a specific method for reducing the chromatic aberration of the zoom lens, when the refractive power of the i-th lens unit having a diffractive optical element is Fi, at least one surface satisfying the following expression is required.
It is preferable to have a surface.

Fi · C _i <0 (i = 1, 2) (2) where C _i represents a paraxial refractive power of the diffractive optical surface in the i-th lens unit Li. When the objective refractive power C _i has a positive value, the refractive power of the i-th group has a negative value, and when the refractive power C _i has a negative value, the refractive power of the i-th group has a positive value. In both the positive lens unit and the negative lens unit, the curvature of the lens unit can be reduced, which is effective for aberration correction.

What can be seen from equation (1) is that the phase can be adjusted by the distance h from the optical axis. The greater the lens diameter, the greater the effect of higher order coefficients. In the consumer zoom lens described in the present embodiment, particularly the zoom lens for video, miniaturization is being promoted, and there are few lenses that are too large, that is, lenses with a large distance h. In addition, it is preferable that the following conditional expression is satisfied in order to achieve effective aberration correction by efficiently using the coefficient even with a small lens. Here, C _2i and C _3i are 4 in the expression (1) of the diffractive optical element in the i-th group, respectively.
These are the coefficients of the next and sixth order terms.

1 · 10 ⁻⁴ <| C _2i / C _i | <1 · 10 ⁻¹ (3) 1 · 10 ⁻⁷ <| C _3i / C _i | <1 · 10 ^-2. (4) As described above, these expressions are for effectively correcting aberrations at a small diameter. If these conditional expressions are not satisfied, not only is it difficult to correct aberrations, but also it becomes difficult to manufacture a diffractive optical surface, which is not appropriate.

As described above, the first lens unit L1 and the second lens unit L1
By the diffractive optical elements arranged in the lens unit 2, the chromatic aberration (secondary spectrum) generated in each lens group is suppressed to be small in cooperation, and the fluctuation due to the zooming of the chromatic aberration due to the movement of the second unit L2 is also suppressed to be small. At this time, the fourth unit L4
Further, a fixed fifth negative lens group can be arranged on the image plane side of the zoom lens. At this time, the fifth group may be configured so as to be of a telephoto type as a whole to further reduce the size.

As in the embodiment, the first unit L1 and the second unit L
In the case where the chromatic aberration correction in place of the achromatism such as the bonding of 2 is performed by the diffractive optical element, the refractive power is not so necessary.

Here, some off-axis aberrations, in particular, curvature of field,
Refractive power may be provided for distortion correction.
In this case, when the focal lengths of the diffractive optical elements of the first and second units L1 and L2 are Fbo1 and Fbo2, and the focal lengths of the first and second units L1 and L2 are F1 and F2, the following conditions must be satisfied. If it is not difficult to manufacture, it is good for aberration correction including chromatic aberration.

0.05 <F1 / Fbo1 <0.7 (5) 0.05 <F2 / Fbo2 <0.7 (6) In particular, the lens group having the diffractive optical element is as follows. It is preferably within the numerical range. 1.0 <F1 / (Fw · Ft) ^1/2 <3.0 (7) In the case of a high zoom ratio of 10 times or more, in particular, when the zoom ratio is about 10 times, 1 0.0 <F1 / (Fw · Ft) ^1/2 <2.5 (7 ′) where Fw and Ft are the focal lengths of the entire system at the wide-angle end and the telephoto end, respectively. Within this range, the function of the diffractive optical element can be effectively brought out. If the lower limit of the expression (7) is deviated, the refractive power of the first lens unit L1 is too strong, so that the chromatic aberration cannot be corrected by the diffractive optical system, and the production becomes difficult. If the value exceeds the upper limit, the chromatic aberration can be easily removed without using a diffractive optical element. In addition, in order to obtain a lens having a desired focal length, the refractive power of the second lens unit L2 becomes particularly strong, and the amount of aberration generated in the second lens unit L2 increases, which is not applicable. That is, the Petzval sum becomes negatively large, and the field curvature becomes overcorrected.

When the diffractive optical element has only one surface, it is preferable that the following expression is satisfied.

| Fi / Rboi | <1.8 (8) where Rboi is the radius of curvature of the surface in the i-th group forming the diffractive optical element. When Rbo1 = ∞, the base surface is flat. If the value deviates from the expression (8), the aberration generated on the curved surface of the base cannot be completely corrected by the diffractive optical system, and the effect of the diffractive optical system cannot be sufficiently brought out.

Generally, a diffractive optical element produces chromatic aberration opposite to chromatic aberration caused by ordinary refraction. For example, when removing a lens that has been achromatized by a conventional bonding surface or the like and reducing the number of lenses, a surface having chromatic aberration sharing opposite to the chromatic aberration sharing that occurred on the bonding surface is diffractive optically. It is preferable to use an element. If you do that,
Chromatic aberration opposite to the chromatic aberration caused by normal refraction occurs on the diffractive optical element, and the direction is the same as the direction of chromatic aberration occurring on the original bonding surface, and achromatization such as bonding is possible on a single lens Becomes

From the viewpoint of the chromatic aberration coefficient (published by Kyoritsu Shuppan Co., Ltd., Yoshiya Matsui, “Lens Design Method”, page 89),
On a surface on the object side of the stop S, the diffractive optical element is arranged on a surface having the same sign on the axial chromatic aberration coefficient L and the magnification chromatic aberration coefficient T,
It is preferable to dispose the diffractive optical element on the surface on the image side of the stop S on the opposite side.

As a result, the lenses constituting the first lens unit L1 are reduced in chromatic aberration by the diffractive optical element, the number of constituent lenses can be reduced, and further downsizing can be achieved while maintaining good performance.

In particular, when the thickness of the lens constituting the first unit L1 on the optical axis is t1, it is preferable that the following conditional expression is satisfied.

0.1 <t1 / F1 <0.33 (9) In particular, the thickness of the lens forming the second unit L2 on the optical axis is t
When 2, it is preferable to satisfy the following conditional expression. In particular, when the magnification is 10 times or more, 0.05 <t2 / F1 <1.5 (10) When the magnification is about 10 times, 0.55 <t2 / F1 <0.4 (0.4) 10 ') Equations (9) and (10) show the range in which the diffractive optical element is used effectively. When the diffractive optical element is used, the curvature is loose as described in the section of equation (2). Also obtains a desired refractive power. A concave lens (first lens unit L) for correcting chromatic aberration
1) If the combination with the convex lens (the second lens unit L2) can be eliminated by the diffractive optical element, the lens thickness is further reduced and the lens is effectively used.

When the magnification is 10 times or more, the thickness tends to be large in order to suppress the fluctuation of aberration. However, the thickness can be reduced by the diffractive optical element.

If the values deviate from the upper limits of the expressions (9) and (10), the thickness is possible even with a normal glass lens, and the diffractive optical element is not effectively used. On the other hand, if the value falls below the lower limit, refracting power due to diffraction is greatly required, and the occurrence of aberrations increases, which is not appropriate.

Although not described in this embodiment, the first
It is also possible to achieve the group lens group L1 or the second lens group L2 with a single lens using a diffractive optical element.

As a configuration of the diffractive optical element used in the present embodiment, a single-layer kinoform-shaped single-layer configuration shown in FIG. 10 or a different (or the same) grating thickness as shown in FIG. A two-layer structure in which two layers are stacked is applicable.

FIG. 11 shows the diffractive optical element 101 shown in FIG.
3 shows the wavelength dependence of the diffraction efficiency of the first-order diffracted light. The actual configuration of the diffractive optical element 101 is such that an ultraviolet curable resin is applied to the surface of the base material 102, and the resin portion has a grating thickness d such that the diffraction efficiency of the first-order diffracted light at a wavelength of 530 nm is 100%.
03 is formed.

As is apparent from FIG. 11, the diffraction efficiency of the design order decreases as the distance from the optimized wavelength of 530 nm increases, while the diffraction efficiencies of the zero-order diffraction light and the second-order diffraction light of orders near the design order increase. An increase in diffracted light other than the design order causes a flare, leading to a decrease in the resolution of the optical system.

FIGS. 12 (A) and 12 (B) show the relationship between the spatial frequency and the M in the case where Numerical Embodiment 1 is created with the lattice shape shown in FIG.
4 shows TF characteristics. In the figure, the MTF in the low frequency region is slightly lowered.

FIG. 14 shows the wavelength dependence of the diffraction efficiency of the first-order diffracted light of the laminated diffractive optical element in which the two layers 104 and 105 shown in FIG. 13 are laminated.

In FIG. 13, the first layer 10 made of an ultraviolet curable resin (nd = 1.499, νd = 54) is formed on the substrate 102.
4 is formed thereon, and another ultraviolet curable resin (nd = 1.
598, νd = 28). In this combination of materials, the lattice thickness d1 of the first layer 104 is d1 = 18.8 μm, and the lattice thickness d2 of the second layer 105 is d2 = 10.5 μm.

As can be seen from FIG. 14, the diffraction efficiency of the design order has a high diffraction efficiency of 95% or more over the entire use wavelength range by using the diffractive optical element having the laminated structure.

FIGS. 15A and 15B show the relationship between the spatial frequency and the M in the case where Numerical Embodiment 1 is created with the lattice shape shown in FIG.
4 shows TF characteristics. Using a diffractive optical element with a laminated structure,
The low-frequency MTF is improved, and a desired MTF characteristic is obtained. As described above, when the laminated structure is used as the diffractive optical element according to the present invention, the optical performance can be further improved.

The material of the diffractive optical element having the above-mentioned laminated structure is not limited to an ultraviolet-curable resin, but other plastic materials or the like may be used. May be formed. Further, the lattice thicknesses do not necessarily have to be different, and depending on the combination of materials, the lattice thicknesses of the two layers 104 and 105 may be equal as shown in FIG.

In this case, since the lattice shape is not formed on the surface of the diffractive optical element, it is excellent in dustproofness and the workability of assembling the diffractive optical element can be improved.

Next, numerical examples of the present invention will be described. In the numerical examples, ri is the radius of curvature of the i-th lens surface in order from the object side, di is the i-th lens thickness and air spacing in order from the object side, and ni and νi are the i-th lens in order from the object side. Are the refractive index and Abbe number of the glass. Table 1 shows the relationship between the above-described conditional expressions and the numerical examples.

The aspheric surface has an X-axis in the optical axis direction, a Y-axis in a direction perpendicular to the optical axis, a positive traveling direction of light, R is a paraxial radius of curvature,
When K, B, C, D, E, and F are each an aspheric coefficient,
It is represented by the following expression. “D-0X” means “10 ^−X ”.

[0097]

(Equation 7) It is represented by the following expression. “D-0X” means “10 ^−X ”.

[0098]

[Outside 1]

[0099]

[Outside 2]

[0100]

[Outside 3]

[0101]

[Table 1]

[0102]

According to the present invention, (c-1) in a four-group type or a five-group type rear focus zoom lens, by appropriately setting the lens configuration of each lens group, telephoto from the wide-angle end A zoom lens having a large aperture ratio and a high zoom ratio having good optical performance can be achieved over the entire zoom range to the end and over the entire object distance from an object at infinity to an object at a very close distance.

(C-2) In a four-group type or a five-group type rear focus zoom lens, diffractive optical elements are introduced into the first and second groups, respectively. The number of lenses in the first and second groups is reduced while reducing the chromatic aberration generated in the first and second groups, and the overall length of the lens is reduced, and the weight of the first group is reduced. A zoom lens having good optical performance over the entire zoom range up to the telephoto end can be achieved.

[Brief description of the drawings]

FIG. 1 is a sectional view of a lens according to a numerical example 1 of the present invention.

FIG. 2 is an aberration diagram at a wide-angle end according to Numerical Embodiment 1 of the present invention.

FIG. 3 is an aberration diagram at a telephoto end in Numerical Example 1 of the present invention;

FIG. 4 is a sectional view of a lens according to a numerical example 2 of the present invention.

FIG. 5 is an aberration diagram at a wide-angle end according to Numerical Example 2 of the present invention.

FIG. 6 is an aberration diagram at a telephoto end in Numerical Example 2 of the present invention.

FIG. 7 is a sectional view of a lens according to a numerical example 3 of the present invention.

FIG. 8 is an aberration diagram at a wide angle end according to Numerical Example 3 of the present invention.

FIG. 9 is an aberration diagram at a telephoto end in Numerical Example 3 of the present invention.

FIG. 10 is an explanatory view of a diffractive optical element according to the present invention.

FIG. 11 is an explanatory diagram of a wavelength dependence characteristic of the diffractive optical element according to the present invention.

FIG. 12 is an MTF characteristic diagram of the diffractive optical element according to the present invention.

FIG. 13 is an explanatory view of a diffractive optical element according to the present invention.

FIG. 14 is an explanatory diagram of a wavelength dependence characteristic of the diffractive optical element according to the present invention.

FIG. 15 is an MTF characteristic diagram of the diffractive optical element according to the present invention.

FIG. 16 is an explanatory diagram of a diffractive optical element according to the present invention.

[Explanation of symbols]

 L1 First lens unit L2 Second lens unit L3 Third lens unit L4 Fourth lens unit L5 Fifth lens unit SP stop IP image plane ΔM meridional image plane ΔS sagittal image plane dd line g g line 101 diffractive optical element 102 substrate 103, 104, 105 layer

Continued on the front page F term (reference) 2H087 KA02 KA03 MA15 NA14 PA07 PA12 PA16 PA18 PB08 PB17 QA02 QA07 QA14 QA17 QA21 QA25 QA26 QA34 QA37 QA41 QA42 QA45 QA46 RA05 RA12 RA13 RA32 RA41 RA42 SA43 SA49SA27 SA53 SA55 SA63 SA65 SA72 SA74 SA76 SB03 SB05 SB13 SB15 SB23 SB25 SB33 SB44 UA01

Claims (7)

[Claims] 1. A lens system comprising: a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit in order from the object side. In a zoom lens that performs zooming by moving a second group and a fourth group, each of the first group and the second group has at least one diffraction optical element that is rotationally symmetric with respect to the optical axis. When the number of lenses of the first group is N1C, the zoom ratio of the entire system is Z, and the Abbe number of an arbitrary lens material of the lenses of the first and second groups is ν, A zoom lens characterized by satisfying the following. 2. The zoom lens according to claim 1, wherein said fourth group has a positive refractive power. 3. The zoom lens according to claim 1, wherein said fourth group has a negative refractive power. 4. A zoom lens according to claim 1, wherein said diffractive optical element comprises a laminated diffraction grating. 5. A first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit and a fourth lens unit having a positive refractive power, and a fifth lens unit having a positive refractive power. 5 lens groups, the second
In a zoom lens that performs zooming by moving a group and a fourth group, each of the first group and the second group has at least one diffractive optical element that is rotationally symmetric with respect to the optical axis. When the number of lenses in the group is N1C, the zoom ratio of the entire system is Z, and the Abbe number of an arbitrary lens material of the lenses in the first and second groups is ν, A zoom lens characterized by satisfying the following. 6. The zoom lens according to claim 1, wherein the focusing is performed by moving the fourth unit on an optical axis. 7. The zoom lens according to claim 1, wherein the diffractive optical element has a single-layer structure or a two-layer structure made of materials having different dispersions.