JP2000019400A - Zoom lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a zoom lens being optimum for a liquid crystal projector having excellent optical performance extending over a whole variable power range to a telephoto end from a wide angle end and extending over a whole object distance to an extremely closest distance object from an infinite distance object. SOLUTION: This zoom lens is provided with five lens groups being the 1st group L1 whose refractive power is negative, the 2nd group L2 whose refractive power is positive, the 3rd group L3 whose refractive power is negative, the 4th group L4 whose refractive power is positive and the 5th group L5 whose refractive power is positive in order from a 1st conjugate point whose distance is long. Then, a variable power action to the telephoto end from the wide angle end is executed by moving the 2nd and the 4th groups to the 1st conjugate point. Besides, at least one lens group out of the respective lens groups is provided with at least one diffraction type optical element being asymmetric with respect to an optical axis.

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 telecentric system suitable for a liquid crystal video projector having a color synthesizing prism, a color synthesizing mirror and various filters at the rear of the lens system. The present invention relates to a zoom lens having a long back focus and excellent optical performance.

[0002]

2. Description of the Related Art Conventionally, there are various lens systems (projection lens systems) having a relatively long back focus for liquid crystal video projectors in which various optical members such as a color combining prism, a polarizing filter, and a color filter are arranged behind a lens system. It has been proposed.

Such a lens system is disclosed in, for example,
It has been proposed in Japanese Patent Application Laid-Open No. 2-291613 and Japanese Patent Application Laid-Open No. 3-145613.

The lens system disclosed in Japanese Patent Application Laid-Open No. 62-291613 has a distortion of about -5%. Further, the lens system disclosed in JP-A-3-145613 tends to have large astigmatism.

[0005] Further, since the size deviation of the image due to each of the RBG colors, that is, the magnification chromatic aberration is large, the color deviation sometimes becomes conspicuous when an image of a high-definition liquid crystal display device is projected.

In order to correct various aberrations and reduce the number of lenses, it is conventionally known to use an aspherical surface as a part of a lens system. The use of an aspherical surface is effective because the number of lenses can be reduced and an aberration correction effect that cannot be obtained with a spherical surface can be expected.

In a lens system corresponding to a high number of pixels, correction of chromatic aberration is particularly important in removing various aberrations. However, good correction of chromatic aberration is difficult with an aspherical surface.

Recently, many zoom lenses have been proposed in which the number of constituent lens groups is increased in order to suppress the aberration of the lens system, and the degree of freedom is increased by reducing the aberration sharing of each lens group.

When a zoom lens is used as a lens system, it is necessary to correct chromatic aberration in the first lens unit, and particularly to suppress occurrence of chromatic aberration in the first lens unit. Otherwise, the fluctuation of the chromatic aberration accompanying zooming increases due to the movement of the second and subsequent lens units, which are the main zooming units.

It is also necessary to reduce fluctuations in chromatic aberration and the like in the second and subsequent zooming units. Therefore, in the related art, achromatizing is performed by frequently using lenses made of a plurality of materials having different dispersion values as lenses constituting each lens group.
In addition, correction was performed by bonding a negative lens and a positive lens.
Therefore, the number of lenses constituting each lens group is increased, and the entire lens system tends to be large.

On the other hand, as a method of suppressing the occurrence and fluctuation of chromatic aberration, it has recently been proposed to apply a diffractive optical element (diffractive optical element) having a diffractive effect to an image pickup optical system.

For example, JP-A-4-213421, JP-A-6-32
No. 4262 proposes an optical system using a diffractive optical element.

[0013]

The above-mentioned optical system using a diffractive optical element is an application of a diffractive optical element to a single lens, and although there is a reference to chromatic aberration, fluctuation of chromatic aberration peculiar to a zoom lens due to zooming is mentioned. There is no consideration or description of removal or the like, and no application to a zoom lens is performed.

With respect to application to a zoom lens, US Pat. No. 5,268,790 proposes a zoom lens composed of four lens groups as a whole. This conventional example proposes using a diffractive optical element for the second unit that is the main zooming unit or the third unit that is the correction unit, and the first unit has a conventional lens configuration. In this configuration, the chromatic aberration generated in the first group remains unchanged, and the chromatic aberration increases or fluctuates due to the movement of the second-unit equal-magnification unit during zooming, and is not efficient.

The present invention comprises a total of five lens groups. Of these, by appropriately setting a diffractive optical element in a predetermined lens group, it is possible to reduce astigmatism and distortion and to obtain a high-definition liquid crystal. It is an object of the present invention to provide a telecentric zoom lens having a long back focus and excellent correction of chromatic aberration of magnification corresponding to (1).

[0016]

The zoom lens according to the present invention comprises: (1-1) a first unit having a negative refractive power, a second unit having a positive refractive power, in order from the first conjugate point having a longer distance, The zoom lens has five lens units, a third unit having a negative refractive power, a fourth unit having a positive refractive power, and a fifth unit having a positive refractive power, and performs zooming from the wide-angle end to the telephoto end. And moving the fourth group to the first conjugate point side, wherein at least one of the lens groups has at least one diffractive optical element symmetrical with respect to the optical axis. And

[0017]

FIG. 1, FIG. 4, FIG. 7, FIG. 10, FIG.
3 is a lens sectional view of each of Numerical Examples 1 to 5 of the present invention. In the figure, S is a screen surface, and LCD is a liquid crystal display element as an image to be projected. The screen surface S is on the first conjugate point side with the longer distance. The liquid crystal display element LCD is located at the shorter second conjugate point.

In the drawing, L1 denotes a first group (first lens group) having a negative refractive power, L2 denotes a second group (second lens group) having a positive refractive power, and L3 denotes a third group having a negative refractive power. (Third lens group), L
Reference numeral 4 denotes a fourth group (fourth lens group) having a positive refractive power, and L5 denotes a fifth group (fifth lens group) having a positive refractive power. GB is a glass block such as an infrared cut filter. The liquid crystal display element LCD corresponds to an image plane.

Arrows indicate the trajectories of the movements of the respective lens units when zooming from the wide-angle end to the telephoto end.

In the present invention, at the time of zooming from the wide-angle end to the telephoto end, the second unit L2 and the fourth unit L4 are moved toward the first conjugate point.

At this time, the interval between the second and third units is increased, and the interval between the third and fourth units is reduced. The third lens unit is fixed or moved monotonously to the second conjugate point side or moved while having a convex locus to the first conjugate point side during zooming. The first and fifth units are fixed during zooming.

The focus is performed by moving the first lens unit L1. At least one diffractive optical element rotationally symmetric with respect to the optical axis is provided in at least one of the first to fifth lens groups.

The diffractive optical element according to the present embodiment is manufactured in a binary manner by a lithographic technique which is a technique for producing a holographic optical element (HOE). The diffractive optical element may be manufactured by binary optics (BINARY OPTICS). 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 this 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 ( and adjusts the phase by ^{C1 · h 2 + C2 · h} 4 + ‥‥‥ Ci · h 2i) ‥‥‥ (a) (a) the distance from the optical axis as if light from the equation h. The larger the lens diameter, the greater the effect of higher order coefficients.

Next, numerical examples of the present invention will be described. Figure
Numerical Example 1 of 1 is an example in which a diffractive optical element is introduced into the fixed first lens group during zooming. In zooming from the wide-angle end to the telephoto end, the second unit and the fourth unit move toward the first conjugate point, and the third unit moves monotonously toward the second conjugate point. The fifth group is fixed during zooming.

Numerical embodiment 2 shown in FIG. 4 is also an example in which a diffractive optical element is introduced into the fixed first lens group during zooming. For zooming from the wide-angle end to the telephoto end, the second unit and the fourth unit are the first.
Movement to the conjugate point side, movement of the third unit to the first conjugate point side, and then return to the second conjugate point side (trajectory convex to the first conjugate side)
It is what you do. The fifth group is fixed during zooming.

Numerical embodiment 3 shown in FIG. 7 is an example in which diffractive optical elements are introduced into the fixed first and fifth lens groups during zooming. In the zooming from the wide-angle end to the telephoto end, the second and fourth units move to the first conjugate point side, the third unit moves to the first conjugate point side, and then returns to the second conjugate point side (No. (A locus convex toward one conjugate point). The fifth group is fixed during zooming.

Embodiment 4 of FIG. 10 is an example in which diffractive optical elements are introduced into the first lens group and the second lens group. In the zooming from the wide-angle end to the telephoto end, the second and fourth units move to the first conjugate point side, the third unit moves to the first conjugate point side, and then returns to the second conjugate point side (No. (A locus convex to the side of one large conjugate point). The fifth group is fixed during zooming. The ninth diffractive optical element is one in which a diffractive surface is formed on a flat glass plate.

Numerical Embodiment 5 in FIG. 13 is an example in which diffractive optical elements are introduced into the first lens unit and the fourth lens unit. For zooming from the wide-angle end to the telephoto end, the second unit and the fourth unit are the first.
It moves to the conjugate point side, and the first group, the third group,
The fifth group is fixed.

In the figure, Li represents the ith group (i = 1 to 5).
L1 to L5 are zoom lens units, and the connecting member C
Through the LCD video projector body. Therefore, the liquid crystal display element LCD side after the glass block GB is included in the projector main body.

In each of the numerical embodiments described above, the magnification is changed from the wide-angle end to the telephoto end as indicated by arrows. Further, the first unit is moved on the optical axis to perform focusing.

Further, it is preferable to provide an aspherical surface in the first lens group, whereby the performance can be further improved.

For better correction of aberrations, especially off-axis flare, distortion and chromatic aberration, at least one aspherical lens and at least one optical axis must be included in the first lens unit. It is effective to have a rotationally symmetric diffractive optical element. As described above, as the resolution of the liquid crystal display element has been increased, the resolution which has not been a problem in the past has become a problem, and it has become necessary to satisfactorily correct aberrations largely caused by the problem. By arranging an aspherical surface in the fifth lens group, off-axis flare can be favorably corrected. In addition, since the fourth lens group passes relatively off-axis rays away from the optical axis, arranging the diffractive optical element makes it possible to satisfactorily correct lateral chromatic aberration, and further improve performance. Is improved.

In order to satisfactorily correct aberrations, especially chromatic aberration, it is necessary to include at least one diffraction optical element rotationally symmetric with respect to the optical axis in the fifth lens unit. As described above, chromatic aberration which has not been a problem in the past, particularly chromatic aberration of magnification, can be satisfactorily corrected as the definition of the liquid crystal display element becomes higher.

It should be noted that the zoom lens aimed at by the present invention is achieved by having the above-mentioned components, but it is more preferable that at least one of the following conditions be satisfied.

[0036] (a-1) to the focal length of the entire system at the wide-angle end and the telephoto end fw, and f _T, 0.8 when the focal length of the i group and fi <| f1 / f2 | < 2.3 ‥‥‥ (1)

[0037]

(Equation 2) It is better to satisfy the following conditional expression.

Equations (1) and (2) above are the main variable power groups.
2 The relationship between the lens group and the first lens group is appropriately defined.

If the value deviates from the lower limit of the expression (1), the diameter of the front lens determined by the first lens unit increases, and the distortion at the wide-angle end increases, which is not appropriate. If the upper limit is deviated, it is necessary to increase the amount of movement of the second lens unit in order to obtain a desired zoom ratio.

Equation (2) is to make the power of the main zooming unit appropriate. If the power exceeds the lower limit, the image plane is over-corrected, which is not appropriate. If the value exceeds the upper limit, the desired zoom ratio is obtained in order to obtain the desired zoom ratio.
2 It is necessary to increase the amount of movement of the lens group, and the entire system is large and unsuitable.

(A-2) In the present invention, it is preferable that the following conditional expression is satisfied in order to maintain substantially telecentricity of the entire lens system.

| Tk | / fw> 4 ‥‥‥ (3) where Tk is the distance from the short-distance second conjugate point to the exit pupil of the zoom lens (the panel whose absolute value during zooming is the minimum (image plane) ) To the exit pupil).

The substantially telecentric property referred to here means that the exit pupil is located at a distance in order to eliminate the influence of the light distribution characteristics of the liquid crystal element or the angle dependence of the color combining dichroic mirror when combining a plurality of color lights. Indicates that there is. Specifically, it is a necessary condition to eliminate the angle dependency.

More preferably, the numerical value of the conditional expression (3) should be within the following range.

| Tk | / fw> 9.0 (3a) (A-3) In order to properly correct the distortion, it is preferable that the following expression is satisfied.

1 <| f1 | / fw <2 (4) When the value exceeds the upper limit of this equation, the distortion at the wide-angle end becomes insufficient. When the value exceeds the lower limit, the distortion at the telephoto end becomes impossible.

(A-4) When the diffractive optical element is arranged in the first group, by appropriately selecting the phase of the diffractive optical element, chromatic aberration of magnification occurring in the first group, such as d-line and g-line, The chromatic aberration of magnification at two wavelengths is suppressed to a small value, and the fluctuation of the chromatic aberration of magnification as a whole due to zooming is suppressed to a small value. Moreover, the amount of axial chromatic aberration (secondary spectrum) remaining at the telephoto end does not deteriorate.

(A-5) It is preferable to arrange a diffractive optical element in the second and subsequent lens units. By appropriately selecting the phase of the diffractive optical element, chromatic aberration of magnification occurring after the second lens group, for example, chromatic aberration of magnification of two wavelengths such as d-line and g-line, can be suppressed to be small, and zooming of chromatic aberration of magnification as a whole can be achieved. Fluctuations can be kept small. Moreover, the amount of axial chromatic aberration (secondary spectrum) remaining at the telephoto end does not deteriorate.

(A-6) By configuring so that both the above (A-4) and (A-5) are simultaneously satisfied, a high degree of chromatic aberration correction can be achieved and good optical performance can be maintained. Further miniaturization can be achieved.

(A-7) The surface of the diffractive optical element according to the present invention is represented by the above-mentioned equation (a). In a telecentric zoom lens, efficient aberration correction can be performed by efficiently utilizing the coefficient. To achieve this, the following conditions are preferably satisfied.

Fi · C1 <0 ‥‥‥ (5) where C1 is a coefficient of the diffractive optical element in the first group.
C1 represents a paraxial refractive power of the diffractive optical element, and has a negative refractive power when C1 has a positive value, and has a positive refractive power when C1 has a negative value. If this expression is satisfied, the curvature of the diffractive optical element can be made gentler both in the positive lens group and in the negative lens group, which is effective in correcting aberrations.

(A-8) It is preferable to set the coefficient of the diffractive optical element so as to satisfy the following condition.

Outside ^{<1 × 10 -1 ‥‥‥ (7} ) The condition | [0053] ^{1 × 10 -4 <| C2 /} C1 | <1 ‥‥‥ (6) 1 × 10 -7 <| C3 / C1 In this case, not only is it difficult to correct aberration, but also it is difficult to produce a diffractive optical element, which is not appropriate.

(A-9) In the first group, in the second group, or in the third group
Preferably, a diffractive optical element is arranged in at least one lens group in the group. With this diffractive optical element, the chromatic aberration of magnification occurring in each group can be suppressed small, and the fluctuation due to zooming of the chromatic aberration of magnification due to the movement of the second group can be suppressed small.

(A-10) The diffractive optical element according to the present invention comprises:
Although applied on an optical surface, the base may be a spherical or planar surface, an aspherical surface, or a quadratic surface.

Further, a film of plastic or the like may be attached to those optical surfaces as a surface of the diffractive optical element (so-called replica aspherical surface). According to this, high optical performance can be easily obtained.

(A-11) If the power of the diffractive optical element is increased, the difference in the pitch between the center and the periphery of the sawtooth (lattice shape) becomes large, making fabrication difficult, and the diffraction efficiency of the finished product is not good.

As in the present invention, when the chromatic aberration is corrected by the diffractive optical system in place of the achromatic lens such as bonding of the first group, the second group, or the third group, much less power is required.

Here, power may be provided for correcting some off-axis aberrations, particularly curvature of field and distortion. In that case, set the focal length of the diffractive optical surface of the i-th lens group to Fboi
When the focal length of the i-th lens group is Fi, if the following conditions are satisfied, the manufacture is not difficult, and the correction of aberration including chromatic aberration is excellent.

0.05 <Fi / Fboi <3.0 ‥‥‥ (8) where i = 1 to 5.

(A-12) It is preferable that the following formula is satisfied for the second lens unit which is the main zooming unit. The magnification at the wide-angle end and the telephoto end of the second lens group are respectively β2W and β2T, the magnification change β2t / β2w of the second lens group is Z2, and the change ft / fw of the focal length ft / fw of the entire system is Z. The moving amounts of the second lens unit and the fourth lens unit during zooming are M2 and M, respectively.
Assuming that 4, 0.8 <Z2 / Z <1.1 (9) 0.9 <M2 / M4 <15 (10) 0.4 <M2 / (ft−fw) <1.5 ‥‥‥ (11) Equation (9) appropriately defines the ratio of zooming between the second lens group and the fourth lens group that are zooming groups. The third lens group is preferably in this range in order to reduce the magnification upon zooming.

The expressions (10) and (11) make the length of the entire lens and the amount of movement of each zooming unit appropriate. Especially the second
Since the power of the fourth lens group is apt to be weaker between the lens group and the fourth lens group, this range is preferable in order to appropriately allocate the magnification. In particular, the movement amount of the second lens group is the fourth.
More preferably, the movement amount of the lens group is exceeded (1).
≤ M2 / M4 <1.6).

(A-13) As described above, the power of the second lens group and the fourth lens group tends to be weaker in the fourth lens group, and it is particularly preferable to satisfy the following expression.

0.4 <f2 / f4 <1.5 (12) (12) is an equation necessary for appropriately setting the Petzval sum while appropriately setting the power distribution and zooming of the main zooming group. It is.

(A-14) In order to appropriately set the exit pupil and the distortion of the entire system, it is preferable that the following expression is satisfied.

0.1 <bf / f5 <0.5 ‥‥‥ (13) 0.5 <| f1 | / bf <2.2 ‥‥‥ (14) bf is a display element (LCD) from the fifth lens group. ) And the air-equivalent length excluding the dichroic prism. The expression (13) is necessary to make the entire system appropriately telecentric. Exceeding the upper limit increases the size,
If the lower limit is exceeded, distortion occurs. Equation 14 is also a condition for making the exit pupil longer and making it telecentric while appropriately taking distortion.

(A-15) It is preferable to satisfy the following expression in order to make the movement amount of each group appropriate while making the power arrangement of each group appropriate and to reduce the size.

[0068]

(Equation 3) Distortion is sufficiently suppressed in the first lens unit,
It is necessary to ensure a sufficient back focus.

If the value exceeds the upper limit, the amount of movement for focusing is increased, the overall length is increased, and the back focus is undesirably shortened. Conversely, if the lower limit is exceeded, the amount of movement for focusing is reduced, but it is not preferable because it becomes difficult to correct distortion and the Petzval sum becomes large negatively and the image surface falls.

(A-16) When the focal length of the i-th lens unit is fi, and the focal lengths at the wide-angle end and the telephoto end are fw and ft, respectively.

[0071]

(Equation 4) Is to satisfy.

Equations (16) and (17) show an appropriate power arrangement of the lens group that contributes to zooming. If each of the upper limits is exceeded, the amount of movement for obtaining a desired zoom ratio becomes large, and the entire lens system becomes large, which is not appropriate.
If the lower limit value is exceeded, the amount of movement of each lens unit becomes small, but the fluctuation of aberrations due to zooming, especially the fluctuation of curvature of field increases, which is not appropriate.

Expression (18) is a condition necessary for making the exit pupil longer and telecentric together with expression (13). If the lower limit value is exceeded, distortion will occur in the fifth lens unit even if it is telecentric, which is not appropriate. If the ratio exceeds the upper limit, the size of the entire system becomes large, which is not suitable.

(A-17) In order to reduce the chromatic aberration of magnification during zooming and to suppress the fluctuation, it is preferable that the third lens group includes a lens having an Abbe number ν3 of the following range.

Ν3> 55 ° (19) It is particularly preferable that there is a lens in the range of ν3> 60 ° (19a).

(A-18) The average Abbe number ν1n of the negative lens constituting the first lens group is preferably set to ν1n> 60 ‥‥‥ (20). With this configuration, it is possible to reduce the chromatic aberration and its fluctuation due to zooming.

(A-19) It is preferable to satisfy the following conditions in order to optimize the distance from the lens to the panel while optimally making the system telecentric.

2 <f5 / fw <7 (21) If the value exceeds the lower limit, the optimum telecentricity cannot be satisfied.

(A-20) In general, chromatic aberration opposite to chromatic aberration caused by ordinary lens refraction occurs on the surface of a diffractive optical element. For example, in the case of removing a lens that has been achromatized by a pasted lens surface or the like and reducing the number of lenses, a lens having chromatic aberration sharing opposite to the chromatic aberration sharing generated on the bonded lens surface is used. Preferably, the surface is a diffractive optical element surface. Then, chromatic aberration opposite to the chromatic aberration caused by ordinary refraction occurs on the surface of the diffractive optical element, and the direction becomes the same direction as the chromatic aberration generation direction on the originally bonded lens surface, and the achromaticity of bonding etc. Becomes possible on a single lens.

From the viewpoint of the chromatic aberration coefficient (Note), the axial chromatic aberration coefficient L
It is preferable to arrange a diffractive optical surface on a lens surface having the same sign as the magnification and chromatic aberration coefficient T, and to arrange a diffractive optical surface on a lens surface having the opposite sign on the lens surface on the image plane side from the stop. Coefficient: Kyoritsu Publishing Co., Ltd. “Lens Design Method”
Yoshiya Matsui, p89 ~).

In order to improve the diffraction efficiency, a diffractive optical element having a laminated structure as described below is preferably used.

FIG. 17 shows the wavelength dependence of the first-order diffraction efficiency of the single-layer diffractive optical element shown in FIG. The actual structure of the diffraction grating is such that an ultraviolet-curing resin is applied to the surface of the substrate 102, and the first-order diffraction efficiency is 100% at a wavelength of 530 nm on the resin portion.
A grating 103 having a grating thickness d is formed.

As is apparent from FIG. 17, the diffraction efficiency at the design order decreases as the wavelength goes away from the optimized wavelength of 530 nm, while the 0th-order and second-order diffracted lights near the design order increase. This increase in diffracted light other than the design order causes a flare, which leads to a reduction in the resolution of the optical system.

FIG. 18 is an explanatory view of a laminated diffractive optical element used in the present embodiment. FIG. 19 shows the wavelength dependence of the first-order diffraction efficiency of the diffractive optical element having this configuration. As a specific configuration, a UV curable resin (nd = 1.499,
νd = 54), and a second diffraction grating 105 made of another UV-curable resin (nd = 1.598, νd = 28) is formed thereon. With this combination of materials, the grating thickness d1 of the first diffraction grating part is d1 =
13.8 μm, the grating thickness d2 of the second diffraction grating portion is d2 = 10.5
μm.

As can be seen from FIG. 19, the diffraction efficiency of the design order can be increased over the entire wavelength range by using a diffraction grating having a laminated structure.
It has a high diffraction efficiency of 95% or more.

As described above, the optical performance is improved by using the diffraction grating having the laminated structure as the diffractive optical element of the embodiment of the present invention.

The material of the diffractive optical element is not limited to the ultraviolet curable resin, but other plastic materials can be used. Depending on the base material, the first diffraction grating 10 may be used.
4 may be formed directly on the substrate. In addition, the lattice thicknesses do not necessarily need to be different, and two lattice thicknesses can be made equal by a combination of materials as shown in FIG. In this case, since the grating shape is not formed on the surface of the diffractive optical element, the dust-proof property is excellent, the workability of assembling the diffractive optical element is improved, and a cheaper optical system can be provided.

In this embodiment, chromatic aberration can be reduced by using the diffractive optical element having the above structure, the number of constituent lenses can be reduced, and further miniaturization can be achieved while maintaining good optical performance.

Hereinafter, numerical examples of the present invention will be described.
In the numerical examples, Ri is the i-th order from the first conjugate point side.
The radius of curvature of the lens surface, Di is the i-th lens thickness and air spacing in order from the first conjugate point, and Ni and νi are the refractive index of the glass of the i-th lens in order from the first conjugate point, respectively. Abbe number.

The flat lens closest to the image plane in Numerical Examples 1 to 5 includes a color combining prism, a polarizing filter,
2 shows a glass block such as a color filter. Also, the relation between the above-mentioned conditional expressions and various numerical values in the numerical examples is shown in Table.
It is shown in FIG.

The diffractive optical element surface has a phase φ (h) of φ (h) = 2π / λ (C1 · h ² + C2 · h ⁴ + C3 · h ⁶ + ‥‥ + Ci ·
h ²ⁱ ) where λ: wavelength Ci: coefficient representing phase h: height from optical axis In addition, for example, the display of “ ^eZ ” means “10 ^−Z ”.

[0092]

[Outside 1]

[0093]

[Outside 2]

[0094]

[Outside 3]

[0095]

[Outside 4]

[0096]

[Outside 5]

[0097]

[Outside 1]

[0098]

As described above, according to the present invention, as a whole, there are five lens groups, and astigmatism and distortion can be reduced by appropriately setting a diffractive optical element in a predetermined lens group. A telecentric zoom lens with a long back focus and small chromatic aberration of magnification corresponding to a small and high-definition liquid crystal has been satisfactorily corrected.

In particular, according to the present invention, by employing the above-described configuration, astigmatism and distortion are reduced while securing a large aperture of about F1.8 with a zoom ratio of 1.3 or more, and The chromatic aberration of magnification is well corrected while ensuring sufficient back focus space for optical elements such as prisms for color synthesis and optical elements of various filters.
A telecentric zoom lens having good performance over all object distances can be realized, and a liquid crystal video projector suitable for the zoom lens can be realized.

[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 a sectional view of a lens according to a numerical example 4 of the present invention.

FIG. 11 is an aberration diagram at a wide angle end according to Numerical Example 4 of the present invention.

FIG. 12 is an aberration diagram at a telephoto end in Numerical Example 4 of the present invention.

FIG. 13 is a sectional view of a lens according to a numerical example 5 of the present invention.

FIG. 14 is an aberration diagram at a wide angle end according to Numerical Example 5 of the present invention.

FIG. 15 is an aberration diagram at a telephoto end in Numerical Example 5 of the present invention.

FIG. 16 is a sectional view of a main part of a conventional diffractive optical element.

FIG. 17 is an explanatory diagram of diffraction efficiency of a conventional diffractive optical element.

FIG. 18 is an explanatory view of a diffractive optical element according to the present invention.

FIG. 19 is an explanatory diagram of the diffraction efficiency of the diffractive optical element according to the present invention.

FIG. 20 is an explanatory diagram of a diffractive optical element according to the present invention.

[Explanation of symbols]

 L1 First group L2 Second group L3 Third group L4 Fourth group L5 Fifth group LCD Image plane d d line gg line ΔS Sagittal image plane ΔM Meridional image plane S screen

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H049 AA04 2H087 KA06 LA01 MA12 NA02 PA09 PA19 PA20 PB11 PB12 QA02 QA06 QA14 QA22 QA26 QA32 QA41 QA46 RA01 RA41 RA43 RA46 SA44 SA46 SA50 SA52 SA55 SA63 SA14 SA04 SB04 SB SB22 SB23 SB34 SB42

Claims (8)

[Claims] 1. A first group having a negative refractive power, a second group having a positive refractive power, a third group having a negative refractive power, and a fourth group having a positive refractive power are sequentially arranged from a first conjugate point having a longer distance. Group, and a fifth lens group of a fifth group having a positive refractive power. The zooming from the wide-angle end to the telephoto end is performed by moving the second and fourth units toward the first conjugate point, A zoom lens, wherein at least one of the lens groups has at least one diffractive optical element symmetrical with respect to an optical axis. 2. In zooming from the wide-angle end to the telephoto end, the distance between the second and third units is wider at the telephoto end than at the wide-angle end.
2. The zoom lens according to claim 1, wherein an interval between the third and fourth units is smaller at a telephoto end than at a wide-angle end. 3. The zoom lens according to claim 2, wherein the third unit is fixed at the time of zooming. 4. The zoom lens according to claim 1, wherein said first and fifth units are fixed during zooming. 5. The zoom lens according to claim 1, wherein said first unit is moved on an optical axis to perform focusing. 6. When the focal length of the i-th lens unit is fi and the focal lengths at the wide-angle end and the telephoto end of the entire system are fW and fT, respectively, 1.1 <| f1 / f2 | <2.3. ] The zoom lens according to any one of claims 1 to 5, wherein the following condition is satisfied. 7. The zoom lens according to claim 6, wherein, when a distance from the exit pupil of the entire system to the second conjugate point having a short distance is TK, a condition of 4 <TK / fW is satisfied. 8. The zoom lens according to claim 1, wherein said zoom lens is made of a telecentric system.