JPH1152244A - Small-sized zoom lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small-sized zoom lens having excellent optical performance over a whole variable power range from a wide-angle end to a telescopic end. SOLUTION: In order from an object side, a first lens group L1 of a positive refractive power, a second lens group L2 of a negative refractive power, a third lens group L3 of a positive refractive power and a fourth lens group L4 of a positive refractive power are arranged. Then, the first lens group L1 is composed of two lenses of a negative meniscus lens and a bi-convex lens, by making it so as to have a diffraction optical surface to the lens surface on the side of the image plane of the negative meniscus lens and properly setting the phase, a chromatic aberration generated in the first lens group L1 is reduced and the chromatic aberration over a whole variable power region is excellently compensated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high zoom ratio and a large aperture ratio of about 10 or more and an F number of about 1.8 or more, which are particularly suitable for video cameras, electronic still cameras, and silver halide photography cameras. The present invention relates to a zoom lens having miniaturization while maintaining good optical performance.

[0002]

2. Description of the Related Art Recently, as home video cameras and the like have become smaller and lighter, remarkable progress has been made in miniaturization of zoom lenses for imaging. Emphasis is placed on simplification.

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 lens 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 that performs focusing by moving the first lens group, so that the entire lens system can be easily reduced in size, and Since it is easy to take a picture, particularly a very close-up picture, and because the small and light lens group is moved, there is an advantage that the driving force of the lens group is small and quick focusing can be performed.

[0004] As such a rear focus type zoom lens, for example, Japanese Patent Application Laid-Open Nos. 62-24213 and 63-247316 disclose a first lens group having a positive refractive power and a negative lens in order from the object side. The zoom lens has four lens groups, a second lens group having a refractive power, a third lens group having a positive refractive power, and a fourth lens group having a positive refractive power. The four lens groups are moved to perform image plane fluctuation and focusing due to zooming.

In Japanese Patent Application Laid-Open Nos. Hei 2-39011 and Hei 6-18782, the first lens group of a zoom lens having a zoom ratio of 6 to 8 is made up of two positive and negative lenses. Thus, the number of lenses is reduced.

On the other hand, as a method for suppressing the occurrence of chromatic aberration, in recent years, proposals have been made to apply a diffractive optical element to an imaging optical system. For example, in JP-A-4-218421 and JP-A-6-324262, chromatic aberration is reduced by applying a diffractive optical element to a single lens.

Also, US Pat. No. 5,268,790 proposes to use a diffractive optical element for the second lens group or the eighth lens group of the zoom lens, which contributes to a reduction in the number of lenses and a reduction in size compared to the conventional example. However, reduction of the number of lenses and miniaturization have not been sufficiently achieved.

[0008]

Generally, if the number of lenses is reduced while increasing the refracting power in order to reduce the size of the optical system, the lens thickness increases and the effect becomes insufficient.

In particular, in a zoom lens having a high zoom ratio with a zoom ratio of 10 times or more, if the chromatic aberration generated in the first lens unit is not corrected to some extent, it is impossible to correct the fluctuation of the chromatic aberration due to the zooming. Have difficulty. Therefore, if the number of lenses is simply reduced by using an aspherical surface, the refractive power of the positive lens becomes too strong, resulting in a shape that cannot be realized, and eventually it becomes necessary to reduce the refractive power of the variable power unit. No reduction in the overall length of the lens is achieved.

An object of the present invention is to introduce a diffractive optical surface into the first lens group, reduce the chromatic aberration generated in the first lens group by combining the diffractive optical function and the achromatic effect of the refraction system, By reducing the number of lenses while maintaining the refractive power of the zoom section, the overall length of the lens can be reduced, and a compact zoom with good optical performance over the entire zoom range from the wide-angle end to the telephoto end. It is to provide a lens.

[0011]

According to the present invention, there is provided a small-sized zoom lens comprising: a first lens unit having a positive refractive power and a second lens having a negative refractive power in order from the object side. A lens group, a third lens group having a positive refractive power, and a fourth lens group having a positive refractive power. At least the second lens group and the fourth lens group are moved to perform zooming and perform focusing. The fourth lens group is moved, and the first lens group is composed of two lenses, one each for positive and negative, and has at least one diffractive optical surface rotationally symmetric with respect to the optical axis. It is characterized by.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail with reference to the illustrated embodiment. FIG. 1 is a sectional view of a lens according to a first embodiment of the present invention. The first lens unit L having a positive refractive power is arranged in order from the object side.
1, the second lens unit L2 having a negative refractive power, and the third lens unit L2 having a positive refractive power
A lens unit L3 and a fourth lens unit L4 having a positive refractive power are arranged. Upon zooming from the wide-angle end to the telephoto end, at least the second lens unit L2 is moved to the image plane side,
The image plane fluctuation due to zooming is corrected by moving the fourth lens unit L4 as shown by the arrow.

Also, a rear-focusing type for performing focusing by moving the fourth lens unit L4 on the optical axis is adopted, and the solid line locus 4a and the dotted line locus 4a of the fourth lens unit L4 are adopted.
b indicates a movement locus for correcting an image plane variation accompanying zooming from the wide-angle end to the telephoto end when focusing on an object at infinity and an object at a short distance.

In this embodiment, the first lens unit L1 and the third lens unit L3 are fixed at the time of zooming and focusing, but at least the second lens unit L2 has at least a variable zoom ratio. It is also possible to move the first lens unit L1.

In this embodiment, the fourth lens unit L4 is moved to correct the image plane fluctuation caused by zooming, and the fourth lens unit L4 is moved for focusing. In particular, at the time of zooming from the wide-angle end to the telephoto end as shown by the curves 4a and 4b, the lens is moved so as to have a convex locus toward the object side. Thereby, the third lens unit L
By effectively utilizing the space between the third and fourth lens units L4, the overall length of the lens is effectively reduced.

In the first embodiment, the first lens unit L1 is composed of two lenses, a negative meniscus lens and a biconvex lens, and has at least one surface, specifically, a diffractive optical surface closest to the image plane. By properly setting the phase, chromatic aberration generated in the first lens unit L1 is reduced, and chromatic aberration is favorably corrected over the entire zoom range.

Even if the first lens unit L1 comprises only a positive lens and is provided with a diffractive optical surface, chromatic aberration can be suppressed when chromatic aberration of only two wavelengths, such as d-line and g-line, is considered. However, since the diffractive optical surface has anomalous dispersion, the chromatic aberration for other wavelengths, that is, the so-called secondary spectrum becomes large, especially at the telephoto end, and the chromatic aberration cannot be corrected within the entire visible wavelength range. .

If it is attempted to correct chromatic aberration only by using a refracting surface without using a diffractive optical surface, the refractive power of the positive lens and the negative lens increases due to achromatism. It is difficult to configure the first lens unit L1 with a small number of lenses, such as two positive and negative single lenses.

In order for the achromatizing effect of the first lens unit L1 to be shared by the diffractive optical surface, it is desirable that the diffractive optical surface has a positive refractive power. If the refractive power of the diffractive optical surface becomes negative, the chromatic aberration generated by the ordinary refractive optical system becomes the same, the achromatic effect by the diffractive optical surface does not occur, and it is not possible to sufficiently correct the chromatic aberration over the entire optical system .

In order to reduce the number of lenses while correcting the chromatic aberration generated in the first lens unit L1, it is necessary to set the focal lengths of the entire first lens unit L1 and its negative lens to f1 and f1n, respectively. It is preferable to satisfy the following conditional expressions.

2.2 <| f1n / f1 | <4.5 (1)

When the refractive power of the negative lens becomes too strong beyond the lower limit of the conditional expression (1), the refractive power of the positive lens also becomes strong.
The curvature becomes large, and it becomes difficult to form the first lens unit L1 into two lenses. Conversely, if the refractive power of the negative lens becomes too weak, the achromatizing effect on the diffractive optical surface becomes too large, and the optical performance is deteriorated due to the secondary spectrum, which is not preferable.

In order to sufficiently correct spherical aberration, coma, distortion, and other aberrations throughout the zooming range, it is necessary to correct spherical aberration, coma, distortion, and the like generated in the first lens unit L1. For this purpose, it is preferable to provide at least one aspheric surface in the first lens unit L1.

In this aspherical shape, X is in the optical axis direction, H is in the direction perpendicular to the optical axis, the traveling direction of light is positive, R is the paraxial radius of curvature, and A, B, C, D, and E are aspherical, respectively. When the spherical coefficient is used, it is expressed by the following equation.

X = (H ² / R) / [1+ ｛1- (1 + K) (H / R) ² } ^1/2 ] + BH ⁴ + CH ⁶ + DH ⁸ + EH ¹⁰ (2)

In the first embodiment, by providing an aspherical surface on the lens surface closest to the object in the first lens unit L1, spherical aberration and coma occurring particularly at the telephoto end are favorably corrected. The shape of the aspherical surface of the first lens unit L1 whose positive refractive power decreases as it goes to the periphery is effective for correcting spherical aberration and coma.

It is preferable that the second lens unit L2 is composed of two negative lenses and one positive lens so that chromatic aberration and other various aberrations generated in the second lens unit L2 can be satisfactorily corrected. Further, the second lens unit L2 includes a negative meniscus lens having a strong concave surface facing the image side in order from the object side, a biconcave lens,
It is desirable to use a positive lens with a strong convex surface facing the object side.

The most object side lens surface of the third lens unit L3 and the most object side lens surface of the fourth lens unit L4 are aspherical.

The lens surface closest to the image in the first lens unit L1 is a diffractive optical surface.

FIGS. 2 and 3 are sectional views of lenses of Examples 2 and 3, respectively. In the second and third embodiments, the second lens unit L2 includes two negative lenses. The aspheric surfaces in the second and third embodiments are different from those in the first embodiment in that the image-side lens surface of the object-side lens of the second lens unit L2 is aspheric.

A diffractive optical surface is provided on the lens surface closest to the object side of the first lens unit L1 and the object side lens surface of the lens on the image plane side of the second lens unit L2. Is corrected to reduce the occurrence of chromatic aberration over the entire zoom range. Also, the second lens unit L2 is set to 2
Since it can be composed of a small number of lenses, for example, the total length of the lens can be further reduced.

It is effective that the diffractive optical surface of the second lens unit L2 has a negative refractive power in that chromatic aberration is corrected in the second lens unit L2. In particular, when the diffractive optical surface is provided only in the second lens unit L2, chromatic aberration can be corrected only for two wavelengths, but it becomes difficult to correct the secondary spectrum.

On the other hand, as described above, by providing the first lens unit L1 as a two-lens structure of a positive lens and a negative lens and providing a diffractive optical surface, the influence of the secondary spectrum of the color generated in the second lens unit L2 is obtained. Can be canceled out, and chromatic aberration can be favorably corrected in the entire zoom range and in the entire visible wavelength range.

In order to correct distortion and astigmatism generated in the second lens unit L2 and to suppress the fluctuation of these aberrations due to zooming in a lens having the same configuration as that of the second and third embodiments, The two-lens unit L2 is preferably composed of a negative meniscus lens having a strong concave surface facing the image surface side and a negative meniscus lens having a strong concave surface facing the object side in order from the object side.

Further, the object side surface of the image-side negative lens in the second lens unit L2 and the image-side radius of curvature (in the case of an aspherical surface, the reference spherical surface determined by the on-axis and effective diameter) are Ra,
When Rb is satisfied, it is preferable that the following conditional expression is satisfied.

1 <(Rb + Ra) / (Rb−Ra) <2 (3)

If the lower limit of conditional expression (3) is exceeded, the distortion generated at the wide-angle end becomes too negative, whereas if the upper limit is exceeded, the distortion at the telephoto end cannot be corrected completely.

If at least one aspherical surface is provided in the second lens unit L2 separately from the diffractive optical surface as in Embodiments 2 and 3, or if the base surface of the diffractive optical surface is made aspherical, Further, the optical performance is improved.

In Examples 1 to 3, when the phase of the diffractive optical surface is φ (h), λ is the reference wavelength (d-line), and h is the distance from the optical axis, it is expressed by the following equation.

Φ (h) = 2π / λ (C ₂ · h ² + C ₄ · h ⁴ + ·· + C _{2 · i} · h ^{2 · i} ) (4)

In order for the first lens unit L1 to perform sufficient chromatic aberration correction, the focal lengths and Abbe numbers of the two lenses of the first lens unit L1 must be f1i, v1i (i = 1, 2), the coefficients of the second order term of the diffractive optical surface of the first lens group L1 when the C _2, it is preferable that the following conditional expression is satisfied:.

| 0.5797 · C ₂ + Σ (1 / (f1i · ν1i) | · f1 <0.02 (5)

Conditional expression (5) is a condition for the first lens unit L1 in which the achromatic effect on the refracting optical surface and the diffractive optical surface is combined to sufficiently correct chromatic aberration. Conditional expression (5)
Exceeds the range, it is not preferable because correction of chromatic aberration generated in the first lens unit L1 becomes insufficient.

In general, the Abbe number (dispersion value) of a refractive optical system
Represents the refractive power at each wavelength of the d, C, and F lines as Nd, N
When C and NF are set, they are expressed by the following equations.

Νd = (Nd-1) / (NF-NC)

On the other hand, the dispersion value ν due to d-line on the diffractive optical surface
d is represented by νd = λd / (λF-λC), where λd, λC, and λF are d-line, C-line, and F-line wavelengths, respectively.
= −3.45.

The refractive power 近 of the paraxial first-order diffracted light at the main wavelength of the diffractive optical surface is given by the following formula representing the phase of the diffractive optical surface, where the coefficient of the second-order term is C ₂ ψ = −2 - represented by C _2.

Since the chromatic aberration generated in a certain lens group is proportional to ψ / ν, the amount corresponding to this is as follows on the diffractive optical surface.

−2 · C ₂ /(−3.45)=0.5797·C ₂ (6)

In a refractive optical system, this amount is Σ1 / (f
· Ν). Therefore, it can be seen that the closer the sum is to 0, the more the chromatic aberration of the lens group is sufficiently corrected.

In the embodiment, in order to reduce the amount of movement of the second lens unit L2 and to shorten the overall length of the zoom unit, the focal lengths of the second lens unit L2, the wide-angle end and the telephoto end of the entire system must be respectively set. When f2, fw, and ft are satisfied, it is preferable to satisfy the following conditional expressions.

0.25 <| f 2 / (fw · ft) ^1/2 | <0.35 (7)

Conditional expression (7) relates to the refractive power of the second lens unit L2, and is intended to effectively obtain a predetermined zoom ratio while reducing aberration fluctuations caused by zooming. If the refractive power of the second lens unit L2 exceeds the lower limit and becomes too strong, it is advantageous for miniaturization. However, the Betzwal sum increases in the negative direction, the field curvature increases, and aberrations associated with zooming increase. It is not good because the fluctuation becomes too large. Conversely, if the value exceeds the upper limit, the amount of movement of the second lens unit L2 becomes too large, and the overall length of the lens becomes longer.

Next, Numerical Embodiments 1 to 3 of Embodiments 1 to 3 will be described. In these 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 or air gap, and ni and νi are the refractive indexes of the glass of the i-th lens. Abbe number. The seventeenth and eighteenth surfaces having no refracting power closest to the image plane in the numerical examples represent optical filters, face plates, and the like.

Numerical Example 1 f = 4.19000 to 41.78 fno = 1: 1.85 to 2.37 2ω = 59.6 ° to 6.6 ° r1 = 14.921 (aspherical surface) d1 = 0.70 n1 = 1.84666 ν1 = 23.8 r2 = 11.300 d2 = 4.90 n2 = 1.69680 ν2 = 55.5 r3 = -263.343 (diffraction surface) d3 = variable r4 = 11.229 d4 = 0.50 n3 = 1.83481 ν3 = 42.7 r5 = 3.919 d5 = 2.44 r6 = -5.438 d6 = 0.50 n4 = 1.67003 ν4 = 47.3 r7 = 5.605 d7 = 1.80 n5 = 1.84666 ν5 = 23.8 r8 = 350.015 d8 = variable r9 = 0.000 (aperture) d9 = 1.00 r10 = 5.130 (aspheric) d10 = 3.02 n6 = 1.58313 ν6 = 59.4 r11 = -2039.298 d11 = 0.08 r12 = 7.029 d12 = 0.55 n7 = 1.84666 ν7 = 23.8 r13 = 4.337 d13 = Variable r14 = 8.962 (aspheric surface) d14 = 2.09 n8 = 1.58313 ν8 = 59.4 r15 = -9.351 d15 = 0.50 n9 = 1.84666 ν9 = 23.8 r16 = -15.459 d16 = 0.75 r17 = ∞ d17 = 3.27 n10 = 1.51633 ν10 = 64.2 r18 =

Aspherical surface coefficient 1 surface K = -5.34084 · 10 ⁻¹ B = 6.74185 · 10 ^-6 C = −9.64841 · 10 ^-9 D = 2.43360 · 10 ^-10 E = 0.00000 · 10 ⁰ 10 surface K = −1.31216・ 10 ⁰ B = 4.96251 ・ 10 ^-4 C = -2.61862 ・ 10 ^-7 D = 3.09567 ・ 10 ^-8 E = 1.87661 ・ 10 ^-9 14 surface K = -1.97016 ・ 10 ^-1 B = -2.58615 ・ 10 ^-4 C = 2.43614 ・ 10 ^-7 D = 4.02213 ・ 10 ^-7 E = -1.92905 ・ 10 ^-8

Phase coefficient 3 plane C ₂ = -6.49067 · 10 ^-4 C ₄ = 3.43622 · 10 ^-6

The focal length 4.19 4.07 41.78 d3 0.52 7.32 11.66 d8 11.95 5.15 0.80 d13 5.45 2.36 5.37

Numerical Example 2 f = 4.19000 to 41.75 fno = 1: 1.85 to 2.45 2ω = 59.6 ° to 6.6 ° r1 = 16.059 (aspherical surface) d1 = 0.70 n1 = 1.84666 ν1 = 23.8 r2 = 12.399 d2 = 0.10 r3 = 12.343 d3 = 4.60 n2 = 1.69680 ν2 = 55.5 r4 = -115.928 (diffractive surface) d4 = variable r5 = 20.044 d5 = 0.50 n3 = 1.83481 ν3 = 42.7 r6 = 4.936 (aspheric surface) d6 = 2.20 r7 = -6.451 (diffractive surface) ) D7 = 0.70 n4 = 1.60311 ν4 = 60.7 r8 = -26.325 d8 = variable r9 = 0.000 (aperture) d9 = 1.00 r10 = 4.702 (aspherical surface) d10 = 2.70 n5 = 1.58313 ν5 = 59.4 r11 = -254.183 d11 = 0.08 r12 = 7.971 d12 = 0.55 n6 = 1.84666 ν6 = 23.8 r13 = 4.274 d13 = variable r14 = 7.324 (aspheric) d14 = 2.40 n7 = 1.48749 ν7 = 70.2 r15 = -7.852 d15 = 0.50 n8 = 1.84666 ν8 = 23.8 r16 = -9.884 d16 = 0.75 r17 = ∞ d17 = 3.27 n9 = 1.51633 ν9 = 64.2 r18 =

Aspherical surface coefficient 1 surface K = -7.63654 · 10^-1 B = 8.96265 / 10^-6 C = 3.99567 · 10^-9 D = 0.00000 ・ 10⁰ E = 0.00000 ・ 10⁰ 6 sides K = -1.93285 ・ 10^-1 B = 1.97008 ・ 10^-Four C = 1.70009 ・ 10^-Five D = 0.00000 ・ 10⁰ E = 0.00000 ・ 10⁰ 10 K = -1.23635 ・ 10⁰ B = 5.72033 ・ 10^-Four C = 4.16392 / 10^-6 D = -4.05099 ・ 10^-7 E = 1.41200 ・ 10^-8  14 K = -2.64097 ・ 10⁰ B = -5.22539 ・ 10^-Five C = 2.27981 · 10^-Five D = -1.19783 ・ 10^-6 E = 2.91817.10^-8

Phase coefficient 4 plane C ₂ = -1.13061 ・ 10 ^-3 C ₄ = 6.25966 ・ 10 ^-6 7 plane C ₂ = -8.67646 ・ 10 ^-3 C ₄ = -1.66169 ・ 10 ^-4

[0062] Focal length 4.19 13.50 41.75 d4 0.50 7.32 11.68 d8 12.00 5.18 0.82 d13 5.67 3.44 7.53

Numerical Example 3 f = 4.19000 to 41.75 fno = 1: 1.85 to 2.44 2ω = 59.6 ° to 6.6 ° r1 = 15.874 d1 = 0.70 n1 = 1.84666 ν1 = 23.8 r2 = 12.311 d2 = 0.10 r3 = 12.308 d3 = 4.60 n2 = 1.69680 ν2 = 55.5 r4 = -110.908 (diffractive surface) d4 = variable r5 = 17.955 d5 = 0.50 n3 = 1.83481 ν3 = 42.7 r6 = 4.856 (aspheric surface) d6 = 2.20 r7 = -6.316 (diffractive surface) d7 = 0.70 n4 = 1.60311 ν4 = 60.7 r8 = -31.296 d8 = variable r9 = 0.000 (aperture) d9 = 1.00 r10 = 4.743 (aspheric surface) d10 = 2.70 n5 = 1.58313 ν5 = 59.4 r11 = 516.605 d11 = 0.08 r12 = 7.272 d12 = 0.55 n6 = 1.84666 ν6 = 23.8 r13 = 4.165 d13 = variable r14 = 7.478 (aspheric surface) d14 = 2.40 n7 = 1.48749 ν7 = 70.2 r15 = -7.398 d15 = 0.50 n8 = 1.84666 ν8 = 23.8 r16 = -9.458 d16 = 0.75 r17 = ∞ d17 = 3.27 n9 = 1.51633 ν9 = 64.2 r18 =

Aspherical surface coefficient 4 surface K = -1.97516 · 10^Two B = 1.81711 ・ 10^-Five C = -7.53338.10^-9 D = -1.04227 ・ 10^-Ten E = 0.00000 ・ 10⁰ 6 K = -7.82659 ・ 10^-1 B = 6.74067 / 10^-Four C = 5.62605 / 10^-Five D = 0.00000 ・ 10⁰ E = 0.00000 ・ 10⁰ 10 K = -1.29090 ・ 10⁰ B = 6.39965 / 10^-Four C = 3.53800 ・ 10^-6 D = -1.82729 ・ 10^-8 E = 1.82729 ・ 10^-8  14 K = -3.70606 / 10⁰ B = 2.46517 / 10^-Four C = 1.39476 / 10^-Five D = -1.20129 ・ 10^-6 E = -1.76827 ・ 10^-9

Phase coefficient 4 plane C ₂ = -1.10208 ・ 10 ^-3 C ₄ = 4.86972 ・ 10 ^-6 7 plane C ₂ = 8.20287 ・ 10 ^-3 C ₄ = -2.26796 ・ 10 ^-4

Focal length 4.19 13.60 41.75 d4 0.46 7.20 11.51 d8 11.87 5.13 0.82 d13 5.52 3.24 7.36

The following table shows the above-mentioned conditional expressions (1), (3),
The relationship between (5) and (7) and Numerical Examples 1 to 3 is shown.

Numerical Example 1 Numerical Example 2 Numerical Example 3 Conditional expression (1) 2.844 3.410 3.479 Conditional expression (3) 1.649 1.506 Conditional expression (5) 9.64 · 10 ^-3 1.062 · 10 ^-2 1.078 · 10 ^-2 Conditional expression (7) 0.296 0.323 0.318

FIGS. 4 to 12 show the wide-angle state of the first to third embodiments.
It is an aberration figure of an intermediate state and a telephoto state.

The diffractive optical surface is formed by a lithographic technique which is a technique for manufacturing a holographic optical element (HOE).
The optical element may be manufactured by a binary optics (BINARY OPTICS) which is an optical element manufactured in a value manner. In this case, in order to further increase the diffraction efficiency, a saw-like shape called a kinoform as shown in FIG. 12 can be used. Further, it may be manufactured by molding using a mold manufactured by these methods.

FIG. 13 is a cross-sectional view of the diffractive optical element, in which an ultraviolet curable resin is applied to the surface of the substrate 1, and the resin portion 2 has a grating thickness d such that the primary diffraction efficiency becomes 100% at a wavelength of 530 nm. Is formed. FIG. 14 shows the wavelength dependence of the first-order diffraction efficiency of the diffractive optical element. The diffraction efficiency at the design order decreases as the distance from the optimized wavelength 530 nm increases, while the 0th-order and second-order diffracted light near the design order. Is increasing. This increase in diffracted light other than the design order causes a flare, which leads to a decrease in the resolution of the optical system.

FIG. 15 shows an MTF (Modulation Transfer Functi) with respect to the spatial frequency on the axis at the telephoto end in the case where the above-mentioned numerical example 1 is prepared for the lattice shape of FIG.
on) characteristic, indicating that the MTF in the low frequency region is lower than a desired value.

In order to further improve the diffraction efficiency, it is preferable to use a diffractive optical element having a laminated structure as described below. Therefore, it is conceivable to use a stacked diffraction grating as shown in FIG. 16 as the grating shape of the diffractive optical element in the embodiment. An ultraviolet curable resin (Nd = 1.4) is provided on the base material 1.
99, νd = 54) is formed, on which another ultraviolet curable resin (nd = 1.598,
νd = 28) is formed.

In this combination of materials, the grating thickness dl of the first diffraction grating 4 is dl = 13.8 μm, and the grating thickness d2 of the second diffraction grating 5 is d2 = 10.5 μm. FIG. 17 shows the wavelength dependence of the first-order diffraction efficiency of the diffractive optical element having this configuration. By using a diffraction grating having a laminated structure as described above, the diffraction efficiency of the design order can be as high as 95% or more over the entire used wavelength range. The impeachment rate is obtained.

FIG. 18 shows M with respect to the spatial frequency in this case.
By exhibiting TF characteristics and using a diffraction grating with a laminated structure,
The low-frequency MTF is improved, and a desired MTF characteristic is obtained. As described above, by using the diffraction grating having the laminated structure as the diffractive optical element of the embodiment, the optical performance is further improved.

The material of the above-mentioned diffractive optical element is not limited to an ultraviolet curable resin, but other plastic materials can be used. Depending on the base material, the first diffraction grating 4 may be formed directly on the base material. May be. Also, the thicknesses of the gratings do not necessarily have to be different, and depending on the combination of materials, the two gratings can have the same thickness 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 dust resistance, the workability of assembling the diffractive optical element is improved, and a less expensive optical system can be obtained.

[0077]

According to the compact zoom lens of the present invention, the first lens group is composed of two positive and negative lenses, and at least one lens rotationally symmetric with respect to the optical axis. With the configuration having the diffractive optical surface, the number of lenses can be reduced, and the overall length of the lens can be reduced while maintaining good optical performance.

[Brief description of the drawings]

FIG. 1 is a sectional view of a lens according to a first embodiment.

FIG. 2 is a sectional view of a lens according to a second embodiment.

FIG. 3 is a sectional view of a lens according to a third embodiment.

FIG. 4 is an aberration diagram of a wide-angle state according to the first embodiment.

FIG. 5 is an aberration diagram of the intermediate state of the first embodiment.

FIG. 6 is an aberration diagram of a telephoto state according to the first embodiment.

FIG. 7 is an aberration diagram of a wide-angle state according to the second embodiment.

FIG. 8 is an aberration diagram of the intermediate state of the second embodiment.

FIG. 9 is an aberration diagram of a telephoto state according to the second embodiment.

FIG. 10 is an aberration diagram of a wide-angle state according to the third embodiment.

FIG. 11 is an aberration diagram of the intermediate state of the third embodiment.

FIG. 12 is an aberration diagram of a telephoto state according to the third embodiment.

FIG. 13 is a sectional view of a diffractive optical element.

FIG. 14 is a graph of a first-order diffraction efficiency wavelength characteristic.

FIG. 15 is a graph showing MTF characteristics.

FIG. 16 is a cross-sectional view of a diffractive optical element having a multilayer structure.

FIG. 17 is a graph of a first-order diffraction efficiency wavelength characteristic multilayer structure.

FIG. 18 is a graph showing MTF characteristics.

FIG. 19 is a sectional view of another diffractive optical element having a multilayer structure.

[Explanation of symbols]

 L1 First lens group L2 Second lens group L3 Third lens group L4 Fourth lens group 1 Base material 2 Resin section 3, 4, 5 Diffraction grating ΔM Meridional image plane ΔS Sagittal image plane

Claims (8)

[Claims] 1. A first lens having a positive refractive power in order from the object side.
A lens group, a second lens group having a negative refractive power, a third lens group having a positive refractive power, a fourth lens group having a positive refractive power, and at least the second lens group and the fourth lens group Is moved to change the magnification, and focusing is performed by moving the fourth lens group. The first lens group is composed of two lenses, one each for positive and negative, and A zoom lens having at least one rotationally symmetric diffractive optical surface. 2. The zoom lens according to claim 1, wherein the diffractive optical surface of the first lens group has a positive refractive power. 3. The conditional expression of 2.2 <| f1n / f1 | <4.5 is satisfied, where f1 and f1n are the focal lengths of the entire first lens unit and its negative lens, respectively. A zoom lens according to claim 1. 4. The zoom lens according to claim 1, wherein the first lens group has at least one aspheric surface other than the diffractive optical surface. 5. The zoom lens according to claim 1, wherein the second lens group includes at least two negative lenses and one positive lens. 6. The zoom lens according to claim 1, wherein the second lens group includes only at least two or more negative lenses. 7. The zoom lens according to claim 5, wherein the second lens group has at least one diffractive optical surface. 8. The compact zoom lens according to claim 1, wherein the diffractive optical surface is a laminated diffraction grating formed by laminating two diffraction gratings on a substrate glass.