JP2000121940A - Zoom lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a zoom lens of a rear focus system of a 4-group type having good optical performance over the entire variable magnification range from a wide angle end to a telephoto lens and the entire part of an object distance from an infinite object to an ultra-close object. SOLUTION: This zoom lens has four lens groups; successively from an object side, a first group L1 of positive refracting power, a second group L2 of negative refracting power, a third group L3 of positive refracting power and a fourth group L4 of positive refracting power and executes variable magnification by moving the second group and the third group and executes focusing by moving the fourth group. In such a case, the first group has a diffraction optical element rotationally symmetrical with the optical axis. The zoom lens satisfies the conditions 3.8<bfw/fw<5.2 when the air computation distance from the final lens face to the image plane at the wide angle end is defined as bfw and the focal length of the entire system at the wide angle end as fw.

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 zoom lens which uses a diffractive optical element as a part of a lens system to satisfactorily correct various aberrations, particularly chromatic aberration, and to use a multi-plate prism, a reflector and the like as a lens. A zoom lens with a large aperture ratio and a high zoom ratio used for photographic cameras, video cameras, and broadcast cameras that have a long back focus that can be placed behind the system. It is.

[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 also been reduced in size.
In particular, the overall length of the lens is reduced, the diameter of the front lens is reduced, and the lens configuration is simplified.

As one means for achieving miniaturization of the entire lens system, a so-called rear focus type zoom lens which performs focusing by moving a lens unit other than the first unit on the object side is known.

In general, a rear focus type zoom lens has a smaller effective diameter of the first lens group than a zoom lens which performs focusing by moving the first lens group, so that the entire lens system can be easily miniaturized, and close-up photographing 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.

[0005] Such a rear focus type zoom lens is disclosed in, 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.

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.

[0008]

Generally, when a rear focus system is adopted in a 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.

[0009] On the other hand, however, there is a problem that aberration fluctuations during focusing become large, 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.

[0011] In particular, a high zoom ratio of 4 or more at a zoom ratio of 10 times or more.
In a zoom lens composed of groups, a cemented lens is often used in order to correct chromatic aberration generated in the first and fourth groups. 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 the fluctuation of 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. For this reason, if the refractive power of the first or fourth group is reduced in the above-described four-group zoom lens, the refractive power of the other lens group must also be reduced accordingly. As the diameter increases, it becomes necessary to increase the lens thickness of the first and fourth units, and the overall length of the lens increases. If the refractive power of the first lens unit is weakened, the back focus at the wide-angle end becomes short, and it becomes difficult to arrange an optical filter, a color separation prism, and the like behind the lens system.

According to the present invention, in a four-unit type zoom lens, by appropriately setting the lens configuration of each lens unit, the zooming lens extends over the entire zoom range from the wide-angle end to the telephoto end, and from an object at infinity to a very close object. It is an object of the present invention to provide a large-aperture-ratio, high-magnification-ratio, long-back-focus zoom lens having excellent optical performance over all object distances up to.

In particular, in a four-group type rear focus zoom lens, a diffractive optical element is introduced into the first group,
By utilizing the diffractive optical effect, the number of lenses in the first group is reduced while reducing the chromatic aberration generated in the first group, the overall length of the lens is reduced, and the weight of the first group is reduced. It is an object of the present invention to provide a rear focus zoom lens having a long back focus and good optical performance over the entire zoom range from the wide-angle end to the telephoto end.

[0016]

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. A zoom lens having four lens groups, a group and a fourth group having a positive refractive power, performing zooming by moving the second and fourth groups, and performing focus by moving the fourth group In the first group, the first group has a diffraction optical element rotationally symmetric with respect to the optical axis, the air operation distance from the final lens surface to the image plane at the wide angle end is bfw, and the focal length of the entire system at the wide angle end is When fw is satisfied, the following condition is satisfied: 3.8 <bfw / fw <5.2 (1)

[0017]

FIG. 1 is a sectional view of a lens at a wide angle end according to a first numerical embodiment of the present invention, and FIGS. 2 to 4 are aberration diagrams at a wide angle end, a middle position, and a telephoto end of the first numerical embodiment of the present invention. is there. FIG. 5 is a sectional view of a lens at a wide angle end according to Numerical Example 2 of the present invention, and FIGS.
FIG. 9 is an aberration diagram at a wide-angle end, a middle position, and a telephoto end in Numerical Example 2 of the present invention.

Next, the features of the lens configuration of Numerical Examples 1 and 2 of FIGS. 1 and 5 will be described. 1 and 5, 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 unit is moved to the image plane side as indicated by the arrow, and the image plane fluctuation due to zooming is changed by projecting the fourth 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 accompanying zooming, and the fourth unit is moved to perform 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, at least one diffractive optical element is provided in the first group, the phase thereof is appropriately set, whereby the chromatic aberration generated in the first group is reduced, and the chromatic aberration over the entire zoom range is favorably reduced. Has been corrected.

In order to reduce the chromatic aberration of the first lens group by using only a refractive surface (lens) without a diffractive optical element, it is necessary to increase the number of lenses or use an extraordinary dispersion glass. The material is generally soft and difficult to process, as represented by, for example, FK01 (trade name).

In particular, in the case of a high-magnification zoom lens that emphasizes image quality, it is highly probable that sufficient correction cannot be performed even with the use of anomalous dispersion glass. In addition, the first group often has a larger lens diameter than the other lens groups. Therefore, if the number of lenses is increased, the weight of the entire lens increases, and the usability deteriorates.

Therefore, in the present invention, chromatic aberration is satisfactorily corrected by using a diffractive optical element in the first group while reducing the number of lenses in the first group. At the wide-angle end, an air calculation distance (distance when a parallel plane plate such as a filter is removed) bfw from the last lens surface to the image surface satisfies the conditional expression (1).

In the case of a video lens that emphasizes image quality, a plurality of image sensors may be used. At this time, a prism for dispersing colors assigned to each image sensor is required. However, when the value goes below the lower limit of the conditional expression (1), the back focus becomes too short, and the space for the prism becomes insufficient. Conversely, when the value exceeds the upper limit of the conditional expression (1), the overall length of the entire lens increases, and as a result, the lens becomes inconvenient.

The diffractive optical element in the present embodiment is binary-manufactured by a lithographic method which is a method of manufacturing 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 ₂ · h ² + C ₄ · h ⁴ + ΔC
_(2i) · i · h ²ⁱ ).

The zoom lens aimed at by the present invention can be attained by satisfying the above-mentioned conditions, but it is more preferable to further satisfy at least one of the following conditions for aberration correction.

(A-1) The focal length of the i-th lens unit is defined as fi (i =
1,2,3,4), the condition 0.31 <f4 / f3 <0.45 (2) is satisfied.

If the refractive power of the third lens unit becomes too strong as the upper limit of conditional expression (2) is exceeded, the distance between the final lens surface and the image surface becomes short, and it becomes impossible to insert an optical member such as a prism. Conversely, if the lower limit of conditional expression (2) is exceeded, if the refractive power of the third lens unit is too weak, the distance between the final lens surface and the image plane will be long, resulting in an increase in the overall length of the entire lens and consequently ease of use. It becomes a bad lens.

(A-2) The diffractive optical element has a positive refractive power.

The first lens unit has a positive refractive power, and the diffractive optical element has a positive refractive power in order to cancel chromatic aberration caused by refraction by the diffractive optical element. If the refractive power of the diffractive optical element is made negative, the chromatic aberration generated by the ordinary refractive optical system will be the same, the achromatic effect of the diffractive optical element will not be obtained, and sufficient chromatic aberration can be corrected throughout the optical system Disappears.

(A-3) When the focal length of the first lens unit is f1 and the focal length of the entire system at the telephoto end is fT,

[0036]

(Equation 2) Satisfying the following conditions.

If the refractive power of the first lens unit is increased as the value goes below the lower limit of conditional expression (3), the chromatic aberration generated by the refractive optical system cannot be sufficiently canceled by the diffractive optical element, and the chromatic aberration can be sufficiently corrected over the entire optical system. Cannot be performed. In addition, it becomes difficult to produce a diffractive optical element. Conversely, if the refractive power of the first lens unit is made weaker as the upper limit of conditional expression (3) is exceeded, the back focus at the wide-angle end becomes too short, and the space for inserting an optical member such as a prism becomes insufficient.

(A-4) The second unit has at least two negative lenses, one positive lens, and a negative lens in order from the object side.

(A-5) The third unit includes, in order from the object side, a meniscus-shaped negative lens and a positive lens whose both lens surfaces are convex.

(A-6) The fourth unit is composed of a positive lens in order from the object side,
That is, the negative lens and the positive lens as a whole have a positive cemented lens.

In the present invention, in order to perform sufficient chromatic aberration correction in the first lens unit, the focal lengths and Abbe numbers of all the lenses in the first lens unit must be f1i and v1i (i = 1, 2).
‥‥), when the coefficient of the second order term of the first group of diffractive optical elements is C ₂₁ | 0.5797 · C ₂₁ + {1 / (f1i · ν1i)} | · f1 <9.8 × 10 ⁻³ (4) It is desirable to satisfy the following condition.

Conditional expression (4) is a condition for combining the achromatizing effect on the refractive optical surface and the diffractive optical surface with respect to the first lens unit and sufficiently correcting chromatic aberration.

In general, the Abbe number (dispersion value) of the refractive optical system is expressed by Nd, NC, and Nd at each wavelength of the d, C, and F lines.
When F is given, it is represented by νd = (Nd−1) / (NF−NC).

On the other hand, the dispersion value νd on the diffractive optical surface is d-line, C
When the wavelengths of the line and the F line are λd, λC, and λF, νd = λd / (λF−λC), and νd = −3.45.

The refractive power ψ of the paraxial first-order diffracted light at the principal wavelength of the diffractive optical surface is given by the following equation representing the phase of the diffractive optical surface, where the coefficient of the second-order term is C ₂ ψ = −2 · C ₂ It is expressed as

The amount corresponding to this because chromatic aberration generated in the one group is proportional to [psi / [nu becomes -2 · C ₂ /(-3.45)=0.5797 · C ₂ is a diffractive optical surface.

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

When the value exceeds the range of the conditional expression (4), the correction of the chromatic aberration generated in the first lens unit becomes insufficient, which is not good.

The diffractive optical element used in this embodiment has a single-layer kinoform-shaped single-layer structure shown in FIG. 9 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. 10 shows the wavelength dependence of the diffraction efficiency of the first-order diffracted light of the diffractive optical element 101 shown in FIG. The actual configuration of the diffractive optical element 101 is such that an ultraviolet-curing resin is applied to the surface of the base material 102, and a layer 10 having a grating thickness d such that the diffraction efficiency of the first-order diffracted light at a wavelength of 530 nm becomes 100% at the resin portion.
3 is formed.

As is apparent from FIG. 10, 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.

FIG. 11 shows the MTF at each angle of view ω with respect to the spatial frequency when Numerical Example 2 is created with the lattice shape of FIG.
Show characteristics.

FIG. 13 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. 12 are laminated.

In FIG. 12, 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 = 13.8 μm, and the lattice thickness d2 of the second layer 105 is d2 = 10.5 μm.

As can be seen from FIG. 13, the diffraction efficiency of the design order can be as high as 95% or more over the entire wavelength range by using a diffractive optical element having a laminated structure.

FIG. 14 shows a numerical example 2 with the lattice shape of FIG.
At each angle of view ω with respect to the spatial frequency when
The F characteristic is shown. When a diffractive optical element having a laminated structure is used, the low-frequency MTF is improved, and desired MTF characteristics can be 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.

Incidentally, 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, the dust-proof property is excellent 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 aspherical 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,

[0061]

(Equation 3) It is represented by the following expression. “D-0X” means “10 ^−X ”.

[0062]

[Outside 1]

[0063]

[Outside 2]

[0064]

[Table 1]

[0065]

As described above, according to the present invention, (a-1) in a four-group type zoom lens, by appropriately setting the lens configuration of each lens group, the entire zoom range from the wide-angle end to the telephoto end is obtained. A zoom lens having a large aperture ratio and a high zoom ratio and a long back focus having excellent optical performance can be achieved over a variable power range and over an entire object distance from an object at infinity to a very close object.

(A-2) In a four-group type rear focus zoom lens, a diffractive optical element is introduced into the first group,
By utilizing the diffractive optical effect, the number of lenses in the first group is reduced while reducing the chromatic aberration generated in the first group, the overall length of the lens is reduced, and the weight of the first group is reduced. It is possible to achieve a rear-focus type zoom lens having a long back focus and good optical performance over the entire zoom range from the wide-angle end to the telephoto end.

[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 intermediate aberration diagram of the numerical example 1 of the present invention.

FIG. 4 is an aberration diagram at a telephoto end in Numerical Example 1 of the present invention;

FIG. 5 is a sectional view of a lens according to a second numerical embodiment of the present invention;

FIG. 6 is an aberration diagram at a wide-angle end according to Numerical Example 2 of the present invention.

FIG. 7 is an intermediate aberration diagram of the numerical example 2 of the present invention.

FIG. 8 is an aberration diagram at a telephoto end in Numerical Example 2 of the present invention.

FIG. 9 is an explanatory view of a diffractive optical element according to the present invention.

FIG. 10 is an explanatory diagram of a wavelength dependence characteristic of the diffractive optical element according to the present invention.

FIG. 11 is an MTF characteristic diagram of the diffractive optical element according to the present invention.

FIG. 12 is an explanatory view of a diffractive optical element according to the present invention.

FIG. 13 is an explanatory diagram of a wavelength dependence characteristic of the diffractive optical element according to the present invention.

FIG. 14 is an MTF characteristic diagram of the diffractive optical element according to the present invention.

FIG. 15 is an explanatory view 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 SP Stop IP image plane ΔM Meridional image plane ΔS Sagittal image plane dd line gg line 101 Diffractive optical element 102 Base 103, 104, 105 layers

 ──────────────────────────────────────────────────続 き Continued on front page F term (reference) 2H087 KA02 KA03 MA15 NA14 PA10 PA19 PB12 QA02 QA07 QA17 QA21 QA25 QA34 QA42 QA45 RA05 RA12 RA32 RA42 RA43 RA46 SA23 SA27 SA29 SA32 SA63 SA65 SA72 SA74 SB04 SB15 SB23 SB34 9A001

Claims (5)

[Claims] 1. 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 having a positive refractive power. And the second group and the fourth group
In a zoom lens that performs zooming by moving a group and focuses by moving a fourth group, the first group has a diffractive optical element that is rotationally symmetric with respect to the optical axis. The air calculation distance from the last lens surface to the image surface is bf
a zoom lens characterized by satisfying a condition of 3.8 <bfw / fw <5.2, where fw is the focal length of the entire system at the wide-angle end. 2. The focal length of the i-th lens unit is fi (i = 1,
2. The zoom lens according to claim 1, wherein, when (2, 3, 4) is satisfied, the following condition is satisfied: 0.31 <f4 / f3 <0.45. 3. The zoom lens according to claim 1, wherein the diffractive optical element has a positive refractive power. 4. When the focal length of the first lens unit is f1 and the focal length of the entire system at the telephoto end is fT, 4. The zoom lens according to claim 1, wherein the following condition is satisfied. 5. The zoom lens according to claim 1, wherein the diffractive optical element has a one-layer structure or a two-layer structure made of materials having different dispersions.