JP4266414B2 - Zoom lens - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a zoom lens, and in particular, corrects various aberrations, particularly chromatic aberration, by using a diffractive optical element as a part of the lens system, and can arrange a multi-plate prism, a reflector, and the like behind the lens system. The present invention relates to a zoom lens for reducing the size of a lens system having a large aperture ratio and a high zoom ratio, which is used for a photographic camera, a video camera, a broadcasting camera, and the like having a long back focus.
[0002]
[Prior art]
Recently, as a home video camera or the like is reduced in size and weight, a zoom lens for imaging is also reduced in size. In particular, the total lens length is shortened, the front lens diameter is reduced, and the lens configuration is simplified.
[0003]
As one means for achieving downsizing of the entire lens system, a so-called rear focus type zoom lens that performs focusing by moving a lens group other than the first group on the object side is known.
[0004]
In general, a rear focus type zoom lens has a smaller effective diameter of the first lens unit than a zoom lens that focuses by moving the first lens unit, which makes it easy to reduce the size of the entire lens system. Close-up photography is facilitated, and the relatively small and light lens group is moved, so that the lens group has a small driving force and can be focused quickly.
[0005]
As such a rear focus type zoom lens, for example, JP-A-62-215225, JP-A-62-206516, JP-A-62-24213, JP-A-63-247316, and In Japanese Patent Laid-Open No. 4-43311, the first group of positive refractive power, the second group of negative refractive power, the third group of positive refractive power, and the fourth group of positive refractive power are arranged in order from the object side. A four-group type rear focus type zoom lens having four lens groups, moving the second group to perform zooming, and moving the fourth group to perform image plane fluctuation and focusing accompanying zooming is provided. Proposed.
[0006]
On the other hand, in many zoom lenses, by providing an aspheric surface in the lens system, various aberrations are corrected favorably, and the entire lens system is reduced in size and high optical performance is obtained.
[0007]
In addition to a method of correcting chromatic aberration by combining glass materials having different dispersions among various aberrations, 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 disclosed in, for example, No. 4-213421, JP-A-6-324262, US Pat. No. 5,268,790, and the like. Among these, US Pat. No. 5,268,790 proposes a zoom lens using diffractive optical elements in the second group and the third group.
[0008]
[Problems to be solved by the invention]
In general, when a rear focus method is used in a zoom lens, the entire lens system can be miniaturized, quick focusing can be performed, and close-up photography can be facilitated.
[0009]
On the other hand, however, the variation in aberration during focusing becomes large, and it becomes very difficult to obtain high optical performance over the entire object distance from an object at infinity to a near object.
[0010]
For example, in a zoom lens with a large aperture ratio and high zoom ratio, the variation in 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. Will arise.
[0011]
In particular, in a zoom lens composed of four groups with a high zoom ratio of 10 times or more, a bonded lens is often used in order to correct chromatic aberration occurring in the first group and the fourth group. A method of reducing the total length of the lens by reducing the number of lenses in the lens group by using an aspherical surface for the lens group is employed.
[0012]
However, if the number of lenses is reduced, the elements for correcting chromatic aberration become insufficient, and it becomes difficult to satisfactorily correct variations in chromatic aberration due to zooming.
[0013]
In general, if low dispersion glass is used for the positive lens, chromatic aberration can be reduced. However, in general, low-dispersion glass tends to have a lens shape that has a low refractive index and is difficult to process. For this reason, if the refractive power of the first group or the fourth group is weakened in the above-described four-group zoom lens, the refractive power of the other lens groups must be weakened accordingly. As the diameter increases, it becomes necessary to increase the lens thickness of the first group and the fourth group, and the total lens length becomes longer. Further, when the refractive power of the first group is weakened, the back focus at the wide angle end is shortened, and it becomes difficult to dispose an optical filter, a color separation prism, and the like behind the lens system.
[0014]
The present invention is a four-group type zoom lens in which an object ranging from an infinitely distant object to an extremely close object is obtained over the entire zooming range from the wide-angle end to the telephoto end by appropriately setting the lens configuration of each lens unit. An object of the present invention is to provide a zoom lens with a long back focus having a large aperture ratio and a high zoom ratio that has good optical performance over a wide range.
[0015]
In particular, a diffractive optical element is introduced into the first group in a four-group type rear focus type zoom lens, and the chromatic aberration generated in the first group is reduced by utilizing the diffractive optical action. Reducing the number of lenses, achieving a reduction in the overall length of the lens, reducing the weight of the first lens group, and having a long back focus with a long back focus that has good optical performance over the entire zoom range from the wide-angle end to the telephoto end. The purpose is to provide a zoom lens.
[0016]
[Means for Solving the Problems]
The zoom lens according to the present invention includes four groups, a first group having a positive refractive power, a second group having a negative refractive power, a third group having a positive refractive power, and a fourth group having a positive refractive power in order from the object side. In a zoom lens configured by a lens group, performing zooming by moving the second group and the fourth group, and focusing by moving the fourth group, the first group is rotationally symmetric with respect to the optical axis Diffractive optical element having a positive refractive power, bfw is the air calculation distance from the final lens surface to the image plane at the wide-angle end, and fo , fT are the focal lengths of the entire system at the wide-angle end and the telephoto end , respectively .
When the focal length of the i-th group is fi (i = 1, 3, 4),
4.46 ≦ bfw / fw <5.20 (1)
0.366 ≦ f4 / f3 <0.45 (2)
[Expression 1]
It is characterized by satisfying the following conditions.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a lens cross-sectional view at the wide angle end according to Numerical Example 1 of the present invention, and FIGS. 2 to 4 are aberration diagrams at the wide angle end, intermediate, and telephoto end of Numerical Example 1 according to the present invention. FIG. 5 is a lens cross-sectional view at the wide angle end according to Numerical Example 2 of the present invention, and FIGS. 6 to 8 are aberration diagrams at the wide angle end, intermediate, and telephoto end according to Numerical Example 2 of the present invention.
[0018]
Next, features of the lens configurations of Numerical Examples 1 and 2 in 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 is an aperture stop, which is disposed 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.
[0019]
In this embodiment, when zooming from the wide-angle end to the telephoto end, the second lens unit is moved to the image plane side as indicated by an arrow, and the image plane variation caused by zooming is changed to a convex locus from the fourth lens unit to the object side. It is corrected by moving it while holding it.
[0020]
In addition, a rear focus type is employed in which focusing is performed by moving the fourth group on the optical axis. The solid curve 4a and the dotted curve 4b of the fourth group shown in the figure show the image plane fluctuations accompanying the zooming from the wide-angle end to the telephoto end when focusing on an object at infinity and an object at close distance, respectively. The movement trajectory for correction is shown. The first group and the third group are fixed during zooming and focusing. The first group may be moved during zooming in order to reduce the sharing of zooming of the second group.
[0021]
In the present embodiment, the fourth group is moved to correct the image plane variation accompanying zooming, and the fourth group is moved to perform focusing. In particular, as shown by the curves 4a and 4b in the figure, the zoom lens is moved so as to have a convex locus toward the object side upon zooming from the wide-angle end to the telephoto end. As a result, the space between the third group and the fourth group is effectively used, and the overall length of the lens is effectively shortened.
[0022]
In the present embodiment, for example, when focusing from an infinitely distant object to a close object at the telephoto end, the fourth group is moved forward as indicated by a straight line 4c in FIG.
[0023]
In this embodiment, at least one diffractive optical element is provided in the first group, and its phase is set appropriately, thereby reducing chromatic aberration generated in the first group and correcting chromatic aberration well over the entire zoom range. Yes.
[0024]
When trying to reduce chromatic aberration with only the refractive surface (lens) of the first group without a diffractive optical element, it is necessary to increase the number of lenses or otherwise use anomalous dispersion glass. For example, as represented by FK01 (trade name), it is generally soft and difficult to process.
[0025]
In particular, in the case of high magnification zoom lens to focus the image quality, even with anomalous dispersion glass may greatly also can not sufficiently corrected. In addition, since the lens diameter of the first group is often larger than that of other lens groups, increasing the number of lenses increases the weight of the entire lens, resulting in poor usability.
[0026]
Therefore, in the present invention, diffractive optical elements are used in the first group, and the number of lenses in the first group is reduced, and chromatic aberration is corrected well. In addition, the air calculation distance (distance when a plane parallel plate such as a filter is removed) bfw from the final lens surface to the image plane at the wide angle end satisfies the conditional expression (1).
[0027]
In the case of a video lens that places importance on image quality, a plurality of image sensors may be used. At this time, a prism for dispersing the colors assigned to each image sensor is required. However, if the lower limit of conditional expression (1) is not reached, the back focus becomes too short, and the space for inserting the prism becomes insufficient. On the contrary, if the upper limit of conditional expression (1) is exceeded, the entire length of the entire lens is extended, resulting in a lens that is not easy to use.
[0028]
The diffractive optical element in the present embodiment is binary-manufactured by a lithographic technique that is a holographic optical element (HOE) manufacturing technique. The diffractive optical element may be made of 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. Moreover, you may manufacture by shaping | molding by the direction manufactured by these methods.
[0029]
Further, the shape of the diffractive optical element in the present embodiment is such that φ (h) = 2π / λ (C ₂ .multidot.C) when the reference wavelength (d-line) is λ, the distance from the optical axis is h, and the phase is φ (h). h ² + C ₄ · h ⁴ + · · · C _(2i) · i · h ²ⁱ )
It is represented by the formula of
[0030]
In the present invention, conditional expressions (2) and (3) are further satisfied.
[0032]
If the refractive power of the third group becomes too strong as the upper limit of conditional expression (2) is exceeded, the distance from the final lens surface to the image plane is shortened, and an optical member such as a prism cannot be inserted. Conversely, if the refractive power of the third lens group becomes too weak as the lower limit of conditional expression (2) is exceeded, the distance from the final lens surface to the image surface becomes longer, resulting in an increase in the overall length of the entire lens, resulting in ease of use. It becomes a bad lens.
[0033]
Before SL diffractive optical element is to have a positive refractive power.
[0034]
The first group has a positive refractive power, and the refractive power of the diffractive optical element has a positive refractive power in order to cancel the chromatic aberration caused by refraction by the diffractive optical element. If the refractive power of the diffractive optical element is negative, the generated chromatic aberration will be the same as that of a normal refracting optical system, and the achromatic effect of the diffractive optical element will not be produced, so that sufficient correction of chromatic aberration can be made throughout the optical system. Disappear.
[0037]
If the refractive power of the first lens unit is increased so as to fall below the lower limit of conditional expression (3), the chromatic aberration generated by the refractive optical system cannot be sufficiently canceled out by the diffractive optical element, and the chromatic aberration can be sufficiently corrected in the entire zooming region. Disappear. In addition, it becomes difficult to create a diffractive optical element. On the contrary, if the refractive power of the first group is weakened so as to exceed the upper limit of conditional expression (3), 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.
[0038]
The second group has at least two negative lenses, one positive lens, and a negative lens in order from the object side.
[0039]
The third lens group has a meniscus negative lens and a positive lens whose convex surfaces are convex in order from the object side.
[0040]
The fourth group includes a positive lens and a positive cemented lens as a whole of the negative lens and the positive lens in order from the object side.
[0041]
In the present invention, in order to perform sufficient chromatic aberration correction in the first group, the focal length and Abbe number of all the lenses in the first group are set to f1i, ν1i (i = 1, 2,..., N ), respectively. When the coefficient of the second-order term of the diffractive optical element is C21,
It is desirable to satisfy the following conditions.
[0042]
Conditional expression (4) is a condition for sufficiently correcting the chromatic aberration by combining the achromatic effects on the refractive optical surface and the diffractive optical surface with respect to the first group.
[0043]
In general, the Abbe number (dispersion value) of a refractive optical system is νd = (Nd−1) / (NF−NC), where Nd, NC, and NF are the refractive indexes at wavelengths of d, C, and F lines.
It is represented by
[0044]
On the other hand, the dispersion value νd on the diffractive optical surface is νd = λd / (λF−λC) where the wavelengths of the d-line, C-line, and F-line are λd, λC, and λF.
And νd = −3.45.
[0045]
Further, the refractive power ψ of the paraxial first-order diffracted light at the principal wavelength of the diffractive optical surface is ψ = −2 · C ₂ when the coefficient of the second-order term is C ₂ from the previous expression representing the phase of the diffractive optical surface.
It is expressed.
[0046]
Chromatic aberration generated in the certain group [psi / amount corresponding thereto is proportional to ν is a diffractive optical surface -2 · C ₂ /(-3.45)=0.5797 · C ₂
It becomes.
[0047]
In a refractive optical system, this amount is Σ1 / (f · ν)
It becomes. Therefore, it can be understood that the closer this sum is to 0, the more chromatic aberration correction of the group is performed.
[0048]
Exceeding the range of the conditional expression (4) is not good because correction of chromatic aberration occurring in the first lens group becomes insufficient.
[0049]
As the configuration of the diffractive optical element used in the present embodiment, one layer having a single kinoform shape shown in FIG. 9 or two layers having different (or the same) grating thicknesses as shown in FIG. A two-layer structure in which layers are stacked can be applied.
[0050]
FIG. 10 shows the wavelength dependence characteristics 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 that an ultraviolet curable resin is applied to the surface of the substrate 102, and a layer 103 having a grating thickness d is formed on the resin portion so that the diffraction efficiency of the first-order diffracted light is 100% at a wavelength of 530 nm. is doing.
[0051]
As is apparent from FIG. 10, the diffraction efficiency of the designed order decreases with increasing distance from the optimized wavelength of 530 nm, while the diffraction efficiency of the 0th order diffracted light and the second order diffracted light near the designed order increases. An increase in the diffracted light other than the design order becomes a flare, which leads to a decrease in the resolution of the optical system.
[0052]
FIG. 11 shows the MTF characteristics at each angle of view ω with respect to the spatial frequency when Numerical Example 2 is created with the lattice shape of FIG.
[0053]
FIG. 13 shows the wavelength dependence characteristics 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.
[0054]
In FIG. 12, a first layer 104 made of an ultraviolet curable resin (nd = 1.499, νd = 54) is formed on a substrate 102, and another ultraviolet curable resin (nd = 1.598, νd = 28) is formed thereon. ) Is formed. 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.
[0055]
As can be seen from FIG. 13, by using a diffractive optical element having a laminated structure, the diffraction efficiency of the designed order has a high diffraction efficiency of 95% or more over the entire operating wavelength range.
[0056]
FIG. 14 shows the MTF characteristics at each angle of view ω with respect to the spatial frequency when the numerical value example 2 is created with the lattice shape of FIG. When a diffractive optical element having a laminated structure is used, the low-frequency MTF is improved and desired MTF characteristics are obtained. Thus, the optical performance can be further improved by using a laminated structure as the diffractive optical element according to the present invention.
[0057]
Note that the diffractive optical element having the above-described laminated structure is not limited to the ultraviolet curable resin, and other plastic materials can be used. Depending on the base material, the first layer 104 is directly formed on the base material. May be. Further, the lattice thicknesses are not necessarily 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.
[0058]
In this case, since the grating shape is not formed on the surface of the diffractive optical element, it is excellent in dust resistance and can improve the assembling workability of the diffractive optical element.
[0059]
Next, numerical examples of the present invention will be shown. In 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. The refractive index and Abbe number of the glass. Table 1 shows the relationship between the above-described conditional expressions and numerical examples.
[0060]
The aspheric shape is the X axis in the optical axis direction, the Y axis in the direction perpendicular to the optical axis, the light traveling direction is positive, R is the paraxial radius of curvature, K is the conic constant , and B, C, and D are the aspheric coefficients. When
[0061]
[Equation 3]
It is expressed by the following formula. “D-0X” means “10 ^−X ”.
[0062]
[Outside 1]
[0063]
[Outside 2]
[0064]
[Table 1]
[0065]
【The invention's effect】
According to the present invention, as described above,
(B-1) In a 4-group type zoom lens, by appropriately setting the lens configuration of each lens group, it covers the entire zoom range from the wide-angle end to the telephoto end, and from an infinitely distant object to an extremely close object. It is possible to achieve a zoom lens having a long back focus with a large aperture ratio and a high zoom ratio with good optical performance over the entire object distance.
[0066]
(A-2) In a four-group type rear focus type zoom lens, a diffractive optical element is introduced into the first group, and the chromatic aberration generated in the first group is reduced by utilizing the diffractive optical action. The rear lens with a long back focus that reduces the number of lenses in the group, achieves a reduction in the overall length of the lens, reduces the weight of the first group, and has excellent optical performance over the entire zoom range from the wide-angle end to the telephoto end. A focus-type zoom lens can be achieved.
[Brief description of the drawings]
1 is a lens cross-sectional view of Numerical Example 1 of the present invention. FIG. 2 is an aberration diagram at the wide angle end of Numerical Example 1 of the present invention. FIG. 3 is an intermediate aberration diagram of Numerical Example 1 of the present invention. FIG. 5 is a lens cross-sectional view of Numerical Example 2 of the present invention. FIG. 6 is an aberration diagram of Wide Angle End of Numerical Example 2 of the present invention. FIG. 8 is an aberration diagram at the telephoto end of Numerical Example 2 according to the present invention. FIG. 9 is an explanatory diagram of a diffractive optical element according to the present invention. FIG. 11 is a diagram illustrating the MTF characteristics of the diffractive optical element according to the present invention. FIG. 12 is a diagram illustrating the diffractive optical element according to the present invention. FIG. 14 is an explanatory diagram of wavelength dependent characteristics of an optical element. FIG. 14 is an MTF characteristic diagram of a diffractive optical element according to the invention. FIG. 15 is a diffractive optical element according to the invention. Illustration DESCRIPTION OF SYMBOLS
L1 1st group L2 2nd group L3 3rd group L4 4th group SP Aperture IP Image plane ΔM Meridional image plane ΔS Sagittal image plane d d line g g line 101 Diffractive optical element 102 Base 103, 104, 105 layers

Claims (2)

In order from the object side, the lens unit includes four lens groups including a first group having a positive refractive power, a second group having a negative refractive power, a third group having a positive refractive power, and a fourth group having a positive refractive power , In a zoom lens in which zooming is performed by moving the second group and the fourth group, and focusing is performed by moving the fourth group, the first group has a positive refractive power diffraction that is rotationally symmetric with respect to the optical axis. has an optical element, each of the focal length of the entire system of air calculated distance from the last lens surface at the wide angle end to the image plane BFW, at the wide-angle end and the telephoto end fw, fT,
When the focal length of the i-th group is fi (i = 1, 3, 4),
4.46 ≦ bfw / fw <5.20
0.366 ≦ f4 / f3 <0.450
A zoom lens characterized by satisfying the following conditions: 2. 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.