Abstract:
A camera apparatus having photographing lenses and an optical low-pass filter for limiting the image information of predetermined spatial frequency components, the filter being disposed at an arbitrary position on the optical axis of the photographing lenses. One of the sides of the optical low-pass filter is formed as a diffraction grating structure having a low pass effect, and the other side is formed in an aspherical shape.

Description:
BACKGROUND OF THE INVENTION 
     1. Field the Invention 
     The present invention relates to a camera apparatus having an optical low-pass filter. More particularly, the present invention relates to a camera apparatus having an optical low-pass filter suited for processing images one-dimensionally and discretely, as in video cameras, electronic still cameras or the like, using color image pick-up elements. 
     2. Description of the Related Art 
     In a photographing lens using image pick-up elements, such as CCDs, which obtain output images by discretely sampling image information, if spatial frequency components exceed the frequency limit of the image pick-up elements then, when an object is photographed, a phenomenon, such as a so-called beat, occurs in the prior art. In this situation, moires which have originally not been present in the object appear, or compacted moires appear as thick moires having uneven densities. That is, frequency components which cannot be picked up by a camera apparatus cannot be reproduced as image information, and therefore a phenomenon called a so-called waveform distortion (areaging) occurs. 
     To suppress this areaging, in a photographing lens of, for example, a video camera, an optical low-pass filter is disposed in the light path of the photographing lens so that the light flux from the object is separated into a plurality of directions. This is accomplished by permitting the light flux to pass through the optical low-pass filter, resulting in a single point image formed on an image photographing plane that is separated into a plurality of point images. As a result, the influence of the areaging is suppressed by limiting the high-band frequency characteristics of the object. 
     As for optical low-pass filters, various filters using birefringence produced by uniaxial crystals, such as crystallized quartz, or filters in which a diffraction grating is disposed in the light path of a photographing lens and the diffraction effect of the diffraction grating is utilized, are used. Since optical low-pass filters, in particular those using a diffraction grating, can be produced easily by molding plastics, they can be obtained at quite a low price. Therefore, they have been widely used in the photographing lenses of recent video cameras or the like. Optical low-pass filters using a diffraction grating have been disclosed in, for example, Japanese Patent Laid-Open Nos. 53-119063 and 63-287921. 
     In addition, there has been a demand for a camera apparatus, for use with a commercial-use video camera, which has an optical low-pass effect and high optical performance, and in which the entire photographing lens is made compact since the entire video camera has become compact. 
     Generally, to maintain a satisfactory optical performance of a photographing lens and at the same time reduce the number of lenses and make the entire lens system compact, it is effective to use aspherical lenses. For example, Japanese Patent Laid-Open No. 2-39011 discloses a compact photographing lens using aspherical lenses. However, aspherical lenses have a problem in that their optical characteristics change greatly due to changes in the environment, such as temperature or humidity, though they can be produced relatively easily by using plastic molds. 
     By contrast, if aspherical lenses are produced by using glass molds, variations in their optical characteristics due to changes in the environment, such as temperature or humidity, decrease. However, there is a problem in that it becomes difficult to produce them, since, for example, the types of grinding materials which can be used are limited, and maintaining the surface precision satisfactorily is difficult. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a camera apparatus which is capable of easily obtaining a predetermined low-pass effect by having a properly arranged optical low-pass filter therein and lenses, where the number of lenses can be reduced by using aspherical surfaces. 
     Another object of the present invention is to provide a camera apparatus having an optical low-pass filter in which variations in the optical characteristics are small when a high optical performance is obtained, and having the capability of maintaining a high optical performance at all times. 
     The camera apparatus having an optical low-pass filter of the present invention has photographing lenses and an optical low-pass filter for limiting image information of predetermined spatial frequency components, the filter being disposed at an arbitrary position on the optical axis of the photographing lenses. One of the sides of the optical low-pass filter is formed as a diffraction grating structure having a low pass effect, and the other side is formed in an aspherical shape. In the present invention, in particular, the paraxial refracting power of the aspherical shape is substantially zero, and the above-described optical low-pass filter is disposed in the vicinity of the aperture diaphragm of the photographing lenses. 
     According to another aspect of the present invention, the camera apparatus includes a photographing lens with a plurality of movable lens units. In this apparatus, an optical low pass filter is disposed on the image plane side of at least one of the plurality of movable lens units of the photographing lens. One of the sides of the optical low pass filter has formed on it a diffraction grating structure having a low pass effect. The other side of the optical low pass filter is formed in an aspherical shape. The camera apparatus has an image pick-up device disposed on the image plane of the photographing lens. The pick-up device has a plurality of photoelectric conversion elements. A number of lens units are disposed between the optical low pass filter and the image pick-up device. These lens units guide light to the image pick-up device. 
     The aforementioned and other objects, features and advantages of the present invention will become clear when reference is made to the following description of the preferred embodiments of the present invention, together with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of an essential portion of an optical system according to a first embodiment of the present invention; 
     FIG. 2 is a cross-sectional view of an essential portion of the optical low-pass filter of FIG. 1; 
     FIG. 3 is a view which illustrates the optical function of the optical low-pass filter of FIG. 1; 
     FIG. 4 is a schematic view of an essential portion of an optical system according to a second embodiment of the present invention; 
     FIG. 5 is a cross-sectional view which illustrates lenses according to numerical data 1 of the present invention; 
     FIG. 6 is a cross-sectional view which illustrates lenses according to numerical data 2 of the present invention; 
     FIG. 7 is a view showing aberrations at the wide angle end position according to numerical data 1 of the present invention; 
     FIG. 8 is a view showing aberrations in the intermediate section between the wide angle end position and the telephoto end position according to numerical data 1 of the present invention; 
     FIG. 9 is a view showing aberrations at the telephoto end position according to numerical data 1 of the present invention; 
     FIG. 10 is a view showing aberrations at the wide angle end position according to numerical data 2 of the present invention; 
     FIG. 11 is a view showing aberrations in the intermediate section between the wide angle end position and the telephoto end position according to numerical data 2 of the present invention; 
     FIG. 12 is a view showing aberrations at the telephoto end position according to numerical data 2 of the present invention; 
     FIG. 13 is a view showing aberrations at the wide angle end position according to numerical data 3 of the present invention; 
     FIG. 14 is a view showing aberrations in the intermediate section between the wide angle end position and the telephoto end position according to numerical data 3 of the present invention; 
     FIG. 15 is a view showing aberrations at the telephoto end position according to numerical data 3 of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a schematic view of an essential portion of an optical system according to a first embodiment of the present invention. 
     In this embodiment, a so-called five-group type zoom lens, including five lens groups as a whole, is used as a photographing lens. An optical low-pass filter is disposed in the vicinity of the aperture diaphragm of the photographing lens. As a result, a predetermined low-pass effect is obtained. 
     In FIG. 1, reference numeral 3 denotes a first lens unit having a focusing function and a positive refracting power; reference numeral 4 denotes a second lens unit having a magnification function and a negative refracting power; reference numeral 5 denotes a third lens unit for compensating for the shift of the image plane as a magnification is changed; reference numeral 6 denotes a fourth lens unit for causing a light flux from the third lens unit 5 to emit as a substantially afocal light flux; reference numeral 1 denotes an optical low-pass filter; reference numeral 2 denotes an aperture diaphragm which controls the amount of light to be transmitted; and reference numeral 7 denotes a fifth lens unit for forming an image. 
     A change in magnification from a wide angle end position to a telephoto end position is made by the photographing lens of the present invention by making the second lens unit 4 and the third lens unit 5 move as shown by the arrows. The optical low-pass filter 1 is disposed in the vicinity of the aperture diaphragm 2. 
     As shown in FIG. 2, one of the surfaces 1a of the optical low-pass filter of this embodiment manifests a low-pass effect, and is formed as a diffraction grating, the cross section of which is in the shape of a triangle, saw-tooth or trapezoid, or in the form of a phase type diffraction grating. The other surface 1b is formed in an aspherical shape to achieve satisfactory compensation for aberrations and to obtain a high optical performance. The refracting power of the aspherical shape at this time is substantially zero from a paraxial point of view. The refracting power being substantially zero refers to a diffraction grating having a focal distance larger than 50 f, where the focal distance of the entire system is denoted as f. The materials in the optical low-pass filter of this embodiment are synthetic resins. 
     As described above, in this embodiment, because a diffraction grating is used for one of the surfaces of the optical low-pass filter, a low-pass effect is obtained. By making the other surface aspherical, the number of lenses of the fifth lens unit, of the photographing lens, that is used for forming an image, is, for example, about three to five. As a result, various aberrations, such as spherical or comatic aberrations, can be compensated for satisfactorily. The paraxial refracting power of the aspherical surfaces at this time is made substantially zero, so that variations in the optical characteristics caused by the shift of focus are reduced. These variations are caused by a change in the environment, such as in the temperature or humidity, when the optical low-pass filter is formed by molding plastics. 
     FIG. 5 is a cross-sectional view which illustrates the lenses according to numerical data 1 which will be described later, in which a zoom type lens of FIG. 1 is used. 
     FIG. 3 is a view which illustrates the image separation produced by the optical low-pass filter of the present invention. In FIG. 3, reference numeral 8 denotes an optical low-pass filter; and reference numeral 9 denotes an image plane of a CCD (charge coupled device) composed of a plurality of photoelectric conversion elements. 
     If the image separation width on the image plane 9, by the optical low-pass filter 8, is denoted as δ, the prism angle and the refraction factor of the optical low-pass filter 8 are denoted as θ and n, respectively, and the distance between the optical low-pass filter 8 and the image plane 9 is denoted as L, the image separation width δ can be expressed by the following equation: 
     
         δ=2 L tan (n-1)θ                               (1) 
    
     In FIG. 3, the fifth lens unit 7 is omitted for simplicity. 
     The image separation width δ by the optical low-pass filter 8 should be substantially one half of the pitch of a color separation stripe filter in order to remove spurious color signals of an image reproduced by a single-plate color photographing element. 
     However, when the optical low-pass filter is disposed inside a zoom lens, because it must be disposed at a position such that it does not obstruct the movement of the lens, the seeming position of the optical low-pass filter relative to the image plane changes due to the zooming. 
     That is, even if the image separation width δ when the zoom lens is at a wide angle end position is proper in zooming, the image separation width δ when the zoom lens is at a telephoto end position may be improper. Therefore, the optical low-pass filter 8 must be disposed at a position where a change in the image separation width δ, caused by the zooming, becomes smallest. In this embodiment, the optical low-pass filter 8 is disposed nearer to the vicinity of the aperture diaphragm of the image plane than to the magnification section so that the image separation width by the low-pass effect becomes constant even if the magnification is changed. 
     FIG. 4 is a schematic view of an essential portion of an optical system according to a second embodiment of the present invention. In this embodiment, a zoom lens, including four groups of lens units, as a whole is used as a photographing lens. The optical low-pass filter, which is the same as in the first embodiment in FIG. 1, is disposed in the vicinity of the aperture diaphragm of the photographing lens. A predetermined low-pass effect and aberration compensation effect are obtained as in the first embodiment. 
     In FIG. 4, reference numeral 43 denotes a fixed, first lens unit having a positive refracting power; reference numeral 44 denotes a second lens unit having a magnification function and a negative refracting power; reference numeral 1 denotes an optical low-pass filter having the same shape as that in the first embodiment; reference numeral 2 denotes an aperture diaphragm; reference numeral 45 denotes a fixed, third lens unit having a positive refracting power; and reference numeral 46 denotes a fourth lens unit having functions for both compensating for the shift of an image plane in consequence of a change in magnification, and for focusing. 
     In this embodiment, a change in magnification from a wide angle end position to a telephoto end position is made by making the second lens unit 44 and the fourth lens unit 46 move as indicated by the arrows. It is brought into focus by making the fourth lens unit 46 move on the light path. 
     In this embodiment, the number of lenses are reduced by making the other surface of the optical low-pass filter aspherical as in the first embodiment shown in FIG. 1. In particular, the number of lenses of the third lens unit 45 is reduced, and thus the entire lens system is made simpler. 
     In this embodiment, the optical low-pass filter is disposed nearer to the object than to the fourth lens unit which is moved during a change in magnification. As a result, the optical distance between the optical low-pass filter and the image plane changes due to the change in magnification, and the low-pass effect varies. Therefore, changes in the image separation width on the image plane are decreased so as not to pose a problem in practical use by properly setting various constants, such as refracting powers, distances between principal points, or conditions for movement of each lens group. 
     FIG. 6 is a cross-sectional view which illustrates the lenses, in which the zoom lens of FIG. 4 is used, according to numerical data 2 which will be described later. 
     Next, numerical data of the present invention will be shown. In the numerical data, Ri denotes a curvature radius of the surface of an i-th lens in order from an object; Di denotes the thickness of an i-th lens from the object and air space; Ni and νi denote the refraction factor and the Abbe number of an i-th lens from the object, respectively. 
     The aspherical shape is expressed by an equation described below when the direction of the optical axis is denoted as the X axis, the direction perpendicular to the optical axis is denoted as the H axis, the direction in which light travels is assumed to be positive, R denotes a paraxial curvature radius, and A, B, C, D and E each are an aspherical factor: ##EQU1## 
     Many different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in this specification. To the contrary, the present invention is intended to cover various modifications and equivalent arrangements included with the spirit and scope of the claims. The following claims are to be accorded a broad interpretation, so as to encompass all such modifications and equivalent structures and functions. 
     
         __________________________________________________________________________Numeric values of Example 1F = 1˜7.6   FNO = 1:2.04˜2.25   2ω = 48.0°˜6.6°__________________________________________________________________________R1 =    6.898 D1 =         0.166 N1 =                   1.80518                        ν1 =                            25.4R2 =    3.480 D2 =         0.833 N2 =                   1.51633                        ν2 =                            64.1R3 =    -13.260     D3 =         0.027R4 =    3.156 D4 =         0.500 N3 =                   1.60311                        ν3 =                            60.7R5 =    13.219     D5 =         VariableR6 =    5.279 D6 =         0.111 N4 =                   1.77250                        ν4 =                            49.6R7 =    1.232 D7 =         0.381R8 =    -1.710     D8 =         0.111 N5 =                   1.77250                        ν5 =                            49.6R9 =    1.325 D9 =         0.319 N6 =                   1.84666                        ν6 =                            23.8R10 =    4306.543     D10 =         VariableR11 =    -2.083     D11 =         0.111 N7 =                   1.77250                        ν7 =                            49.6R12 =    -5.425     D12 =         VariableR13 =    2.071 D13 =         0.583 N8 =                   1.51633                        ν8 =                            64.1R14 =    -2.284     D14 =         0.208R15 =    Aspherical     D15 =         0.138 N9 =                   1.49171                        ν9 =                            57.4    surfaceR16 =    Grating     D16 =         0.138    surfaceR17 =    Aperture     D17 =         0.28    diaphragmR18 =    1.057 D18 =         0.458 N10 =                   1.48749                        ν10 =                            70.2R19 =    4.070 D19 =         0.104R20 =    5.023 D20 =         0.111 N11 =                   1.76182                        ν11 =                            26.5R21 =    0.919 D21 =         0.804R22 =    3.354 D22 =         0.291 N12 =                   1.60311                        ν12 =                            60.7R23 =    -2.018__________________________________________________________________________ 
    
     
                       TABLE 1______________________________________Focal LengthVariable Distance         1.00        3.80   7.60______________________________________D5            0.11        1.98   2.48D10           2.69        0.48   0.56D12           0.41        0.75   0.17______________________________________15th Surface Aspherical SurfaceR.sub.0 =    ∞       B = -7.896 × 10.sup.-2C =      -2.86 × 10.sup.-2                  D = -7.533 × 10.sup.-3______________________________________ 
    
     
         __________________________________________________________________________Numeric values of Example 2F = 1˜7.6   FNO = 1:2.04˜2.25   2ω = 48.0°˜6.6°__________________________________________________________________________R1 =    7.843 D1 =         0.166 N1 =                   1.80518                        ν1 =                            25.4R2 =    3.578 D2 =         0.833 N2 =                   1.51633                        ν2 =                            64.1R3 =    -14.766     D3 =         0.027R4 =    3.392 D4 =         0.500 N3 =                   1.69680                        ν3 =                            55.5R5 =    15.466     D5 =         VariableR6 =    5.028 D6 =         0.111 N4 =                   1.77250                        ν4 =                            49.6R7 =    1.225 D7 =         0.381R8 =    -1.696     D8 =         0.111 N5 =                   1.77250                        ν5 =                            49.6R9 =    1.308 D9 =         0.319 N6 =                   1.84666                        ν6 =                            23.8R10 =    1009.983     D10 =         VariableR11 =    -2.084     D11 =         0.111 N7 =                   1.77250                        ν7 =                            49.6R12 =    -5.427     D12 =         VariableR13 =    2.060 D13 =         0.583 N8 =                   1.51633                        ν8 =                            64.1R14 =    -2.298     D14 =         0.208R15 =    Aspherical     D15 =         0.138 N9 =                   1.49171                        ν9 =                            57.4    surfaceR16 =    Grating     D16 =         0.138    surfaceR17 =    Aperture     D17 =         0.28    diaphragmR18 =    1.078 D18 =         0.458 N10 =                   1.51633                        ν10 =                            64.1R19 =    3.759 D19 =         0.129R20 =    4.807 D20 =         0.111 N11 =                   1.80518                        ν11 =                            25.4R21 =    0.924 D21 =         0.762R22 =    3.096 D22 =         0.291 N12 =                   1.60311                        ν12 =                            60.7R23 =    -2.024__________________________________________________________________________ 
    
     
                       TABLE 2______________________________________Focal LengthVariable Distance         1.00        3.80   7.60______________________________________D5            0.17        2.03   2.53D10           2.69        0.48   0.56D12           0.41        0.76   0.17______________________________________15th Surface Aspherical SurfaceR.sub.0 =    ∞       B = -7.815 × 10.sup.-2C =      -2.924 × 10.sup.-2                  D = -6.632 × 10.sup.-3______________________________________ 
    
     
         __________________________________________________________________________Numeric values of Example 3F = 1˜5.75   FNO = 1:2.05˜2.61   2ω = 50.4°.about.9.4°__________________________________________________________________________R1 =    9.759 D1 =         0.132 N1 =                   1.80518                        ν1 =                            25.4R2 =    2.884 D2 =         0.500 N2 =                   1.60311                        ν2 =                            60.7R3 =    -9.827     D3 =         0.029R4 =    2.312 D4 =         0.323 N3 =                   1.71999                        ν3 =                            50.3R5 =    7.262 D5 =         VariableR6 =    19.640     D6 =         0.088 N4 =                   1.78590                        ν4 =                            44.2R7 =    0.820 D7 =         0.347R8 =    -1.194     D8 =         0.088 N5 =                   1.51823                        ν5 =                            59.0R9 =    1.194 D9 =         0.264 N6 =                   1.80518                        ν6 =                            25.4R10 =    -75.158     D10 =         VariableR11 =    Grating     D11 =         0.147 N7 =                   1.49171                        ν7 =                            57.4    surfaceR12 =    Aspherical     D12 =         0.147    surfaceR13 =    Aperture     D13 =         0.18    diaphragmR14 =    2.707 D14 =         0.323 N8 =                   1.60311                        ν8 =                            60.7R15 =    -5.323     D15 =         VariableR16 =    3.297 D16 =         0.102 N9 =                   1.84666                        ν9 =                            23.8R17 =    1.304 D17 =         0.426 N10 =                   1.51633                        ν10 =                            64.1R18 =    -2.478     D18 =         0.022__________________________________________________________________________ 
    
     
                       TABLE 3______________________________________Focal LengthVariable Distance         1.00        2.65   5.75______________________________________D5            0.19        1.25   1.87D10           1.85        0.79   0.16D12           1.01        0.67   1.19______________________________________12th Surface Aspherical SurfaceR.sub.0 =    ∞       B =      4.701 × 10.sup.-2C =      2.482 × 10.sup.-2                  D =    -1.341 × 10.sup.-2______________________________________