JPH1198391A - Video camera and zoom lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the problem of double image from being generated when a filter for infrared ray cut or the like is inserted into a zoom lens by providing light transmissible parallel planes between a 3rd lens group and a 4th lens group and specifying a distance between a position, where the image of light reflected to the side of an object on the image pickup plane of an imaging device and next reflected to the side of an image on the planes is secondarily formed, and the image pickup plane. SOLUTION: This device is composed of a 1st lens group I having positive diffraction power, 2nd lens group II having negative diffraction power, 3rd lens group III having positive diffraction power and 4th lens group IV having positive diffraction power successively from the side of the object. Then, such a zoom lens corrects the fluctuation of an image point at the time of magnification due to the move of the 2nd lens group II and the move of the 4th lens group IV. Between these 3rd and 4th lens groups III and IV, light transmissible parallel planes 11 are provided and the inequality of 1<βII.βIII/f<2 is satisfied. In this case, the βII is the lateral magnification of the 2nd lens group, the βIII is the lateral magnification of the 3rd lens group and the (f) is the focal distance of the entire system.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a home video camera and an inner focus type zoom lens suitable for the video camera. In particular, the present invention relates to a video camera and a zoom lens in which a filter for cutting infrared rays or the like can be inserted into the zoom lens.

[0002]

2. Description of the Related Art An inner focus type zoom lens having four positive / negative / positive / positive lens groups is a typical zoom lens for home video cameras. In this type of zoom lens, the power of the lens is arranged so that the space between the third lens group and the fourth lens group is substantially afocal light for the following reasons. (1) It is difficult to sufficiently secure the back focus of the lens if the convergent light is a certain level or more. (2) If the diverging light is a certain level or more, the positive power of the fourth lens group must be increased, and the effect of achromatism is reduced. From this balance, it is placed near the afocal. (3) By making it afocal, it is possible to obtain a large tolerance between the surfaces, and the sensitivity of the eccentricity at the time of moving the fourth lens unit is relaxed.

[0003]

However, when a flat plate such as a filter is inserted between the third lens unit and the fourth lens unit, the light reflected on the CCD surface is reflected again on the flat plate, and the CCD is turned off. The reflected light forms a secondary image near the surface, which causes a phenomenon that the image is taken twice.

FIG. 6 is a diagram showing the arrangement of an optical system of a video camera and the image forming relationship for explaining the problem of double image in a conventional inner focus type zoom lens. The optical system of the video camera includes an image sensor 21 and a zoom lens that forms an image of a subject on the image sensor. The zoom lens includes, in order from the object side, a first lens group I having a positive refractive power, a second lens group II having a negative refractive power, a third lens group III having a positive refractive power,
The fourth lens group IV has a positive refractive power. The magnification is changed by the movement of the second lens group, and by the movement of the fourth lens group,
The image point fluctuation at the time of zooming is corrected. Further, a translucent parallel flat plate (filter 11) is provided between the third lens group and the fourth lens group.
Is equipped. The filter 11 is usually for cutting infrared rays. The filter 11 is inserted into the optical path during general light imaging, and is removed from the optical path in the case of infrared imaging (night vision) with a monitoring video camera. In the figure, reference numeral 13 placed between the fourth lens group IV and the CCD imaging surface 21 is a low-pass filter made of quartz for removing minute disturbance light. Reference numeral 15 denotes a CCD cover glass.

[0005] In the drawing, what is indicated by a two-dot chain line is an image forming light ray of a normal object image, which is correctly formed on the CCD imaging surface 21. On the other hand, the dashed line indicates that the light primarily formed on the CCD imaging surface 21 is reflected by the
1, returns along the two-dot chain line, is reflected by the object-side surface of the filter 11, and passes again through the fourth lens group IV and the like to the surface 23.
The image was formed by In the example of FIG.
The distance Δ between the lens 1 and the image plane 23 of the reflected light is about 0.12 mm, which is as short as 7% of the focal length f of the entire lens system. It will be hard to see.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and a video camera and a zoom camera in which a double image problem is prevented from occurring when a filter such as an infrared cut filter is inserted into a zoom lens. It is intended to provide a lens.

[0007]

In order to solve the above-mentioned problems, a video camera according to the present invention is a video camera including an imaging device and a zoom lens for forming an image of a subject on the imaging device; The zoom lens includes, in order from the object side, a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a third lens group having a positive refractive power. The zoom lens comprises four lens units, and changes the magnification by moving the second lens unit, and corrects the image point fluctuation during zooming by moving the fourth lens unit; A light-transmissive parallel plane plate is interposed between the image plane and a position where light reflected on the image side of the image sensor toward the object side and then reflected on the image side on the plane plate forms a secondary image and the image plane. The distance Δ is 0.33f or more with respect to the focal length f of the entire lens system. And wherein the door.

As a result of analyzing a double image caused by the reflection of the filter when the filter is inserted into the zoom lens,
If the reflection imaging plane is shifted from the CCD imaging plane to the extent described above, the contrast of the reflection image is reduced and almost no problem occurs on the image. This is the first time that they found out,
It is a matter grasped quantitatively.

From another point of view, the video camera of the present invention is characterized in that the degree of afocal light emitted from the third lens group is convergent light of 0.015 radians to 0.03 radians. I do. In other words, if the light beam in the space in which the filter is placed is afocal, the reflected light from the surface of the filter passes through almost the same locus as the primary light, and
It is easy to form an image near the CD imaging surface. On the other hand, when the degree of non-afocal is increased, the problem described in the section of the related art arises. Thus, the present inventors have analyzed the optical balance in the presence of a new element called a filter between the third lens group and the fourth lens group, and have found that a reasonable solution can be obtained within the above range.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A zoom lens according to a first embodiment of the present invention comprises, in order from the object side, a first lens group having a positive refractive power, a second lens group having a negative refractive power, The zoom lens includes a third lens unit having a positive refractive power and a fourth lens unit having a positive refractive power. The zoom ratio is changed by moving the second lens unit, and the image point is changed by changing the fourth lens unit. And a transmissive plane-parallel plate between the third lens group and the fourth lens group, wherein the following conditional expression is satisfied. 1 <β _II · β _III / f <2 (1) β _II : lateral magnification of the second lens group β _III : lateral magnification of the third lens group f: focal length of the entire lens system.

When the value falls below the lower limit of the expression (1), the back focus becomes short, and the space for the low-pass filter or the like to be inserted on the image side of the fourth lens group IV decreases.
In addition, the amount of movement of the fourth lens unit for focusing becomes large, and it becomes difficult to secure a space between the third lens unit and the fourth lens unit. On the other hand, when the value exceeds the upper limit of the above expression (1), the light emitted from the third lens group approaches an afocal, and the reflected light between the CCD surface and the flat plate re-images near the CCD surface, which is desirable. Absent.

In the zoom lens of this embodiment, the fourth lens group is cemented to a negative meniscus lens having a convex surface facing the object side, which is arranged in order from the object side, and a concave surface on the image side of the meniscus lens. It is preferable to have a convex lens and satisfy the following conditional expressions. _{0.27 <r x / f IV <} 0.33 ......... (2) r x: curvature of the concave on the image side of the meniscus lens radius f _IV: focal length of the fourth lens group

When the value is below the lower limit of the expression (2), coma occurs, and image deterioration particularly at an intermediate zoom position is likely to occur. In addition, the workability of the lens deteriorates. On the other hand, when the value exceeds the upper limit of the expression (2), the correction of the chromatic aberration of magnification tends to be insufficient.

Further, the second lens group includes a negative meniscus lens having a convex surface facing the object side, a biconcave lens, and a convex lens cemented to the image-side concave surface of the biconcave lens. And it is preferable to satisfy the following conditional expressions. ν _Y −ν _Z > 35 (35) | n _Y −n _Z |> 0.23 (4) ν _Y : Abbe number of the material of the biconcave lens ν _Z : above Abbe number of the material of the biconvex lens n _Y : refractive index of the material of the biconcave lens with respect to d-line n _Z : refractive index of the material of the biconvex lens with d-line

When the value of the condition (3) is exceeded, the axial chromatic aberration tends to be excessively corrected at the telephoto end. When the value falls below the condition of the above-mentioned expression (4), it becomes difficult to correct the curvature of field at the telephoto end, and it becomes difficult to secure the edge thickness of the convex lens.

Hereinafter, description will be made with reference to the drawings. FIG.
FIG. 3 is a diagram illustrating an arrangement of an optical system of a video camera and an image forming relationship for explaining a problem of a double image in the inner focus type zoom lens according to one embodiment of the present invention.
The optical system of the video camera includes an image sensor 21 and a zoom lens that forms an image of a subject on the image sensor. The zoom lens includes, in order from the object side, a first lens unit I having a positive refractive power and a second lens group I having a negative refractive power.
It comprises a lens group II, a third lens group III having a positive refractive power, and a fourth lens group IV having a positive refractive power. The magnification is changed by the movement of the second lens group, and the image point fluctuation at the time of zooming is corrected by the movement of the fourth lens group. The third lens group and the fourth lens group
A translucent plane parallel plate (filter 11) is provided between the lens groups. The filter 11 is usually for cutting infrared rays. The filter 11 is inserted into the optical path during general light imaging, and is removed from the optical path in the case of infrared imaging (night vision) with a monitoring video camera. In the figure, reference numeral 13 placed between the fourth lens group IV and the CCD imaging surface 21 is a low-pass filter made of quartz for removing minute disturbance light.
Reference numeral 15 denotes a CCD cover glass.

The specific configuration of each lens group is as follows from the object side to the image side. First lens group: Negative meniscus lens with convex surface facing the object side (first lens) Convex lens (second lens) joined to the meniscus lens Positive meniscus lens with convex surface facing object side (third lens) Second lens group: negative meniscus lens (fourth lens) with convex surface facing the object side Biconcave lens (fifth lens) Positive meniscus lens (sixth lens) joined to the biconcave lens Third lens group : Biconvex lens (seventh lens) Fourth lens group: negative meniscus lens having a convex surface facing the object side (eighth lens) Convex lens (ninth lens) joined to the meniscus lens

In the drawing, a two-dot chain line indicates a light beam of a regular object image, which is correctly formed on the CCD image pickup surface 21. On the other hand, the dashed line indicates that the light primarily formed on the CCD imaging surface 21 is reflected by the
1, returns along the two-dot chain line, is reflected by the object-side surface of the filter 11, and passes again through the fourth lens group IV and the like to the surface 23.
The image was formed by

FIG. 2 shows a third example of the video camera optical system shown in FIG.
FIG. 4 is an enlarged view showing an image side from a lens group. (A) is the wide-angle end state, (B) is the intermediate state, and (C) is the telephoto end state. The circle drawn on the right side of the figure indicates the size of the secondary imaging area on the CCD imaging surface 21 of the light reflected from the CCD imaging surface 21 and the object side surface of the filter 11.

The characteristic of FIG. 2 is that the light reflected from the CCD imaging surface 21 indicated by a broken line is slightly divergent after passing through the fourth lens group IV.
The light is reflected by the object-side surface of the filter 11 and further diverges. Therefore, even if the light passes through the fourth lens group IV again, the light does not converge completely, but forms a secondary image on a surface 23 </ b> A which is farther away from the CCD imaging surface 21 on the image side. This discussion,
This is an imaginary discussion when the reflected light passes through the CCD imaging surface 21. Therefore, when the reflected light impinges on the CCD imaging surface 21, the image becomes a blurred image in a wide area, and the image is of such an extent that a double image cannot be recognized.

The distance Δ between the CCD imaging surface 21 and the reflected light imaging surface 23 is the shortest in the intermediate state (B), but the length is 1.07 mm (Δ / f) in Numerical Example 1 described later. = 0.6
6), 0.54 mm (Δ / f = 0.33) in Numerical Example 2
It is. At this time, the diameter of the spread of the reflection image on the CCD imaging surface 21 was Φ1.5 mm in Numerical Example 1, and Φ0.74 mm in Numerical Example 2, which was sufficiently blurred.

Hereinafter, two specific numerical examples will be given.
FIG. 3 is a diagram showing names of respective parts of the optical system of the zoom lens according to the present embodiment. The meaning of each code in the numerical example is as follows. f: focal length (mm) F _NO : F number 2ω: angle of view ri: radius of curvature of the i-th surface (mm) di: i-th surface interval (mm) Ni: d-line of the i-th lens The refractive index νi: Abbe number of the i-th lens An aspherical surface is obtained by adding an asterisk (R) to the numerical value of R. The surface shape of the lens takes the y-axis in the direction perpendicular to the Z-axis optical axis, It is expressed by the following equation. ^{Z = (y 2 / r)} / [1+ {1- (y / r) 2} 1/2]
However ^{_{+ A 4 y 4 + A 6}} y 6 + A 8 y 8 + A 10 y 10, A 4, A 6, A 8, A 10 is the aspherical coefficient

[0023]

[Outside 1]

[0024]

[Table 1]

[0025]

[Table 2]

Conditional expression (1) β _II · β _III /f=1.03 (2) r _x / f _IV = 0.283 (3) ν _Y −ν _Z = 40.4 (4) | n _Y − n _Z | = 0.25753

[0027]

[Outside 2]

[0028]

[Table 3]

[0029]

[Table 4]

Conditional expression (1) β _II · β _III /f=1.95 (2) r _X / f _IV = 0.311 (3) ν _Y −ν _Z = 37.5 (4) | n _Y − n _Z | = 0.32986

FIGS. 4 and 5 show various aberration diagrams of the first embodiment and the second embodiment of the present invention. Each figure in the drawings shows various aberration diagrams in the infinity in-focus state in the wide-angle end state, the intermediate state, and the telephoto end state, respectively. In each aberration diagram, the solid line in the spherical aberration diagram is the spherical aberration for the d line, the broken line is the spherical aberration for the g line, and the one-dot chain line is the spherical aberration for the c line. The solid line in the astigmatism diagram indicates the sagittal image plane, and the broken line indicates the meridional image plane. From each aberration diagram, it is clear that the present embodiment has excellent correction of various aberrations and excellent imaging performance.

[0032]

As is apparent from the above description, according to the present invention, even if a flat plate such as a filter is put in and out between the third lens unit and the fourth lens unit, re-imaging on the CCD surface is performed. There is an effect that can be avoided. Furthermore, in some embodiments, there is an effect that aberration variation (movement of an image point) at the time of putting in and taking out the filter is small, and the angle of the light ray is almost parallel to the optical axis, so that the optical path length change by the filter is small. it can.

[Brief description of the drawings]

FIG. 1 is a diagram showing an arrangement of an optical system of a video camera and an image forming relationship for explaining a problem of a double image in an inner focus type zoom lens according to one embodiment of the present invention.

FIG. 2 is an enlarged view showing an image side from a third lens group of the video camera optical system of the embodiment in FIG. 1; (A) is the wide-angle end state, (B) is the intermediate state, and (C) is the telephoto end state.

FIG. 3 is a diagram showing names of respective parts of the optical system of the zoom lens according to the embodiment.

FIG. 4 shows various aberration diagrams of the first embodiment and the second embodiment of the present invention. Each figure in the drawings shows various aberration diagrams in the infinity in-focus state in the wide-angle end state, the intermediate state, and the telephoto end state, respectively.

FIG. 5 shows various aberration diagrams of the first embodiment and the second embodiment of the present invention. Each figure in the drawings shows various aberration diagrams in the infinity in-focus state in the wide-angle end state, the intermediate state, and the telephoto end state, respectively.

FIG. 6 is a diagram showing an arrangement of an optical system of a video camera and an image forming relationship for explaining a problem of a double image in a conventional inner focus type zoom lens.

[Explanation of symbols]

11 ... filter, 13 ... low-pass filter, 15 ...
Cover glass, 21: CCD imaging surface, 23: Reflected light imaging surface,

Claims (5)

[Claims] 1. A video camera including an image sensor and a zoom lens for forming an image of a subject on the image sensor; wherein the zoom lens has a first refractive power having a positive refractive power in order from an object side. A lens group, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a fourth lens group having a positive refractive power. The movement of the four lens units corrects the image point fluctuation at the time of zooming; a translucent parallel flat plate is provided between the third lens unit and the fourth lens unit; The distance Δ between the position where the light reflected on the object side and then reflected on the image side on the flat plate to form an image and the distance Δ from the imaging surface is 0.33 f with respect to the focal length f of the entire lens system. A video camera characterized by the above. 2. A video camera comprising: an image sensor; and a zoom lens for forming an image of a subject on the image sensor; wherein the zoom lens has a first refractive power having a positive refractive power in order from an object side. A lens group, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a fourth lens group having a positive refractive power. The movement of the four lens units corrects the image point fluctuation at the time of zooming; a translucent parallel flat plate is provided between the third lens unit and the fourth lens unit; A video camera, wherein the degree of afocal of emitted light is convergent light of 0.015 radian to 0.03 radian. 3. A first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a third lens group having a positive refractive power. A zoom lens comprising four lens groups, wherein the magnification is changed by moving the second lens group, and the image point fluctuation at the time of changing the magnification is corrected by moving the fourth lens group; the third lens group and the fourth lens A zoom lens having a light-transmitting parallel plane plate between groups and satisfying the following conditional expressions. 1 <β _II · β _III / f <2 (1) β _II : lateral magnification of the second lens group β _III : lateral magnification of the third lens group f: focal length of the entire lens system 4. The fourth lens group includes a negative meniscus lens having a convex surface facing the object side and a convex lens joined to a concave surface on the image side of the meniscus lens, the lenses being arranged in order from the object side, The zoom lens according to claim 3, wherein the following conditional expression is satisfied. _{0.27 <r x / f IV <} 0.33 ......... (2) r x: curvature of the concave on the image side of the meniscus lens radius f _IV: focal length of the fourth lens group 5. A negative meniscus lens having a convex surface facing the object side, a biconcave lens, and a convex lens joined to the image-side concave surface of the biconcave lens, wherein the second lens group is arranged in order from the object side. The zoom lens according to claim 3, wherein the zoom lens satisfies the following conditional expression. ν _Y −ν _Z > 35 (35) | n _Y −n _Z |> 0.23 (4) ν _Y : Abbe number of the material of the biconcave lens ν _Z : above Abbe number of the material of the biconvex lens n _Y : refractive index of the material of the biconcave lens with respect to d-line n _Z : refractive index of the material of the biconvex lens with d-line