JPH1144847A - Rear focus type zoom lens and image pickup device using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a zoom lens having excellent performance all over the zoom area and all over the subject distance while sufficiently securing a back focus space by monotonously moving a 2nd group and a 3rd group in a direction opposite to each other so that variable power may be performed and moving a 4th group so that focusing may be performed. SOLUTION: This zoom lens is equipped with a 1st group L1 having positive refractive power, the 2nd group L2 having negative refractive power, the 3rd group L3 having the positive refractive power and the 4th group L4 having the positive refractive power, and the 4th group L4 has a 41st group L41 having the negative refractive power and a 42nd group L42 having the positive refractive power. In the zoom lens, in the case of performing the variable power from a wide-angle end to a telephoto end, the 2nd group L2 is monotonously moved to an image surface side as shown by an arrow, and the movement of an image surface associated with the variable power is corrected by monotonously moving the 3rd group L3 to an object side, which is a reverse direction to the movement of the 2nd group L2 Rear focus that focusing is performed by moving a part or all (41st group L41 in this case) of the 4th group L4 on an optical axis is adopted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rear focus type zoom lens and an image pickup apparatus using the same, and more particularly, to providing a color separation prism on an image plane side used for an image pickup apparatus such as a video camera and a broadcast camera. The present invention relates to a small rear-focus type zoom lens having a long back focus as large as possible, a large aperture ratio, a high zoom ratio, and a short overall lens length, and an image pickup apparatus using the same.

[0002]

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

As one means for achieving these objects, there is known a so-called rear focus type zoom lens which performs focusing by moving a lens group other than the first group on the object side.

In general, a rear focus type zoom lens has a smaller effective diameter of the first lens group than a zoom lens which moves and focuses the first lens group, so that the entire lens system can be easily miniaturized, and close-up photography can be performed. In particular, extremely close-up photography is facilitated, and the relatively small and lightweight lens group is moved, so that the driving force of the lens group is small and quick focusing can be performed.

As such a rear focus type zoom lens, Japanese Patent Laid-Open Publication No. 3-15813 discloses a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, in order from the object side.
The zoom lens has four lens groups, a third group having a positive refractive power and a fourth group having a positive refractive power, and performs zooming by changing the distance between the second group and the third group. A zoom lens that focuses by moving some lens groups is disclosed.

In Japanese Patent Application Laid-Open Nos. 61-296317 and 61-296318, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a positive refractive power are sequentially arranged from the object side. The zoom lens has four lens groups, a third group of power and a fourth group of positive refractive power, and performs zooming by changing the distance between the second and third groups. And an aperture stop is arranged between
A zoom lens that performs focus by moving a group is disclosed.

On the other hand, with the recent high performance (digitalization) of video decks, various improvements have been made to the image quality of video cameras. One of them is color separation of an image by a color separation optical system. Such a zoom lens for a video camera is disclosed in, for example, Japanese Patent Application Laid-Open Nos.
Japanese Patent Application Laid-Open Nos. 51199, 7-199069, 7-270684, and the like.

[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.

On the other hand, on the other hand, while ensuring a long back focus enough to dispose a color separation prism, fluctuations in aberrations during focusing are reduced, and high optical performance is obtained over the entire object distance from an object at infinity to an object at a short distance. If so, the lens configuration becomes very difficult.

Particularly, in a zoom lens having a large aperture ratio and a high zoom ratio, it is very difficult to obtain high optical performance over the entire zoom range and over the entire object distance while securing a long back focus.

In the above-mentioned Japanese Patent Application Laid-Open No. 3-158613, the size of the entire lens system is reduced by using a rear focus system. However, in order to move the diaphragm integrally with the third lens unit, drive control of the diaphragm is performed. The IG meter that moves is also moved along with zooming, so that the mechanism tends to be complicated.

In general, when the refractive power of each lens unit is increased in a zoom lens, the amount of movement of each lens unit for obtaining a predetermined zoom ratio is reduced, and a high zoom ratio can be achieved while shortening the overall length of the lens. It becomes possible. However, if the refractive power of each lens group is simply increased, the aberration fluctuation accompanying zooming becomes large, and it becomes difficult to obtain good optical performance over the entire zooming range especially when achieving high zooming.

The present invention employs a rear focus system, and has a long back focus such that a color separation prism, an optical filter, and the like can be arranged on the image plane side.
And with a large aperture ratio, it has a high zoom ratio of 14 to 16 times, over the entire zoom range from the wide-angle end to the telephoto end, and over the entire object distance from infinite objects to super close objects,
It is an object of the present invention to provide a rear focus type zoom lens having excellent optical performance and an imaging apparatus using the same.

[0014]

According to the present invention, there is provided a rear focus type zoom lens comprising: (1-1) a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a positive refractive power in order from the object side. The zoom lens has four lens groups, a third group of power, and a fourth group of positive refractive power. The second and third groups are monotonously moved in opposite directions to perform zooming. Is characterized by moving a part or all of the focus.

(1-2) First positive refractive power in order from the object side
The zoom lens includes four lens groups, 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. To move monotonically to zoom,
The fourth unit is a first unit having a negative refractive power and a second unit having a positive refractive power.
It has two lens groups, and is characterized in that focusing is performed by moving one or both of the 41st and 42nd groups.

The image pickup apparatus of the present invention has (2-1) a rear focus type zoom lens having the constitutions (1-1) and (1-2), and a color separation optical system on its image plane side. It is characterized by:

[0017]

FIG. 1 is a sectional view of an essential part of an image pickup apparatus having a rear focus type zoom lens according to a first embodiment of the present invention, and FIGS. FIG. 8 is an aberration diagram at a zoom position at a middle or telephoto end.

FIG. 5 is a sectional view of a main part of a second embodiment of an image pickup apparatus having a rear focus type zoom lens according to the present invention.
FIGS. 6, 7 and 8 are aberration diagrams of the second embodiment at zoom positions at the wide-angle end, the middle position, and the telephoto end.

FIG. 9 is a sectional view of a main part of a third embodiment of an image pickup apparatus having a rear focus type zoom lens according to the present invention.
FIGS. 10, 11, and 12 show the wide-angle end, the middle,
FIG. 7 is an aberration diagram at a zoom position at a telephoto end.

FIG. 13 is a sectional view of an essential part of an image pickup apparatus having a rear focus type zoom lens according to a fourth embodiment of the present invention. FIGS. 14, 15, and 16 show a wide angle end, an intermediate position, and a telephoto end of the fourth embodiment. 4 is an aberration diagram at a zoom position of FIG.

In the figure, 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. , A negative 41st lens unit and a sexual 42nd lens unit. SP denotes an aperture stop, which is arranged in front of the third lens unit L3. GA is a protective glass provided as needed for protecting the zoom lens, and GB is a glass block such as a color separation prism, a face plate, and a filter. IP is an image plane on which an image sensor such as a CCD is arranged.

The first to fourth units L1 to L4 constitute one element of a zoom lens (zoom lens unit) ZL. The glass block GB and the imaging device are housed in the camera body CB. The zoom lens unit ZL is detachably attached to the camera body CB via a mount member C.

In this embodiment, when zooming from the wide-angle end to the telephoto end, the second lens unit is monotonously moved to the image plane side as indicated by an arrow, and the image plane fluctuation due to zooming is changed to the third lens unit. Correction is made by moving monotonously to the object side.

Also, a rear focus system is adopted in which a part or the whole of the fourth unit (the 41st unit L41 in this embodiment) is moved on the optical axis to perform focusing.

In the figure, the first unit L41 is composed of a negative lens having both concave lens surfaces, and the negative lens is indicated by an arrow 4a.
And focuses from an object at infinity to an object at a short distance. In the present embodiment, the fourth
The second lens unit L42 is fixed at the time of focusing.
It may be moved at a different speed together with or independently of the first lens unit L41.

The video camera (imaging apparatus) of the present invention comprises at least the above-mentioned zoom lens, a color separation element, an image pickup element corresponding to each color divided by the color separation element, an image pickup signal processing circuit and the like. .

In the present embodiment, the second and third units sandwiching the stop SP are monotonously moved in opposite directions during zooming, so that the second unit has a zooming effect and the third unit is also zoomed. A high zoom ratio is achieved by giving a doubling effect. Also,
The space between the second unit and the third unit is effectively used, and the movement amount of the second unit and the third unit on the optical axis is reduced as a whole to shorten the overall length of the lens.

In a general zoom lens, the first group accounts for 50 to 80% of the weight of the lens. Therefore, in order to reduce the weight of the zoom lens, it is effective to reduce the volume by reducing the material of the first lens group or reducing the diameter of the first lens group. Therefore, in the present embodiment, the diameter of the first lens unit is reduced to reduce the weight of the entire zoom lens.

That is, as compared with a zoom lens in which the stop SP is disposed behind the third unit, the stop is closer to the second unit than the second unit.
The lens diameter of the first group is reduced by arranging it at a substantially intermediate position of the optical system between the group and the third group.
In this embodiment, the lens diameter of the first group determined by the incident light at the wide-angle end and the axial light at the telephoto end (F-number light)
The aperture and the lens group are arranged such that the lens effective diameter becomes smaller at both the lens diameters of the first group determined by the following formula.

In order to lengthen the back focus, the fourth lens unit is composed of two lens units, a first lens unit having a negative refractive power and a second lens unit having a positive refractive power. As a result, the fourth lens unit is made a retrofocus type, and a long back focus is secured. The first unit is fixed during zooming and focusing.

As described above, the zoom lens of this embodiment is composed of four lens groups as a whole, and the moving conditions of each lens group during zooming and focusing, the lens configuration of the fourth group, and the like are appropriately set. As a result, high optical performance is obtained over the entire zoom range and the entire object distance while securing a predetermined back focus.

Next, the features of another lens configuration of the rear focus type zoom lens of the present invention will be described.

[A1] When the distance between the second group and the third group and the distance between the second group and the aperture stop at the zoom position at the wide-angle end are D23W and D2SW, respectively, 0.3 <D2SW / D23W <0. 65 ‥‥ (1) is satisfied.

This effectively achieves a reduction in the diameter of the first lens unit, a reduction in the overall length of the lens unit, and excellent aberration correction.

In particular, by arranging the second and third lens units and the aperture so as to satisfy the conditional expression (1), each of them can reduce the overall length of the lens while achieving high magnification without causing mechanical interference. And a reduction in the diameter of the first group lens.

When the stop is moved closer to the first lens unit, the lens diameter of the first lens unit decreases. On the contrary, the distance of the final lens unit farther from the stop increases. This conditional expression (1) is mainly for reducing the overall lens diameter while maintaining a good balance between the lens diameter of the first lens unit and the lens diameter of the final lens unit.

If the lower limit of conditional expression (1) is exceeded, the lens diameter of the first lens unit will decrease, but conversely, the lens diameter of the final lens unit will increase. Then, since the position of the off-axis ray incident on the fourth unit changes greatly due to the movement of the third unit at the variable power, it becomes difficult to satisfactorily correct the aberration fluctuation. Further, if the aperture position is farther from the first group than the upper limit value, the lens diameter of the first group becomes large, which is not preferable.

[A2] The focal length of the second lens unit is f2, and the F-number and focal length of the entire system at the wide-angle end are FN.
W, fW, the focal length of the entire system at the telephoto end is fT,

[0039]

(Equation 3) 0.53 <| f2 | × FNW / fM <0.84 (2) is satisfied.

Thus, the fluctuation of the coma caused by the zooming is effectively corrected. This conditional expression (2)
Restricts the focal length of the second lens unit, and is greatly related to the F-number FNW at the wide-angle end. The second lens unit moves on the optical axis during zooming because it mainly has a zooming function. For this reason, the aberration variation that occurs must be corrected well. In particular, coma aberration fluctuates greatly with zooming. Conditional expression (2) is for correcting this satisfactorily.

Beyond the lower limit of conditional expression (2), F at the wide-angle end
If the NW is made brighter or the focal length f2 of the second lens unit is shortened, a high-order coma flare is largely generated and correction becomes difficult. Conversely, if the focal length of the second lens unit is excessively increased beyond the upper limit or the FNW at the wide-angle end is darkened, the optical performance is increased, but the overall length of the lens is increased, and miniaturization cannot be achieved.

[A3] The amount of air conversion from the fourth lens unit to the image plane when focusing on an object at infinity at the wide-angle end is represented by BF.
W, where fW, FNW, and ωw are the focal length of the entire system at the wide-angle end, the F-number, and the half angle of view, respectively.

[0043]

(Equation 4) The following conditions are satisfied.

Thus, a predetermined length of back focus is ensured. If the F-number at the wide-angle end is made brighter than the lower limit of conditional expression (3), many high-order spherical aberrations and coma occur, making it difficult to satisfactorily correct them.
Conversely, when the F-number exceeds the upper limit and the F-number becomes darker, the axial ray bundle becomes thinner, and the color separation prism disposed between the final lens surface of the fourth group and the image plane can be downsized.
Although it is not necessary to lengthen the back focus, it must be lengthened, which leads to an increase in the overall length of the lens, which is not preferable.

[A4] One of the objects of the present invention is to obtain a zoom lens having a high zoom ratio. For this reason, it is desirable that the chromatic aberration generated due to zooming be mainly canceled in the first and second units.

However, the manner of occurrence of chromatic aberration of magnification accompanying zooming differs greatly between the first and second groups, and tends to be excessively corrected at the wide-angle end. Accordingly, the chromatic aberration of magnification of the fourth group is insufficiently corrected, so that the balance of chromatic aberration as a whole is maintained.

On the other hand, axial chromatic aberration can be corrected without a large loss of balance when the zoom ratio is small.
When aiming for a high zoom ratio and a large aperture as in the present invention, axial chromatic aberration is insufficiently corrected as a whole, and it is difficult to maintain high optical performance.

Therefore, in the present invention, the third lens unit has a cemented lens of a positive lens having an appropriate refractive power and Abbe number and a negative meniscus lens having a strong concave surface facing the object side, so that it is optimally provided over the entire zoom range. Chromatic aberration is corrected, and the F / #
It has a large diameter of about 1.6 NW and maintains high optical performance.

Basically, if a lens is joined in each lens group, eccentricity in the group can be effectively suppressed, and product performance can be stabilized.
One degree of freedom in design is reduced, and it becomes difficult to achieve sufficient initial performance while satisfying the specifications of a large aperture and a small zoom.

Therefore, in this embodiment, the third lens unit has a cemented lens, and the third lens unit employs an aspherical surface, thereby effectively suppressing intra-group eccentricity as shown in Numerical Examples 2-3. To obtain a large-aperture zoom lens with higher optical performance.

The aspherical surface provided in the third lens unit is mainly used to correct a higher-order flare component of spherical aberration at the wide-angle end, and for that purpose, it is effective to apply it to a stronger convex surface. is there. Therefore, it is best to employ an aspheric surface for the positive lens of the third group having the largest positive refractive power.

[A5] By adopting an aspherical surface in the fourth lens unit, a zoom lens having high optical performance is achieved despite being a large-diameter, ultra-high-magnification zoom lens as shown in Numerical Example 3.

The aspherical surface provided in the fourth lens group is mainly used for correcting a higher-order flare component of spherical aberration and astigmatism. For that purpose, it is effective to apply a stronger convex surface. is there. Therefore, it is best to employ an aspheric surface for the positive lens of the fourth group having the largest positive refractive power.

[A6] In the order from the object side, the lens sub-unit in the order from the object side, the positive 421st lens having a convex lens surface on the image side, the negative 422 lens having a meniscus shape with the convex surface facing the object side, and both lens surfaces being convex The 422th lens comprises a positive 423th lens of
When the radii of curvature of the lens surface on the image side of the lens and the lens surface on the object side of the 423rd lens are Ra and Rb, respectively, and the focal length at the wide-angle end of the entire system is fW, 0 ≦ | 1 / Ra−1 / Rb | · FW <0.11 ‥‥ (4) is satisfied.

This conditional expression (4) is for suppressing high-order astigmatism and spherical aberration components occurring in the fourth lens unit between the positive lens and the negative lens. is there. The lower limit has the same effect as a cemented lens or
It will be in a very stable state. When the value exceeds the upper limit, the correction of the higher-order flare component is concentrated on the higher-order term of the aspheric surface provided in the fourth lens unit.

Since the aspherical surface provided in the fourth lens group is basically intended to correct spherical aberration, it is desirable that the positive refractive power becomes weaker toward the periphery of the lens.

Next, numerical examples of the present invention will be described. In the numerical examples, Ri is the radius of curvature of the ith lens surface in order from the object side, Di is the ith lens thickness and air spacing in order from the object side, and Ni and νi are the ith lens in order from the object side. Are the refractive index and Abbe number of the glass. The aspherical shape has a radius of curvature R at the center of the lens surface, the optical axis direction (the traveling direction of light) is the X axis, the direction perpendicular to the optical axis is the Y axis,
When B, C, D and E are each aspheric coefficients

[0058]

(Equation 5) This is represented by “E-0X” means “× 10 ^−X ”. Table 1 shows the relationship between the above-described conditional expressions and various numerical values in the numerical examples.

Further, R25 to R25 in Numerical Examples 1 to 3
R28 and R28 to R31 in Numerical Example 4 indicate glass blocks such as a color separation prism, an optical filter, and a face plate.

[0060]

[Outside 1]

[0061]

[Outside 2]

[0062]

[Outside 3]

[0063]

[Outside 4]

[0064]

[Table 1]

[0065]

As described above, according to the present invention, by setting each element, it is possible to arrange a color separation prism and an optical filter on the image plane side while adopting the rear focus method. Has a long back focus,
And with a large aperture ratio, it has a high zoom ratio of 14 to 16 times, over the entire zoom range from the wide-angle end to the telephoto end, and over the entire object distance from infinite objects to super close objects,
A rear focus type zoom lens having excellent optical performance and an imaging apparatus using the same can be achieved.

[Brief description of the drawings]

FIG. 1 is a sectional view of a main part of a first embodiment of an imaging apparatus having a rear focus zoom lens according to the present invention;

FIG. 2 is an aberration diagram at a wide-angle end of an imaging device having a rear-focusing zoom lens according to a first embodiment of the present invention.

FIG. 3 is an intermediate aberration diagram of Embodiment 1 of the imaging apparatus having the rear focus zoom lens of the present invention.

FIG. 4 is an aberration diagram at a telephoto end of an image pickup apparatus including a rear-focus type zoom lens according to a first embodiment of the present invention.

FIG. 5 is a sectional view of a principal part of a second embodiment of an imaging apparatus having a rear focus zoom lens according to the present invention;

FIG. 6 is an aberration diagram at a wide-angle end of an imaging apparatus having a rear focus zoom lens according to a second embodiment of the present invention.

FIG. 7 is an intermediate aberration diagram of Embodiment 2 of the imaging apparatus having the rear focus type zoom lens of the present invention.

FIG. 8 is an aberration diagram at a telephoto end of an image pickup apparatus having a rear focus zoom lens according to a second embodiment of the present invention.

FIG. 9 is a sectional view of a main part of a third embodiment of an imaging apparatus having a rear focus type zoom lens according to the present invention;

FIG. 10 is an aberration diagram at a wide-angle end of an imaging device having a rear-focusing zoom lens according to a third embodiment of the present invention.

FIG. 11 is an intermediate aberration diagram of Embodiment 3 of the imaging apparatus having the rear focus zoom lens of the present invention.

FIG. 12 is an aberration diagram at a telephoto end of an imaging apparatus having a rear-focusing zoom lens according to a third embodiment of the present invention.

FIG. 13 is a cross-sectional view of a principal part of a fourth embodiment of the imaging apparatus having the rear focus zoom lens according to the present invention;

FIG. 14 is an aberration diagram at a wide-angle end of an imaging device including a rear-focusing zoom lens according to a fourth embodiment of the present invention.

FIG. 15 is an intermediate aberration diagram of Embodiment 4 of the imaging apparatus having the rear focus zoom lens of the present invention.

FIG. 16 is an aberration diagram at a telephoto end of an imaging apparatus including a rear-focusing zoom lens according to a fourth embodiment of the present invention.

[Explanation of symbols]

 L1 First group L2 Second group L3 Third group L4 Fourth group L41 First group L42 Second group SP Aperture IP Image plane d d-line g g-line S Sagittal image plane M Meridional image plane GB Glass block

Claims (8)

[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. The second group and the third group
The group is monotonously moved in the opposite directions to perform zooming, and the fourth
A rear-focus type zoom lens that performs focusing by moving part or all of a group. 2. Four lens groups of 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 this order from the object side. The second group and the third group
The group is monotonously moved in the opposite directions to perform zooming, and the fourth
The two groups are the 41st group of negative refractive power and the 42nd group of positive refractive power.
A rear focus type zoom lens having two lens groups and performing focusing by moving one or both of the first and second groups. 3. The distance between the second lens unit and the third lens unit and the distance between the second lens unit and the aperture stop at the zoom position at the wide-angle end are D23.
3. The rear focus type zoom lens according to claim 1, wherein a condition of 0.3 <D2SW / D23W <0.65 is satisfied when W and D2SW are satisfied. 4. The focal length of the second lens unit is f2, and the F-number and focal length of the entire system at the wide-angle end are FNW and fN, respectively.
W, fT is the focal length of the entire system at the telephoto end, and 4. The rear focus zoom lens according to claim 1, wherein the following condition is satisfied: 0.53 <| f2 | × FNW / fM <0.84. 5. An air conversion amount from the fourth lens unit to the image plane when focusing on an object at infinity at the wide-angle end is BFW, and the focal length of the entire system at the wide-angle end, the F-number, and the half angle of view are FW. , FNW, ωw 4. The rear focus type zoom lens according to claim 1, wherein the following condition is satisfied. 6. The lens system according to claim 2, wherein the first lens unit includes a negative lens having both concave lens surfaces, and the first lens unit is moved to perform focusing.
Rear focus type zoom lens as described in the item. 7. The first unit comprises a negative lens having both lens surfaces concave, and the second unit comprises a positive lens having a convex lens surface on the image surface side, and a meniscus negative lens having a convex surface facing the object side. 6. A rear focus system according to claim 2, wherein both lens surfaces are composed of three positive lenses having a convex surface, and said first lens unit is moved to perform focusing. Zoom lens. 8. An image pickup apparatus comprising: the rear focus type zoom lens according to claim 1; and a color separation optical system on an image plane side thereof.