JPH0627374A - Zoom lens of rear focusing type - Google Patents

Abstract

PURPOSE:To appropriately cut frame flare of high order, to perform aberration correction satisfactorily, and to secure sufficient light quantity by providing a light shielding diaphragm moving integrally with a positive lens group at the image surface side of a positive lens. CONSTITUTION:A first lens group 12 is provided with a positive power as a whole, and is fixed on a fixed lens-barrel 10. A second lens group 14 is provided with a negative power as a whole, and is guided so as to move in a direction of optical axis by a guide bar 22, and is driven by a screw shaft 24, then, performs a variable power operation. A third lens group 18 is provided with the positive power as a whole, and is fixed on the fixed lens-barrel 10. A fourth lens group 20 is provided with the positive power as a whole, and is guided so as to travel in the direction of optical axis by guide bars 26, 28, and is travelled in the direction of optical axis by driving a screw shaft 32 rotated by a motor 30, and performs a compensator fucntion to perform the correction of an image surface according to a variable power and a focusing function. The fourth lens group 20 is held with a lens holding frame 20A, and is provided with a light shielding part 20B extended from the holding frame 20A.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvements in zoom lens and barrel structures in optical systems such as video cameras and still cameras.

[0002]

2. Description of the Related Art In recent years, various zoom lenses for photographic cameras, video cameras, etc. have been proposed which employ a so-called rear focus type in which a lens other than the first lens unit on the object side is moved for focusing. There is.

In the rear focus type zoom lens, the effective diameter of the first lens group is smaller than that of a general conventional zoom lens in which the first lens group is moved to perform focusing, so that it is easy to downsize the entire lens system. Also, close-up photography, especially very close-up photography, becomes easier, and since the relatively small and lightweight lens group is moved, the driving force of the lens group can be small,
It has features such as quick focusing.

As such a rear focus type zoom lens, there is, for example, JP-A-58-160913.

[0005]

Generally, when a rear focus system is adopted in a zoom lens, the entire lens system is small and lightweight as described above, and quick focusing is possible. However, on the other hand, the aberration variation during focusing becomes large, and it becomes difficult to maintain high optical performance over the entire object distance from an infinitely distant object to a short-distance object.

In particular, when the focal length is on the telephoto side, the amount of movement of the focus lens group with respect to a short-distance object is large, and as a result, the off-axis ray bundle passes through the optical system more than usual, as shown in FIG. These ray bundles cause high-order coma flare and deteriorate optical performance. It is an object of the present invention to appropriately cut these high-order coma flares, perform good aberration correction, and secure a sufficient amount of light.

[0007]

A feature of the present invention resides in that in a zoom lens for correcting and focusing an image plane variation by at least a positive lens group on the image plane side of a variable power lens group, A light-shielding diaphragm that moves integrally with the positive lens group is provided on the image plane side of the lens group.

In particular, the zoom lens is composed of four lens groups, in order from the object side, a positive first lens group, a negative second lens group, a positive third lens group, and a positive fourth lens group. The second lens group moves on the optical axis due to the magnification change, and the variation of the image plane due to the magnification change is corrected by moving the fourth lens group. Further, the fourth group moves for focusing, The light blocking diaphragm is arranged on the image plane side of the four lens group.

[0009]

EXAMPLES Examples of the present invention will be described below.

FIG. 1 is a side sectional view showing a zoom lens apparatus according to an embodiment of the present invention.

In the figure, reference numeral 10 indicates a fixed lens barrel. The first lens group 1 is attached to the fixed lens barrel 10 from the object side.
2, a second lens group 14, a diaphragm unit 16, a third lens group 18, and a fourth lens group 20 are arranged.

The first lens group 12 has a positive power as a whole and is fixed to the fixed lens barrel 10. Second lens group 1
4 has a negative power as a whole, is guided by the guide bar 22 so as to be movable in the optical axis direction, and is driven by a screw shaft 24 which is rotationally driven by a motor (not shown) to perform a variable power operation. The aperture unit 16 is adjusted to have an appropriate amount of light by changing the aperture diameter of the aperture blades by a drive source (not shown). The third lens group 18 has a positive power as a whole and is fixed to the fixed lens barrel 10.
The fourth lens group 20 has positive power as a whole, is guided to be movable in the optical axis direction by the guide bars 26 and 28, and is moved in the optical axis direction by driving the screw shaft 32 rotated by the motor 30. Performs a compensator and focus function that corrects the image plane associated with zooming.

The fourth lens group 20 includes a lens holding frame 20A.
The holding member has a light blocking diaphragm portion 20B extended from the holding frame 20A.

Reference numeral 20 denotes a fourth lens located at the end of the zoom lens.
As shown in FIG. 3, the lens group compensates for the image plane movement due to the movement of the second lens group, and moves along the optical axis for focusing. This fourth of these zoom lenses
The movement locus of the lens group draws a convex shape on the object side with respect to zooming from the wide-angle end to the telephoto end as shown in, for example, Japanese Patent Laid-Open No. 3-12622, and as for focusing, as it goes from the wide-angle to the telephoto side The amount of extension of the focusing lens (fourth lens group) gradually increases. Then, the fourth lens group is mainly extended to the subject side with respect to the closest object in the intermediate zoom range.

As described above, the fourth lens group as the focus lens group of the zoom lens moves along the optical axis for zooming and focusing, but when the fourth lens group is extended to the object side, the upper ray is moved. Will tend to enter, and high-order flare components will be generated.

Therefore, in this embodiment, as shown in FIGS. 2 and 4, the lens holding frame of the fourth lens group, which is the focus lens and the final lens group, maintains good optical performance and cuts high-order flare. A light blocking diaphragm is provided. In order to effectively cut flare, as shown in FIG. 2, the final diameter of the final lens in the fourth lens group is Φ ₁ , the diameter of the light-shielding diaphragm is Φ ₂ , and the curvature of the final lens surface is The radius is R ₁ , and the distance from the edge of the final lens to the diaphragm is L
When

[0017]

[Outside 2] It is desirable to determine various factors so as to satisfy the following conditions.

The first expression is an expression relating to the radius of curvature of the lens closest to the image side in the fourth lens group, the lens outer diameter, and the distance to the light-shielding stop. The denominator on the right side is from the optical axis center point of the final lens to the lens. It refers to the depth to the outer periphery, and the molecule refers to the distance to the light blocking diaphragm. That is, this conditional expression means that the light-shielding diaphragm can exert its effect only when it is located on the image plane side more than half from the outer periphery of the final lens to the apex of the lens. Further, the second formula refers to the ratio of the outer diameter of the final lens to the diameter of the light-shielding diaphragm, and if it exceeds the lower limit, it becomes difficult to properly secure the amount of light, and if it exceeds the upper limit, the light-shielding diaphragm becomes meaningless. .

The above is the conditional expression of the present invention, but in order to achieve the present invention more effectively, a light-shielding line or the like for preventing the reflection of light rays is provided on the inner wall of the lens barrel of the fourth group. It is good to provide or taper the narrowed portion. Further, in order to reduce the cost, the fourth lens barrel structure and the light blocking diaphragm may be integrated into a plastic structure.

In the above embodiment, positive, negative, positive,
Although the positive four-group rear focus zoom lens has been described, other types of rear focus zoom lenses such as positive, negative, negative, positive, positive rear focus zoom lenses can be used for focusing positive lens groups. A configuration may be used in which a light blocking diaphragm is provided.

[0021]

According to the present invention, it is possible to secure a sufficient amount of light while maintaining high optical performance over the entire object distance from an object at infinity to a near object by appropriately cutting high-order coma flare. Can be achieved.

[Brief description of drawings]

FIG. 1 is a sectional view of a zoom lens device according to the present invention.

FIG. 2 is a diagram showing a configuration of a focus lens group.

FIG. 3 is a diagram showing a moving state of a focus lens group.

FIG. 4 shows a focus lens group and its optical path.

[Explanation of symbols]

 20 4th group lens 20A lens holding frame 20B light-shielding diaphragm

Claims (3)

[Claims] 1. A zoom lens for correcting and focusing an image plane variation by a positive lens group at least on the image plane side of a variable power lens group, wherein the positive lens group is provided on the image plane side of the positive lens group. A rear-focus type zoom lens having a light-shielding diaphragm that moves integrally. 2. The zoom lens comprises, in order from the object side, four lens groups of a positive first lens group, a negative second lens group, a positive third lens group, and a positive fourth lens group,
The second lens group moves on the optical axis for zooming, and the variation of the image surface due to zooming is corrected by moving the fourth lens group. Further, the fourth group moves for focusing, The rear focus type zoom lens according to claim 1, wherein the light-shielding diaphragm is disposed on the image plane side of the fourth lens group. 3. The outermost diameter of the last lens in the fourth lens group, which is Φ ₁ , the diameter of the light-shielding diaphragm, Φ ₂ , the radius of curvature of the last lens surface, R ₁ , and the edge portion of the last lens. Let L be the distance from the aperture to the aperture. The rear focus type zoom lens according to claim 2, wherein the light-shielding diaphragm is configured to satisfy the following condition.