JP2581187B2 - Rear focus zoom lens - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a rear focus type zoom lens,
More particularly, the present invention relates to a rear focus type zoom lens suitable for a zoom lens having a large aperture ratio of about 6 and an F number of about 1.4, which is used for a photographic camera, a video camera, a broadcast camera, and the like, and which has a high zoom ratio.

2. Description of the Related Art Conventionally, various types of zoom lenses such as a photographic camera and a video camera adopting a so-called rear focus type in which a lens group other than the first group on the object side is moved to perform focusing is proposed. ing.

In general, the rear focus method is performed by moving a relatively small and light lens group, and thus has a feature that the driving force of the lens group is small and quick focusing can be performed.

For example, in order from the object side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power for zooming, a third lens unit having a negative refractive power for correcting an image plane variation due to zooming, and a positive lens. Of the fourth group of the refractive power of
In a so-called four-unit zoom lens including one lens group, a rear-focus type zoom lens in which a third unit is moved to perform focusing has been proposed.

In Japanese Patent Application Laid-Open No. 58-136012, a zooming unit is composed of three or more lens groups, of which some of the lens groups are moved to perform focusing or a rear focus type zoom lens is proposed. I have.

However, in these zoom lenses, for example, even when the object distance is the same, the extension amount of the focus lens group varies depending on the difference in the zoom position, that is, the difference in the focal length, and the extension amount changes in a quadratic curve or discontinuously. May come.

Further, in such a zoom lens, if the zoom ratio is increased, a large space for moving the focus lens group must be taken on the wide-angle side, and the lens system increases.
In addition, if the same rear focus method as described above is employed, the amount of extension of the focus lens group for the same object distance may be significantly larger at the telephoto end than at the wide-angle end.

Further, in such a zoom lens, the amount of movement of the image plane with respect to the amount of movement of the focus lens group, that is, the sensitivity increases on the telephoto side, and if this value is increased to some extent, it becomes mechanically difficult to control the movement of the focus lens group. Come.

On the other hand, if the sensitivity on the telephoto side is set to a value that can be controlled, the sensitivity at the wide-angle end will be too small, requiring more space for moving the focus lens group, and increasing the lens system. There were problems such as becoming more and more.

(Problems to be Solved by the Invention) The present invention employs a rear focus system, and when trying to achieve a large aperture ratio and a high zoom ratio, while preventing the entire lens system from being enlarged, the wide-angle end to the telephoto end. It is an object of the present invention to provide a rear focus type zoom lens having good optical performance over the entire zoom range up to and having a simple configuration that facilitates mechanical control of a focusing lens group.

(Means for Solving the Problems) In order from the object side, 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 Wherein the third group is a positive 31st lens,
An aperture stop, a 32nd lens having both lens surfaces convex, a 33rd meniscus lens having a convex surface facing the image surface side, and a 34th positive lens, and the fourth unit is located on the object side. A negative meniscus 41st lens with a convex surface facing, a 42th lens with both lens surfaces convex, and a 43rd lens with both lens surfaces convex. The second unit is moved to change the magnification. The fourth lens unit is moved to correct the image plane movement caused by zooming, and the fourth lens unit is moved to perform focusing. The focal length of the i-th lens unit is fi, and the j-th lens unit of the i-th lens unit is focused. Let the radius of curvature of the lens surface be
i, j, when the Abbe numbers of the materials of the positive lens and the negative lens in the fourth group are νP and νN, respectively, 1.2 <| R2,1 / f2 | <27 (1) 0.4 <| R3,5 /f3|<0.52 (2) 0.71 <| R4,2 / f4 | <0.93 (3) 32 <νP−νN (4)

(Embodiment) FIG. 1 is a schematic view of an embodiment showing a paraxial refractive power arrangement of a rear focus type zoom lens according to the present invention.

In the figure, I is a first group having a positive refractive power, II is a second group having a negative refractive power, III is a third group having a positive refractive power, and IV is a fourth group having a positive refractive power. The third lens group III has a positive refractive power.
1. Aperture stop SP and 3-2 group of positive or negative refractive power
III-2. In zooming from the wide-angle end to the telephoto end, the second lens unit is moved to the image plane side, and the image plane fluctuation caused by zooming is corrected by moving the fourth lens unit.

In addition, a rear focus type in which the fourth unit is moved on the optical axis to perform focusing is adopted. The solid line curve 4a and the dotted line curve 4b of the fourth lens group shown in the same figure show the image plane fluctuation caused by zooming from the wide-angle end to the telephoto end when focusing on an object at infinity and an object at a short distance, respectively. The movement locus for correction is shown. The first and third units are fixed during zooming and focusing.

In the present embodiment, the fourth unit is moved to correct the image plane fluctuation caused by zooming, and the fourth unit is moved to perform focusing. In particular, as shown by the curves 4a and 4b in the same figure, at the time of zooming from the wide-angle end to the telephoto end, the lens is moved so as to have a convex locus toward the object side. Thereby, the space between the third and fourth units is effectively used, and the overall length of the lens is effectively reduced.

In this embodiment, for example, when focusing from an object at infinity to an object at a short distance at the telephoto end, the straight line in FIG.
This is achieved by extending the fourth lens group forward as shown in FIG. 4c.

In the present embodiment, the rear focus method as described above is employed to effectively prevent an increase in the effective lens diameter of the first group, as compared with a conventional four-group zoom lens in which the first group is extended and focused. doing.

By disposing the aperture stop between the third and third lens subunits, the variation in aberration due to the movable lens unit is reduced, and the distance between the lens units in front of the aperture stop is shortened. The reduction of the front lens diameter is easily achieved.

In addition, the diameter of the aperture stop is reduced by disposing the third-first lens group having a positive refractive power in front of the aperture stop. In the above-described lens configuration, the lens shape and the like of each lens group are set as in the conditional expressions (1) to (4) to reduce the size of the entire lens system while zooming and focusing using the fourth group. Is well corrected.

 Next, the technical meaning of each of the above conditional expressions will be described.

Conditional expression (1) mainly relates to the radius of curvature of the lens surface closest to the object side of the second lens unit, and is used mainly to favorably correct the fluctuation of distortion due to zooming. If the lower limit of conditional expression (1) is exceeded, the fluctuation of distortion due to zooming will increase. Conversely, if the upper limit of the conditional expression (1) is exceeded, the fluctuation of coma from the middle to the periphery of the screen will increase.

Conditional expression (2) relates to the radius of curvature of the lens surface having negative refractive power on the object side of the negative third lens in the third group. If the negative refractive power exceeds the lower limit and the negative refractive power becomes too strong, the spherical aberration will be overcorrected over the entire zoom range. In addition to the shortage, coma aberration increases from the wide-angle end to an intermediate magnification range, and it becomes difficult to satisfactorily correct the coma.

Conditional expression (3) relates to the radius of curvature of the lens surface having a negative refractive power on the image plane side of the negative meniscus 41st lens in the fourth unit.
If the negative refractive power exceeds the lower limit and the negative refractive power becomes too strong, the positive astigmatism increases.If the negative refractive power exceeds the upper limit and the negative refractive power becomes too weak, the negative astigmatism increases. It becomes difficult to make a good correction.

Conditional expression (4) is for appropriately setting the Abbe numbers of the materials of the positive lens and the negative lens in the fourth group, and for favorably correcting lateral chromatic aberration over the entire zoom range. This is not good because the fluctuation of the chromatic aberration of magnification increases.

Next, numerical examples of the present invention will be described. In numerical examples
Ri is the radius of curvature of the i-th lens surface in order from the object side,
Is the i-th lens thickness and air gap from the object side, Ni and ν
i is the refractive index and Abbe number of the glass of the i-th lens in order from the object side.

Table 1 shows the relationship with each conditional expression in each numerical example. R26 and R27 are glass materials such as a face plate.

Numerical example 1 Numerical example 2 Numerical example 3 (Effects of the Invention) According to the present invention, as described above, the refracting power of the four lens units, the moving conditions of the second and fourth units in zooming, the lens shape of each lens unit, and the like are set, and at the time of focusing. By adopting a lens configuration for moving the fourth lens unit, it is possible to reduce the size of the entire lens system while achieving good aberration correction over the entire zoom range, and to achieve aberration correction during focusing. It is possible to achieve a rear-focus type zoom lens having an F-number of 1.4 and a large aperture ratio having a high optical performance with little.

[Brief description of the drawings]

FIG. 1 is a schematic view of an embodiment showing a paraxial refractive power arrangement of the present invention, FIG. 2 is a sectional view of a lens of Numerical Embodiment 1 of the present invention, and FIGS. It is a various aberration figure of Examples 1-3. In the aberration diagrams, (A) is the wide-angle end, (B) is the middle,
(C) is an aberration diagram at the zoom position at the telephoto end. In FIGS. 1 and 2, I, II, III, and IV are the first, second, third, and fourth groups, d is the d-line, g is the g-line, ΔM is the meridional image plane, and ΔS is the sagittal image plane. And SP are aperture stops.

Claims (1)

(57) [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. And the third group is a positive
31 lens, aperture stop, 32nd lens with both lens surfaces convex,
A negative meniscus 33rd lens with the convex surface facing the image surface side,
The fourth group has a positive meniscus 41th lens with the convex surface facing the object side, a 42th lens with both lens surfaces convex, and a 4th lens with both lens surfaces convex. No.
The zoom lens has a 43 lens, and performs zooming by moving the second unit, and corrects the image plane variation due to zooming by moving the fourth unit, and also moves the fourth unit to focus. The focal length of the i-th lens unit is fi, the radius of curvature of the j-th lens surface of the i-th lens unit is Ri, j, and the Abbe numbers of the materials of the positive lens and the negative lens in the fourth lens unit are νP, When νN, 1.2 <| R2,1 / f2 | <27 0.4 <| R3,5 / f3 | <0.52 0.71 <| R4,2 / f4 | <0.93 32 <νP-νN Rear focus zoom lens.