JPH06235861A - Rear focus type zoom lens - Google Patents

Abstract

PURPOSE:To shorten a super near distance at a telephoto end to increase a magnification, and to attain a superior optical performance by appropriately setting the lateral magnification of the second group, the synthetic lateral magnification of the third and fourth groups, the focal distance of an entire system, open F number, and a photographic half-field angle. CONSTITUTION:A variable power from a wide angle edge to the telephoto end is performed by moving the second group L2 on an image surface side, an image surface fluctuation accompanied with the variable power is corrected by moving the forth group L4, and a focusing is performed by moving the fourth group L4. The second group L2 is constituted of three single lenses, that is, the negative 21th lens whose concave with a strong refractive power is faced on the image surface side, the negative 22th lens whose both lens surfaces are concaves, and the positive 23th lens whose convex with the strong refractive power is faced to an object side. Then, in the case of an infinite distant object at the telephoto end, the lateral magnification of the second group L2 is defined as beta2T, the synthetic lateral magnification of the third group L3 and the fourth group L4 is defined as beta34T, the focal distances of the entire system at the wide angle end and the telephoto end are respectively fW and fT, and the open F number and photographic half-field angle at the telephoto end are respectively FNOT and WT. then, when S2T is shown by an expression I, one of an expressions II is fulfilled.

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 more particularly to a photographic camera, a video camera,
Further, the present invention relates to a rear focus type zoom lens having a large aperture ratio with a variable power ratio of 13, an F number of about 1.8 and a high variable power ratio, which is used for broadcasting cameras and the like.

[0002]

2. Description of the Related Art Recently, as home video cameras have become smaller and lighter, remarkable progress has been made in the miniaturization of zoom lenses for image pickup. Especially, the total lens length and front lens diameter have been reduced. The effort is focused on simplification.

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

Generally, in a rear focus type zoom lens, the effective diameter of the first lens group is smaller than that of a zoom lens in which the first lens group is moved to perform focusing, which facilitates downsizing of the entire lens system and close-up photography. In particular, since extremely close-up photography is facilitated and the relatively small and lightweight lens group is moved, the driving force of the lens group is small, and quick focusing is possible.

As such a rear focus type zoom lens, for example, JP-A-62-24213, JP-A-63-247316 and JP-A-4-433.
In the publication No. 11, the first group having positive refracting power in order from the object side,
It has four lens groups, 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, and the second lens group is moved to perform zooming. The group is moved to perform image plane variation and focusing due to zooming.

In the rear focus type zoom lens proposed in Japanese Patent Laid-Open No. 4-43311, the fourth lens unit is located closer to the image plane at the telephoto end than at the wide-angle end. As a result, the moving distance during focusing of the fourth lens unit on the telephoto side is increased, and it is possible to focus on a super close-range (tele-macro shooting) object.

[0007]

Generally, when a rear focus system is adopted in a zoom lens, the entire lens system is downsized as described above, quick focusing is possible, and further close-up photography is facilitated. .

On the other hand, however, the aberration variation at the time of focusing becomes large, and it becomes very difficult to obtain high optical performance over the entire object distance from an object at infinity to a near object.

Particularly in a zoom lens having a large aperture ratio and a high zoom ratio, it becomes very difficult to obtain high optical performance over the entire zoom range and over the entire object distance.

In addition to this, if the paraxial refractive power arrangement as a zoom lens is properly arranged and the magnifying power at the super close range at the telephoto end is not sufficiently increased, the magnifying power will decrease.
There is a problem that a powerful enlarged image cannot be obtained.

The present invention adopts the rear focus method, and at the time of achieving a large aperture ratio and a high zoom ratio, shortens the super close distance at the telephoto end and increases the enlargement ratio, and at the same time, from 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 over the entire object distance from an object at infinity to a super close object.

[0012]

The rear focus type zoom lens of the present invention comprises (1-1) a first group having a positive refractive power, a second group having a negative refractive power, and a positive refractive power in order from the object side. It has four lens groups, a third lens group of power and a fourth lens group of positive refracting power, and moves the second lens group to the image side to perform zooming from the wide-angle end to the telephoto end, and zooming. The image plane variation caused by the movement is corrected by moving the fourth lens group, and the fourth lens group is moved to perform focusing. The second lens group has a negative 21st surface with a concave surface having a strong refractive power facing the image surface side. It consists of three single lenses: a lens, a negative 22nd lens whose both lens surfaces are concave surfaces, and a positive 23rd lens with a convex surface having a strong refractive power facing the object side. The lateral magnification of the second group is β _2T , and the lateral magnification of the composite of the third and fourth groups is β
_34T , the focal length of the entire system at the wide-angle end and the telephoto end is f
_{When W} , f _T , the open F number at the telephoto end and the shooting half angle of view are F _NOT and ω _T , respectively, and S _2T = {1- (β _2T ) ² } ・ (β _34T ) ²

[0013]

[Equation 4] It is characterized by satisfying either one of the above.

(1-2) The first group having a positive refracting power, the second group having a negative refracting power, the third group having a positive refracting power, and the fourth group having a positive refracting power in order from the object side. Has two lens groups, the second
The lens unit is moved to the image plane side to change the magnification from the wide-angle end to the telephoto end, and the image plane variation due to the magnification change is corrected by moving the fourth unit and moving the fourth unit. The fourth group has a cemented lens in which at least one set of a negative lens and a positive lens are cemented together, and at least one positive lens, and the fourth group has a wide-angle end and a telephoto end at infinity. When the lateral magnifications of the object are β _4W and β _4T , respectively

[0015]

[Equation 5] It is characterized by satisfying the following condition.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a lens cross-sectional view of Numerical Embodiment 1 to be described later of a rear focus type zoom lens of the present invention, and FIGS.
4 is a numerical example 1, FIGS. 5 to 7 are numerical examples 2, 8
10 is a numerical example 3, FIGS. 11 to 13 is a numerical example 4, FIGS. 14 to 16 are numerical examples 5, FIGS. 17 to 19 are numerical examples 6, and FIGS. 20 to 22 are numerical examples. 16 is a diagram of various types of aberration in Example 7. FIG.

In the aberration diagrams, FIGS. 2, 5, 8, 11, 1
4, 17, 20 are wide-angle end,
18 and 21 are intermediate, FIGS. 4, 7, 10, 13, 16, and 1.
Reference numerals 9 and 22 indicate the telephoto end.

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. . SP is an aperture stop, which is arranged in front of the third lens unit L3. G is a glass block such as a face plate or a filter.

In this embodiment, the second lens unit is moved to the image plane side as indicated by the arrow when the magnification is changed from the wide-angle end to the telephoto end, and the image plane variation due to the magnification change is corrected by moving the fourth lens unit. ing.

Further, a rear focus type is adopted in which the fourth lens unit is moved on the optical axis for focusing. The solid curve 4a and the dotted curve 4b of the fourth group shown in the same figure represent image plane fluctuations due to zooming from the wide-angle end to the telephoto end when focusing on an object at infinity and a near object, respectively. The movement locus for correction is shown. The first and third groups are fixed during zooming and focusing.

In the present embodiment, the fourth lens unit is moved to correct the image plane variation due to zooming, and the fourth lens unit is moved to perform focusing. Curve 4 in the figure
As shown in a and 4b, the zoom lens is moved so as to have a convex locus toward the object side during zooming from the wide-angle end to the telephoto end.
As a result, the space between the third group and the fourth group is effectively used, and the total lens length is effectively shortened.

In the present embodiment, for example, when focusing from an object at infinity to a near object at the telephoto end, the fourth lens unit is moved forward as indicated by a straight line 4c in FIG.

Next, the features of the lens structure will be sequentially described.

First, in the present embodiment, the second lens unit is constructed as described above, so that the amount of fluctuation of the distortion aberration is particularly reduced during zooming. With conventional general zoom lenses, distortion aberration exceeds -5% at the wide-angle end and + 5% at the telephoto end.
It was around.

On the other hand, in this embodiment, the distortion aberration is less than -5% at the wide-angle end and is drastically reduced to + 3% or less at the telephoto end.

Further, the position of the front principal point of the second group is set closer to the object side, the principal point distance from the first group is shortened, and the first group is brought closer to the diaphragm position. As a result, the height of the off-axis light beam entering the first group from the optical axis is lowered, the lens diameter of the first group is reduced, and the overall lens length is shortened and the size and weight are reduced.

In this embodiment, the negative 22nd lens and the positive 23rd lens in the second lens group are composed of a single lens.
Aberration correction is performed by an air lens formed by both lenses. As a result, various aberrations such as spherical aberration, coma and axial chromatic aberration are corrected in good balance.

In the present embodiment, the 22nd lens and the 2nd lens
Since the three lenses are composed of a single lens, the height from the optical axis when the on-axis light flux passing through the second lens group enters the 23rd lens after exiting the 22nd lens, the conventional general bonding It will be higher than when using a lens.

Therefore, the effects of various aberrations corrected by the 23rd lens become too strong. Therefore, the curvature of the object-side lens surface of the 23rd lens, the curvature of the image-side lens surface of the 21st lens, and the curvatures of both lens surfaces of the 22nd lens can be reduced accordingly.

In this embodiment, various aberrations generated from the second lens group are thereby reduced, and aberration fluctuations due to zooming are corrected well.

Next, in the present embodiment, the fourth group is composed of a cemented lens having at least one set of a negative lens and a positive lens cemented together.
By using a single positive lens, the fluctuations of various aberrations such as spherical aberration and astigmatism at the time of focusing in the fourth group are favorably corrected.

Further, the third lens unit has at least one positive lens and one negative lens to favorably correct spherical aberration and coma at the wide-angle end. In particular, the spherical aberration generated by the positive lens is corrected by the negative lens.

Further, coma aberration is satisfactorily corrected by the air lens formed by the positive lens and the negative lens. still,
By providing an aspherical surface on at least one lens surface in the third group, high optical performance is obtained over the entire screen.

At this time, the aspherical shape in the third lens group is the intersection of at least one point with the curvature (reference curvature) connecting the point of the optical axis of the aspherical surface and the point of the maximum ray height passing through the aspherical surface. Has a lens shape.

As described above, in this embodiment, the lens configuration of each lens unit, the lateral magnification of each lens unit, and optical constants such as the focal length of the entire system at the wide-angle end and the telephoto end are properly set. This makes it possible to obtain a zoom lens with a high zoom ratio that has a very short distance (tele-macro distance) at the telephoto end and a large magnifying power, and has good optical performance over the entire zoom range and over the entire object distance. There is.

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

Conditional expression (1) is the ratio f _T / f _W of the focal lengths at the telephoto end and the wide-angle end when focusing on an object at infinity, that is, the square root of the zoom ratio and the lateral magnification of the second lens unit at the telephoto end. Regarding the ratio of β _2T , it is mainly for making the super close range shorter.

In the conventional zoom lens, the lateral magnification of the second lens unit is

[0039]

[Equation 6] Using the neighborhood of.

As a result, the position of the fourth lens unit on the optical axis was substantially the same at the wide-angle end and the telephoto end. Therefore, the movable distance of the fourth lens unit at the telephoto end was shortened, and the super close distance was relatively long.

Conditional expression (1) is for effectively shortening the super close range. If the lower limit value of the conditional expression (1) is exceeded and the value becomes smaller, the value becomes closer to the conventional example described above, which is not good.
On the other hand, when the value exceeds the upper limit and becomes larger, the position of the fourth lens unit on the optical axis at the telephoto end becomes closer to the image plane side than at the wide-angle end, and the movable distance of the fourth lens unit becomes longer, which makes it possible to shorten the super close distance. However, the distance between the fourth lens group and the image plane becomes too short, and the configuration of the lens barrel physically interferes.

Conditional expression (2) defines the ratio of the vertical magnification of the second lens unit at the telephoto end to the F number by the image height. If the F-number at the telephoto end becomes larger than the upper limit of conditional expression (2), the blur on the screen will not be a problem even if the position on the optical axis of the second lens unit deviates more than an arbitrary stop position. It is impossible to achieve a bright F-number zoom lens.

Further, when the image height becomes large, the whole lens system becomes large, which is not good. If the vertical magnification of the second lens unit at the telephoto end becomes larger than the lower limit value and the stop position of the second lens unit deviates more than an arbitrary position, blurring on the screen becomes large, which is not preferable.

Further, in order to improve it, the position of the second lens group must be controlled with high accuracy, which requires a highly accurate lens barrel, actuator, sensor, etc., which is not good.

Conditional expression (3) relates to the lateral magnification ratio β _4T / β _4W at the telephoto end and the wide-angle end when the fourth lens unit focuses on an object at infinity.

In the conventional zoom lens, the position on the optical axis of the fourth lens unit is substantially the same at the wide-angle end and the telephoto end when focusing on an object at infinity. Therefore, the distance between the fourth lens unit and the third lens unit at the telephoto end becomes short, and it is difficult to secure the moving distance of the fourth lens unit for focusing on the closest object.

The conditional expression (3) is for effectively shortening the super close distance, and if it becomes smaller than the lower limit value of the conditional expression (3), it becomes close to the above-mentioned conventional example and is not good.

On the other hand, if the value exceeds the upper limit, the movable distance of the fourth lens unit becomes long and the super close distance can be shortened. However, aberration variation due to focusing becomes large, which is not preferable. Also, the fourth
It is not good because the number of lenses in the group must be increased and the moving lens group becomes large and heavy.

The rear focus type zoom lens, which is the object of the present invention, can be achieved by satisfying the above-mentioned various conditions, respectively, but further downsizing the entire lens system while covering the entire zooming range and the entire object distance. In order to obtain good optical performance, the following conditions should be satisfied.

(2-1) Let f be the focal length of the i-th group._i ,
The focal length of the entire system at the telephoto end is f _T At the telephoto end
Open F number and shooting half angle of view are F_NOT , Ω_T And
When

[0051]

[Equation 7] To satisfy the condition.

If the upper limit of conditional expression (4) is exceeded and the refracting power of the first lens unit becomes weaker, aberration correction will become easier.
The distance between the group and the diaphragm increases, and the lens diameter of the first group for securing the off-axis light beam increases. 1st below the lower limit
When the refracting power of the group becomes strong, the total lens length becomes short, but the interval with the second group becomes short, which causes physical interference, which is not good.

In the present invention, the photographing range at the super close range (tele-macro) is set within 35 mm, and in order to obtain a powerful magnified image, the lateral magnification of the entire system when focusing on the super close range object at the telephoto end is set. When M (M = image height / subject height)

[0054]

[Equation 8] It is better to satisfy the following condition. Outside this range, good images cannot be obtained.

Next, numerical examples of the present invention will be shown. In the numerical examples, Ri is the radius of curvature of the i-th lens surface in order from the first conjugate point side, Di is the i-th lens thickness and air gap from the first conjugate point side, and Ni and νi are the first conjugate points respectively. From the side, the refractive index and the Abbe number of the glass of the i-th lens are in order. However, R22 and R23 represent glass blocks such as a face plate and a filter.

The aspherical shape has an X axis in the optical axis direction, a Y axis in the direction perpendicular to the optical axis, and a traveling direction of light is positive, and γ is a paraxial radius of curvature k, A ₂ , A ₃ , A ₄ , A _5. Where each is an aspherical coefficient

[0057]

[Equation 9] It is expressed by the formula. The display of "D-0X" is "10.
^-X "means.

Table 1 shows the relationship between the above-mentioned conditional expressions and various numerical values in the numerical examples.

[0059]

[Outer 1]

[0060]

[Table 1]

[0061]

[Outside 2]

[0062]

[Table 2]

[0063]

[Outside 3]

[0064]

[Table 3]

[0065]

[Outside 4]

[0066]

[Table 4]

[0067]

[Outside 5]

[0068]

[Table 5]

[0069]

[Outside 6]

[0070]

[Table 6]

[0071]

[Outside 7]

[0072]

[Table 7]

[0073]

[Table 8]

[0074]

According to the present invention, by setting the lens configuration as described above, when a rear focus system is adopted and a large aperture ratio and a high zoom ratio are achieved, the super close range at the telephoto end is achieved. The rear focus type with good optical performance over the entire zoom range from the wide-angle end to the telephoto end, and the entire object distance from the infinity object to the ultra close object while shortening the zoom ratio and increasing the magnification ratio. A zoom lens can be achieved.

[Brief description of drawings]

FIG. 1 is a lens cross-sectional view of Numerical Example 1 of the present invention.

FIG. 2 is an aberration diagram at the wide-angle end according to Numerical Example 1 of the present invention.

FIG. 3 is an intermediate aberration diagram of Numerical Example 1 of the present invention.

FIG. 4 is an aberration diagram at a telephoto end according to Numerical Example 1 of the present invention.

FIG. 5 is an aberration diagram at a wide-angle end according to Numerical Example 2 of the present invention.

FIG. 6 is an intermediate aberration diagram of Numerical example 2 of the present invention.

FIG. 7 is an aberration diagram at a telephoto end according to Numerical Example 2 of the present invention.

FIG. 8 is an aberration diagram at a wide-angle end according to Numerical Example 3 of the present invention.

FIG. 9 is an intermediate aberration diagram of Numerical Example 3 of the present invention.

FIG. 10 is an aberration diagram at a telephoto end according to Numerical Example 3 of the present invention.

FIG. 11 is an aberration diagram at a wide-angle end according to Numerical Example 4 of the present invention.

FIG. 12 is an intermediate aberration diagram of Numerical Example 4 of the present invention.

FIG. 13 is an aberration diagram at a telephoto end according to Numerical Example 4 of the present invention.

FIG. 14 is an aberration diagram at a wide-angle end according to Numerical Example 5 of the present invention.

FIG. 15 is an intermediate aberration diagram of Numerical example 5 of the present invention.

FIG. 16 is an aberration diagram at a telephoto end according to Numerical Example 5 of the present invention.

FIG. 17 is an aberration diagram at a wide-angle end according to Numerical Example 6 of the present invention.

FIG. 18 is an intermediate aberration diagram of Numerical Example 6 of the present invention.

FIG. 19 is an aberration diagram at a telephoto end according to Numerical Example 6 of the present invention.

FIG. 20 is an aberration diagram at a wide-angle end according to Numerical Example 7 of the present invention.

FIG. 21 is an intermediate aberration diagram of Numerical Example 7 of the present invention.

FIG. 22 is an aberration diagram at a telephoto end according to Numerical Example 7 of the present invention.

[Explanation of symbols]

 L1 1st group L2 2nd group L3 3rd group L4 4th group SP Aperture d d line g g line ΔS Sagittal image surface ΔM Meridional image surface

Claims (4)

[Claims] 1. Four lens groups, in order from the object side, 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. And moving the second lens group toward the image plane side to perform zooming from the wide-angle end to the telephoto end, and moving the fourth lens group to correct the image plane variation due to zooming and The second group is moved to perform focusing, and the second group has a negative 21st lens with a concave surface having a strong refractive power facing the image side, a negative 22nd lens with both lens surfaces concave, and a strong refraction toward the object side. It consists of three single lenses, the positive 23rd lens with the convex surface of the power directed, and the lateral magnification of the second group at the telephoto end at infinity is β _2T , and the lateral magnification of the third group and the fourth group is The lateral magnification is β _34T , the focal lengths of the entire system at the wide-angle end and the telephoto end are f _W and f _T , respectively, and the open F number and the shooting half-angle at the telephoto end are F _NOT and ω, respectively. _{Letting T} be S _2T = {1- (β _2T ) ² } ・ (β _34T ) ² [ _Equation 1] A rear focus type zoom lens characterized by satisfying either of the above conditions. 2. Four lens groups, 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 order from the object side. And moving the second lens group toward the image plane side to perform zooming from the wide-angle end to the telephoto end, and moving the fourth lens group to correct the image plane variation due to zooming and The fourth group includes a cemented lens in which at least one negative lens and a positive lens are cemented together, and at least one positive lens, and the fourth group has a wide angle. When the lateral magnifications of an object at infinity at the end and at the telephoto end are β _4W and β _4T , respectively, A rear-focus type zoom lens that satisfies the following conditions. 3. When the focal length of the i-th group is f _i , the focal length of the entire system at the telephoto end is f _T , and the open F number and the shooting half angle of view at the telephoto end are F _NOT and ω _T , respectively, [Equation 3] The rear focus type zoom lens according to claim 1 or 2, wherein the following condition is satisfied. 4. The third lens unit has at least one positive lens and at least one negative lens.
Or 3 rear focus type zoom lens.