JP2000221400A - Rear focus type zoom lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a rear focus type zoom lens which possesses four lens groups as a whole, at which focusing is executed by a lens group on the rear side of a variable power system, by which entire lens system is miniaturized and whose variable power ratio is about 10. SOLUTION: This lens orderly possesses four lens groups of the first group L1 of positive refracting power, the second group L2 of negative refracting power, the third group L3 of the positive refracting power and the fourth group L4 of the positive refracting power from an object side, and variable power from a wide angle end to a telephoto end is executed by moving the second group L2 to an image surface side, and image surface fluctuation in accordance with the variable power is corrected by moving the fourth group L4, and also focusing is executed by moving the fourth group, so that the lens constitution of each lens group is appropriately set.

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 (inner focus type) zoom lens, and more particularly to a rear focus type (inner focus type) zoom lens which is used in a film camera, a video camera, a broadcast camera, etc., and has a high zoom ratio (10 times or more). The present invention relates to a rear-focusing zoom lens that has high optical performance that is compact as a whole lens system while ensuring high zoom performance.

[0002]

2. Description of the Related Art In recent years, as home video cameras have become smaller and lighter, remarkable progress has been made in miniaturization of imaging zoom lenses. Particularly, efforts have been made to shorten the overall length of the lens and simplify the lens configuration. Is peeling.

A first method for achieving these goals is (a-1) a rear focus method or an inner focus method in which a lens group other than the first lens group on the object side is moved to perform focusing.

In general, a zoom lens of the rear focus type or the inner focus type has a smaller effective diameter of the first lens unit than a zoom lens which performs focusing by moving the first lens unit, and the overall size of the lens system can be reduced. It will be easier. In addition, close-up photography, particularly extremely close-up photography, can be performed. Further, since a relatively small and light lens group is moved, the driving force of the lens group is small and quick focusing can be performed. As such a rear focus type or inner focus type zoom lens, for example,
JP-A-62-24213, JP-A-63-2473
Japanese Patent Application Publication No. 16-163 and the like have a positive first lens group, a negative second lens group, a positive third lens group, and a positive fourth lens group in this order from the object side, and zoom by moving the second lens group. The fourth lens group discloses a zoom lens that corrects the image plane fluctuation caused by zooming and performs focusing. (Note: hereafter, focus is performed by the lens group on the image plane side like this type.) This method is called a rear focus method.)

[0005] As a second method, there is a method (a-2) in which the refractive power of the second lens group is increased to reduce the amount of movement of the second lens group for obtaining a desired zoom ratio.

With this configuration, the distance between the lens units (first lens unit and second lens unit), which is a variable power system, is reduced, and the distance from the stop to the first lens unit is shortened. Since the diameter can be reduced and the thickness of the first lens group can be reduced, the entire system can be reduced in size.

However, if the refracting power of the second lens group is too strong, the Petzval sum of the entire lens becomes large at a negative value, and various aberrations such as curvature of field are extremely deteriorated. There was a limit to miniaturization by increasing the refractive power.

As still another method, (A-3)
There is a method of reducing the size of the third lens unit and the fourth lens unit, which are imaging systems.

Specifically, the third lens group is composed of a positive lens and a negative lens in order from the object side, and the third lens group is a so-called telephoto lens type, and the principal point position of the third lens group is shifted to the object side. This is to reduce the actual distance between the third lens unit and the fourth lens unit by moving the lens unit to reduce the size.

[0010] The configuration of this imaging system is often applied to an intermediate lens type camera lens (100 mm to 2 mm in 35 mm format).
This is very similar to a simplified type of a lens having a small telephoto ratio used as a lens having a focal length of about 00 mm.

Japanese Patent Application Laid-Open Nos. 5-19165 and 5-6097 disclose zoom lenses having such a lens configuration.
4, JP-A-5-297275, JP-A-5-297275
60973, JP-A-5-107473, U.S. Pat.
This is known from SP 5,189,558 and the like.

However, in this type, if the principal point position of the third lens group is too close to the object side to make the actual distance between the third lens group and the fourth lens group too short,
A moving space for the fourth lens group that moves for distance adjustment cannot be secured, and it is not possible to secure a sufficient close-up photographing distance particularly in the middle to telephoto range.

As another method, (a-4) the principal point of the fourth lens group is moved to the object side as much as possible to shorten the back focus. Specifically, the fourth lens group also includes a positive lens and a negative lens in order from the object side, and the fourth lens group is a so-called telephoto lens type, in which the principal point position of the fourth lens group is moved to the object side. It is an object to shorten the actual distance between the third lens unit and the fourth lens unit and to shorten the back focus, thereby achieving downsizing. However, even in this type, if the principal point position of the fourth lens group is too close to the object side so as to make the actual distance between the third lens group and the fourth lens group too short, especially in the middle to telephoto range, In addition to being unable to secure a sufficient photographing distance, it is also not possible to secure a dimension of a back focus portion in which members such as a low-pass filter enter.

However, the thickness of the low-pass filter has been considerably improved as described in JP-A-6-289323.

[0015]

In general, when a rear focus system is employed in a zoom lens, the entire lens system can be reduced in size, quick focus can be achieved, and close-up photographing can be easily performed.

However, on the other hand, while trying to increase the zoom ratio,
To reduce aberration fluctuations during focusing and obtain high optical performance over the entire object distance from an object at infinity to an object at a short distance, the lens configuration becomes very difficult.

In particular, with 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.

The present invention employs a rear focus method, has a large aperture ratio, a high zoom ratio of 10 times, and a high zoom ratio, and covers the entire zoom range 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 excellent optical performance over the entire object distance from an object to a very close object.

[0019]

According to a first aspect of the present invention, a rear focus type zoom lens includes a first lens unit having a positive refractive power and a second lens having a negative refractive power movable during zooming, in order from the object side. A rear focus type zoom lens having a lens unit, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power for correcting movement of an image point due to zooming, and performing distance adjustment using the fourth lens unit. Wherein the third lens unit includes a positive 31st lens having convex surfaces with strong refractive power facing the object side and a positive 32nd lens having a convex surface facing the object side, and a negative 32nd meniscus lens having a concave surface facing the image surface side. The fourth lens group is composed of a positive 41st lens having both convex surfaces and a negative 42th lens having a meniscus shape having a concave surface facing the object side. L: Overall length (thickness of low-pass filter, protective glass, etc.) in terms of air) W: maximum image height f _w The focal length of the entire optical system at the wide-angle end bf _w: back focus at the wide-angle Rui (low-pass filter, the thickness of such protective glass is converted into air) bf _t: back focus (low-pass filter at the telephoto end, the thickness of such protective glass air D ₃ : Distance between the 31st lens and the 32nd lens in the third group M ₂ : Movement amount F _i (i = 1, 2, 3, 4) of the second lens group from the wide-angle lens to the telephoto lens ): When the focal length of the i-th lens unit in the order from the object side is

[0020]

(Equation 5)

It is characterized by satisfying the following.

[0022]

FIG. 1 is a sectional view of a rear focus type zoom lens according to a first embodiment of the present invention at a wide angle end according to a numerical example 1 described later. 2 and 3 are aberration diagrams at the wide-angle end and the telephoto end in Numerical Embodiment 1. FIG.

FIG. 4 is a sectional view of a rear focus type zoom lens according to a second embodiment of the present invention at a wide-angle end, which will be described later. 5 and 6 are aberration diagrams at the wide-angle end and the telephoto end in Numerical Example 2. FIG.

FIG. 7 is a sectional view of a rear focus type zoom lens according to a third embodiment of the present invention at a wide-angle end, which will be described later. 8 and 9 are aberration diagrams at the wide-angle end and the telephoto end in Numerical Example 3.

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 denotes 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, and IP is an image plane.
The image plane IP is provided with photosensitive means such as a CCD for a video camera and a film for a film camera.

In this embodiment, at the time of zooming from the wide-angle end to the telephoto end, the second lens unit is moved to the image surface side as indicated by an arrow, and the image surface fluctuation caused by zooming is caused by projecting the fourth lens unit to the object side. It moves while having a trajectory of a shape.

Also, a rear focus type is employed in which the fourth unit is moved on the optical axis to perform focusing. A solid line curve 4a and a 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.

In the numerical embodiments of the present invention, the first lens unit and the third lens unit
The group is fixed at the time of zooming and focusing, but may be moved together with the stop at the time of zooming to further reduce the size of the entire lens system.

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 curves 4a and 4b in the same figure, the zoom lens is moved so as to have a convex locus toward the object side when zooming from the wide-angle end to the telephoto end.

As a result, 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 fourth unit is moved forward as indicated by a straight line 4c in FIG.

In the present embodiment, the third lens group is arranged in order from the object side, and the convex surface having a stronger refractive power is directed toward the object side than the image surface side. It is composed of a negative meniscus 32nd lens with a concave surface of force. This embodies a method of reducing the size of the third lens unit and the fourth lens unit, which are the above-described imaging systems (a-3).

By adopting such a lens configuration, the third lens group is a so-called telephoto lens type, and the principal point position of the third lens group is moved to the object side, thereby realizing the actual operation of the third lens group and the fourth lens group. The distance between them is shortened to reduce the size.

The fourth lens unit includes, in order from the object side, a positive first lens having both convex lens surfaces and a negative meniscus second lens having a concave surface facing the object side. This is achieved by moving the principal point of the fourth lens group to the object side in (A-4) and shortening the back focus, thereby making the fourth lens group a so-called telephoto lens type. By moving the principal point of the group toward the object side, the actual distance between the third lens group and the fourth lens group is shortened, the back focus is shortened, and the size is reduced.

Conditional expression (1) embodies the size of the small zoom lens described so far and is standardized by the maximum image height. If this formula is not satisfied, the zoom lens will be large.

Conditional expression (2) shows the condition of the back focus for facilitating realization of the condition given by conditional expression (1). When this expression is not satisfied, conditional expression (1)
Therefore, it is difficult to reduce the size of the zoom lens, which is the object of the present invention.

When the back focus at the wide-angle end is shorter than that at the telephoto lens at the same zoom ratio, the refractive power of the second and fourth lens units can be reduced. Correction of curvature and distortion is facilitated. The lower limit of conditional expression (3) is embodied. If the lower limit is exceeded, it becomes difficult to correct field curvature and distortion. Conversely, if the upper limit is exceeded, the back focus at the wide-angle end becomes too long, and the compactness is lost.

In each numerical embodiment of the present invention, the third lens group is a telephoto lens type by separating the lenses of the 31st lens and the 32nd lens of the third lens group, and the principal point of the third lens group is positioned on the object side. In order to shorten the actual distance between the third lens unit and the fourth lens unit to reduce the size. Conditional expression (4) embodies this, and if the conditional expression is not satisfied, the distance between the third lens unit and the fourth lens unit is increased, and compactness is lost. A meniscus negative 32nd lens having a strong negative refractive power on the image plane side of the third lens unit.
The arrangement of the lenses facilitates correction of aberrations such as distortion.

Conditional expression (5) is used to make the refractive power of the second lens unit described in (A-2) and the amount of movement of the second lens unit for obtaining a desired zoom ratio have an appropriate relationship. It is.
If the value exceeds the upper limit of conditional expression (5), a desired zoom ratio cannot be obtained. If the value exceeds the lower limit, variation in aberration due to zooming increases or the size increases.

In the present embodiment, a small zoom lens is provided with good performance by making full use of the above-described miniaturization methods (A-1) to (A-4) in a range that does not cause the disadvantages. I have.

In order to provide good performance, it is important not to make the refractive power of each lens unit too strong for miniaturization, but it is also necessary to suppress chromatic aberration. As the zoom ratio increases, the variation in chromatic aberration tends to increase.
For this purpose, each lens group is composed of at least a positive lens and a negative lens, and in particular, the third lens group and the fourth lens group are arranged in the order of the above-described methods (A-3) and (A-4). It offers the optimal compact zoom lens with good performance.

In the present invention, the rear focus type having a simple and compact lens structure having good optical performance from the wide-angle end to the telephoto end and over the entire object distance while preventing the entire lens system from being enlarged. In order to obtain the above zoom lens, it is preferable to satisfy at least one of the following conditions.

(A-1) Rij (i, j = 1,2,2
3. . . ): When the radius of curvature of the j-th lens surface of the i-th i-th lens unit is taken in order from the object side.

[0044]

(Equation 6)

Is satisfied.

Conditional expression (6) is for making the relationship of the radius of curvature of the lens surface of the front lens (first lens group) appropriate. In the case of a rear focus type zoom lens as shown in the numerical examples, the Petzval sum tends to be negative.

By satisfying the lower limit of conditional expression (7), this is improved, and aberration correction such as field curvature and distortion is facilitated. On the other hand, when not satisfied, it is difficult to improve the above aberration.

When the upper limit of conditional expression (6) is not satisfied, the size of the front lens becomes large, and compactness is lost.

(A-2) When the focal length of the i-th lens unit is Fi,

[0050]

(Equation 7)

Is satisfied.

Conditional expression (7) is satisfied for the fourth lens unit with respect to the fourth lens unit.
This is necessary to make the refractive power of the lens group appropriate.
If the upper limit of conditional expression (7) is not satisfied, chromatic aberration is likely to occur. Conversely, if the lower limit is not satisfied, the amount of movement of the fourth lens unit during zooming and focusing increases, resulting in loss of compactness. .

(A-3) When the focal length of the i-th lens unit is Fi

[0054]

(Equation 8)

Is satisfied.

Conditional expression (8) is necessary for appropriately setting the refractive power of the first lens unit and the second lens unit. If the upper limit of conditional expression (8) is not satisfied, it becomes difficult to improve the Betzval sum, and as a result, aberrations such as curvature of field will be likely to occur.

Conversely, if the lower limit is not satisfied, the amount of movement of the second lens unit accompanying zooming increases, resulting in a loss of compactness.

(A-4) the object-side lens surface of the third lens group or the lens surface of the fourth lens group as an example of an object;
Alternatively, both lens surfaces are aspherical.

In order to maintain good performance, it is preferable that the lens surface of the object example of the third lens unit, the lens surface on the object side of the fourth lens unit, or both lens surfaces are aspherical. With such a configuration, the chromatic aberration of the third lens unit and the fourth lens unit is suppressed well, and spherical aberration and coma aberration can be satisfactorily corrected even with the minimum number of positive and negative lenses.

(A-5) The forty-first and forty-second lenses are cemented.

For good aberration correction and miniaturization, it is preferable that the two lenses of the fourth lens group are cemented. Especially for space saving for miniaturization,
The fourth lens unit includes, in order from the object side, a positive lens whose both lens surfaces are convex and a meniscus negative second lens 42 whose concave surface faces the object side.
Since the lens surfaces are formed by lenses, the respective lens surfaces face each other with the same curvature in the same direction.

To hold each lens separately, it is necessary to secure the interval between the lenses, and the thickness of the fourth lens group becomes large, which is not appropriate. In addition, when the respective lenses are tilted relative to each other, the aberration tends to be significantly deteriorated. Therefore, it is preferable that the two lenses are bonded to each other.

(A-6) The second lens unit is composed of a meniscus-shaped negative lens with the convex surface facing the object side, a negative lens with both lens surfaces concave, and a positive lens with both lens surfaces convex. .

Next, numerical examples of the present invention will be described. In the numerical examples, Ri is the radius of curvature of the i-th lens surface in order from the object side, Di is the i-th lens thickness and air spacing from the object side, and Ni and νi are the i-th lens surfaces in order from the object side. The refractive index and Abbe number of glass. In the numerical examples, the last two lens surfaces are glass blocks such as a face plate and a filter. In addition, the table shows the relationship between the above-described conditional expressions and various numerical values in the numerical examples.

The aspherical surface has an X axis in the optical axis direction, an H axis in a direction perpendicular to the optical axis, a positive traveling direction of light, R represents a paraxial radius of curvature, and K, B, C, and D each represent an aspherical coefficient. And when

[0066]

(Equation 9)

This is expressed by the following equation. "E-0x" means "10-x".

[0068]

[Outside 1]

[0069]

[Outside 2]

[0070]

[Outside 3]

[0071]

[Table 1]

[0072]

According to the present invention, when a zoom ratio of 10 and a high zoom ratio are to be achieved while adopting the rear focus method as described above, the lens configuration of each lens group is appropriately set, and A rear-focusing zoom lens with good optical performance over the entire zoom range from the wide-angle end to the telephoto end, and over the entire object distance from infinity to super close-up objects while miniaturizing the entire system Can be achieved.

[Brief description of the drawings]

FIG. 1 is a sectional view of a lens at a wide angle end according to Numerical Embodiment 1 of the present invention.

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

FIG. 3 is an aberration diagram at a telephoto end in Numerical Example 1 of the present invention;

FIG. 4 is a sectional view of a lens at a wide-angle end according to a second numerical embodiment 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 aberration diagram at a telephoto end in Numerical Example 2 of the present invention.

FIG. 7 is a sectional view of a lens at a wide angle end according to Numerical Embodiment 3 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 aberration diagram at a telephoto end in Numerical Example 3 of the present invention.

[Explanation of symbols]

 L1 First lens unit L2 Second lens unit L3 Third lens unit L4 Fourth lens unit SP Aperture IP Image plane d d line g g line ΔS Sagittal image plane ΔM Meridional image plane G Glass block

Continued on the front page F term (reference) 2H087 KA02 KA03 MA15 PA07 PA20 PB10 QA02 QA07 QA17 QA21 QA25 QA37 QA41 QA46 RA05 RA12 RA32 RA42 RA43 SA23 SA27 SA29 SA32 SA63 SA65 SA72 SA74 SB04 SB14 SB23 SB33

Claims (6)

[Claims] 1. A first lens unit having a positive refractive power, a second lens unit having a negative refractive power movable during zooming, in order from the object side,
In a rear focus type zoom lens having a third lens group having a positive refractive power and a fourth lens group having a positive refractive power for correcting movement of an image point due to zooming, and distance adjustment is performed by the fourth lens group, The third lens group includes a positive 31st lens having convex surfaces with strong refractive power facing the object side, and a negative 32nd lens having a meniscus shape having a concave surface facing the image surface side. The fourth lens unit is composed of a positive 41st lens having both convex lens surfaces and a negative 42nd lens having a meniscus shape with the concave surface facing the object side. L: Overall length (the thickness of the low-pass filter, the protective glass, etc. is air. in terms of) W: maximum image height f _w: the focal length of the entire optical system at the wide-angle end bf _w: back focus (low-pass filter at the wide-angle Rui, the thickness of such protective glass is converted into air) bf _t: Bakkufo at the telephoto end Kas (low-pass filter, converted thickness of such protective glass to air) D _3: the third inter-group of the 31 lens and the 32 lens lens distance M _2: movement amount from the wide angle Rui the second lens unit to the telephoto Rui F _i (i = 1, 2, 3, 4): When the focal length of the i-th lens group in order from the object side, A rear focus zoom lens that satisfies the following conditions. 2. Rij (i, j = 1, 2, 3,...):
When the radius of curvature of the j-th lens surface of the i-th i-th lens group is given in order from the object side. 2. The rear focus type zoom lens according to claim 1, wherein: 3. When the focal length of the i-th lens group is Fi, The rear focus type zoom lens according to claim 1 or 2, wherein the following condition is satisfied. 4. When the focal length of the i-th lens group is Fi. 4. The rear focus zoom lens according to claim 1, wherein the zoom lens satisfies the following condition. 5. The lens surface of the third lens group on the object side, the lens surface of the object example of the fourth lens group, or both lens surfaces are aspherical. Rear focus type zoom lens of 3 or 4. 6. The rear focus zoom lens according to claim 1, wherein the 41st lens and the 42nd lens are cemented.