JPH0777655A - Zoom lens - Google Patents

Abstract

PURPOSE:To provide the wide-angle oriented zoom lens of small-sized constitution which has a 75 deg. field angle and an about X2.8 variable power rate. CONSTITUTION:The zoom lens consists of a 1st group 1 with negative refracting power, a 2nd group 2 with positive refracting power, and a 3rd group 3 with negative refracting power in that order from the object side and is varied in power by moving the 1st group 1 and 2nd group 2; and the 1st group 1 has a (11)th negative lens which has a large-curvature concave surface on the image side, a (12)th negative lens, and a (13)th positive meniscus lens 13 which has a convex surface on the object side, the 2nd group 2 has at least two positive lenses and a negative and a positive lenses which have large-curvature concave surfaces on the image sides, and 3rd group 3 consists of one negative lens. The lens system satisfies the following conditions: 1.7<N11, 1.6<N12, 0.32<R2/R1<0.42, 1.1<R4/R5<1.9. and 40<nu31 hold, where N is a refractive index, R is the radius of curvature of a lens, and nu is an Abbe number.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a zoom lens,
In particular, the present invention relates to a zoom lens composed of three lens groups having a variable power ratio of about 2.8, which is preceded by a lens group having a negative refractive power, which is suitable for a single-lens reflex camera, a video camera and the like.

[0002]

2. Description of the Related Art Conventionally, in a single-lens reflex camera, a rotary reflecting mirror for guiding a light beam to a finder system is provided behind a taking lens. Therefore, the taking lens for a single-lens reflex camera is required to be a compact lens having a high optical performance because it can easily obtain a back focus for obtaining a space for disposing the reflecting mirror. There is also a demand for downsizing, weight reduction, and price reduction of the taking lens.

Conventionally, as a zoom lens suitable for a single-lens reflex camera, a zoom lens composed of two lens groups having a negative refractive power and a positive refractive power from the object side is known.
For example, a zoom lens having a three-group structure in which a fixed negative lens is arranged on the object side has been proposed as a further miniaturized one, which is proposed in, for example, Japanese Patent Laid-Open No. Sho 49-11545. It is proposed in the bulletin.

In addition, as an attempt to increase the multiplication, Japanese Patent Laid-Open No.
No. 2-63909, Japanese Patent Laid-Open No. 58-95315, and the like, a four-group structure having negative refractive power, positive refractive power, negative refractive power, and positive refractive power from the object side having a focal length of about 28 mm to 85 mm. Zoom lenses have been proposed.

[0005]

SUMMARY OF THE INVENTION The above zoom lens is
A lens having a negative refracting power precedes and has a so-called retro type configuration, and is suitable for a zoom lens including a wide angle range.

By the way, in recent years, the zoom lens has been increased in magnification,
Miniaturization and cost reduction are desired, and standard focal length (about 5
When trying to achieve a zoom lens that includes
The zoom lens having the two-group configuration and the three-group configuration is suitable for downsizing and cost reduction, but in order to achieve high magnification, the refractive power arrangement and the lens configuration must be set appropriately.
Further, the above-mentioned four-group zoom lens is advantageous for higher magnification and smaller size, but the number of groups is inevitably increased due to the large number of groups, and the lens barrel structure becomes complicated, which causes a problem in terms of cost. . The present invention expands the zoom ratio to a conventional standard zoom while maintaining the optical performance, and the zoom ratio is 2.8.
It is an object of the present invention to provide a compact and low-priced zoom lens that can satisfy an angle of view at the wide-angle end of about 75 °.

[0007]

[Means for Solving the Problems] A first group having a negative refractive power, a second group having a positive refractive power, and a third group having a negative refractive power are arranged in this order from the object side. In the zoom lens for moving the zoom lens, the first group includes a negative meniscus eleventh lens having a strong concave surface facing the image side and a negative twelfth lens, and a positive meniscus lens having a convex surface facing the object side. The second group includes at least two positive lenses, a negative lens having a strong concave surface toward the image side and a positive lens, and the third group includes one negative lens. Expressions (3) to (7) are satisfied, and at this time, it is preferable that conditional expression (1) be satisfied.
It satisfies (2). 0.5 <| f ₁ / f _T | <0.65 ... (1) 0.4 <f ₂ / f _T <0.5 ... (2) 1.7 <N ₁₁ ... (3) 1.6 < N ₁₂ (4) 0.32 <R ₂ / R ₁ <0.42 (5) 1.1 <R ₄ / R ₅ <1.9 (6) 40 <ν ₃₁ (7) f ₁ ; focal length of the first group. f ₂ ; focal length of the second group. f _T : Focal length of the entire system at the telephoto end. N ₁₁ : Refractive index of the glass material of the 11th lens. N ₁₂ : Refractive index of the glass material of the 12th lens. R ₁ ; radius of curvature of the eleventh lens on the object side. R ₂ ; radius of curvature of the eleventh lens on the image side. R ₄ ; paraxial radius of curvature on the image side of the 12th lens. R ₅ : radius of curvature of the thirteenth lens on the object side. ν ₃₁ : Abbe number of the material of the negative lens of the third group.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1, 2 and 3 are lens sectional views at the wide-angle end of Numerical Embodiments 1, 2, and 3 of the present invention, which will be described later. In the figure, 1 is a first lens unit having a negative refracting power, which is moved and focused while drawing a convex axis trace on the image surface side in order to correct the image surface variation due to zooming. The second lens unit 2 has a positive refractive power, which moves toward the object side during zooming from the wide-angle end to the telephoto end. 3 is the third group of negative refractive power,
It is always fixed with respect to the image plane. P is an aperture stop, and FIGS.
SP is a flare cut diaphragm arranged mainly between the second group and the third group for cutting flare of an intermediate angle of view,
It is moved in the direction of the arrow along with the scaling.

In order to reduce the cost, it is desirable to reduce the number of movable groups to simplify the lens barrel structure and reduce the number of lenses, but in the present embodiment, two groups are moved to change the magnification. And the simplest zoom method is used to correct the image plane. In addition, in order to increase the zoom ratio, it is general to increase the amount of movement of the first and second groups, but this method tends to increase the total lens length, while comparing the total lens lengths. In order to obtain a desired zoom ratio while shortening the length, the refractive power of each group must be increased, but in the case of the two-group configuration, the solution is almost uniquely determined, and as a result, sufficient miniaturization can be achieved. The inconvenience of not being able to occur occurs. Therefore, in this embodiment, the third lens unit having a negative refractive power is used to form a three lens unit, and the total length is shortened.

In this embodiment, as described above, the zoom lens is composed of the first, second, and third lens groups, and
By defining the lens configuration of each group, and more preferably by defining the refracting power as described above, it is possible to satisfactorily correct aberration variation due to zooming, achieve a compact, low-cost zoom lens with high optical performance. ing.

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

Conditional expression (1) relates to the focal length of the first lens group with respect to the focal length at the telephoto end, and when the refractive power of the first lens group becomes too weak beyond the upper limit, the first lens group is used to secure an off-axis ray. If the lens outer diameter of the first group increases and the lower limit is exceeded and the refractive power of the first group becomes strong, it becomes difficult to correct distortion in the wide-angle side and spherical aberration in the telephoto side in a well-balanced manner. When the focusing is performed by the first group, the fluctuation of spherical aberration becomes large.

Conditional expression (2) relates to the focal length of the second lens unit with respect to the focal length at the telephoto end, and when the refractive power of the second lens unit becomes weaker than the upper limit, the second lens unit is used to obtain a predetermined zoom ratio. The amount of movement of the group increases, resulting in an increase in the total lens length, which is not preferable. If the refracting power of the second lens unit becomes strong beyond the lower limit, it is advantageous for downsizing, but along with this, it becomes necessary to strengthen the refracting power of the first lens unit and the third lens unit, and the aberration generated in each lens becomes large. Therefore, it becomes difficult to correct this in good balance.

Conditional expressions (3) and (4) are the eleventh and the eleventh, respectively.
This is related to the refractive index of the material of the twelfth lens, and when the refractive index becomes lower than the respective lower limit values, the radius of curvature of each surface becomes tight in order to obtain a predetermined refractive power, and the complementary aberration, especially at the wide-angle end, The distortion and the spherical aberration at the telephoto end become large, and it becomes difficult to set the Petzval sum to an appropriate value.

Conditional expression (5) relates to the size of the radius of curvature of the image side with respect to the radius of curvature of the surface of the eleventh lens on the object side. , The negative refracting power of the twelfth lens must be strengthened, and spherical aberration is generated a lot and flare is generated a lot from the middle to the telephoto end. If the radius of curvature on the image side becomes smaller than the lower limit, the first
The negative refracting power of one lens becomes strong, and the negative distortion on the wide angle side increases, which is not preferable.

Conditional expression (6) relates to the ratio of the radius of curvature of the image-side surface of the twelfth lens to the radius of curvature of the object-side surface of the thirteenth lens. 1
If the spherical aberration of the lens group is in the under direction and exceeds the lower limit value, it becomes large in the over direction, and it becomes difficult to satisfactorily correct various aberrations due to zooming.

Conditional expression (7) relates to the Abbe number of the material of the negative lens of the third lens unit, and if the lower limit value is exceeded, variation of chromatic aberration due to zooming becomes large, which is not preferable.

Although the basic structure of the lens system is determined as described above, it is more preferable that the image-side surface of the twelfth lens is an aspherical surface having a shape in which the negative refracting power becomes weaker from the center toward the periphery. good. When the wide-angle end has a considerably wide angle of view of about 75 ° as in the present embodiment, the distortion aberration at the wide-angle end increases to the negative side, and in order to correct this with the three lenses of the first group, It is effective to use an aspherical surface such as, and it is suitable for compactification. Here, the same effect can be obtained by using an aspherical surface for the other lens surface in the first group, but it is most preferable to use it for the lens surface in terms of aspherical surface processing.

In order to reduce the cost, it is preferable that the negative lens of the third lens group be made of plastic. Furthermore, by using an aspherical surface for the negative lens, it becomes possible to reduce the fluctuation of the image plane from the wide-angle end to the telephoto end.

On the other hand, in order to reduce the outer diameter of the lens barrel and improve the feeling of zooming operation, it is preferable to move the second lens unit non-linearly so as to draw a weak convex toward the object side during zooming. When the outer diameter of the lens barrel is reduced, the diameter of the cam ring for moving the first group and the second group is accordingly reduced. Therefore, when the amount of operation for zooming is constant, the expanded length of the cam becomes smaller,
In particular, the angle of the cams of the first group becomes tight at the wide-angle end, and when the zooming operation is performed, a feeling of being caught at the wide-angle end occurs. Therefore, by moving the second group as described above, this can be prevented.

In the numerical example, R _i is the radius of curvature of the _ith lens surface from the object side, D _i is the ith lens thickness and air gap from the object side, and N _i and ν _i are from the object side, respectively. It is the refractive index and Abbe number of the glass of the i-th lens.

In the numerical examples, the aspherical shape is such that the radius of curvature of the lens surface is R, the optical axis direction (light traveling direction) is the X axis, the direction perpendicular to the optical axis is the Y axis, and B, C, D, E
Where a is an aspherical coefficient,

[0023]

[Outer 1] It is represented by the formula

[0024]

[Outside 2]

[0025]

[Outside 3]

[0026]

[Outside 4]

[0027]

[Table 1]

[0028]

According to the present invention, in the three-group type zoom lens, by setting each lens group configuration as described above, the field angle at the wide-angle end is about 75 ° and the zoom ratio is simple with a simple configuration. However, it is possible to achieve a zoom lens of good optical performance that is small in size of about 2.8 times and can be manufactured at low cost.

[Brief description of drawings]

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

2 is a lens cross-sectional view of Numerical Example 2. FIG.

FIG. 3 is a lens cross-sectional view of Numerical Example 3.

FIG. 4 is an aberration diagram of Numerical example 1 at the wide-angle end and at infinity of an object.

5A and 5B are aberration diagrams of Numerical example 1 at the telephoto end and at infinity of an object.

6A and 6B are aberration diagrams of Numerical example 2 at the wide-angle end and at infinity of an object.

7A and 7B are aberration diagrams of Numerical example 2 at the telephoto end and at infinity of an object.

8A and 8B are aberration diagrams of Numerical example 3 at the wide-angle end and at infinity of an object.

FIG. 9 is an aberration diagram of Numerical example 3 at the telephoto end and at infinity of an object.

[Explanation of symbols]

 1 1st group 2 2nd group 3 3rd group SC Sine condition S Sagittal image image M Meridional image plane

Claims (5)

[Claims] 1. A first group having a negative refracting power, a second group having a positive refracting power, and a third group having a negative refracting power in order from the object side. The first group and the second group are moved. In the zoom lens that performs zooming, the first group includes a negative eleventh lens and a negative twelfth lens having a strong concave surface facing the image side, and a positive meniscus thirteen lens having a convex surface facing the object side, The second group includes at least two positive lenses, a negative lens having a strong concave surface toward the image side, and a positive lens, and the third group includes one negative lens and satisfies the following conditions. This is a zoom lens. 1.7 <N ₁₁ 1.6 <N ₁₂ 0.32 <R ₂ / R ₁ <0.42 1.1 <R ₄ / R ₅ <1.9 40 <ν ₃₁ However, N ₁₁ ; the 11th The refractive index of the glass material of the lens. N ₁₂ : Refractive index of the glass material of the 12th lens. R ₁ ; radius of curvature of the eleventh lens on the object side. R ₂ ; radius of curvature of the eleventh lens on the image side. R ₄ ; paraxial radius of curvature on the image side of the 12th lens. R ₅ : radius of curvature of the thirteenth lens on the object side. ν ₃₁ : Abbe number of the material of the negative lens of the third group. 2. The focal length of the first lens unit is f ₁ , and the second lens unit is the second lens unit.
_2. The zoom lens according to claim 1, wherein the following expression is satisfied, where f _{2 is} the focal length of the group and f _T is the focal length of the entire system at the telephoto end. 0.5 <| f ₁ / f _T | <0.65 0.4 <f ₂ / f _T <0.5 3. The zoom lens according to claim 1, wherein each of the 12 lenses has an aspherical surface having a shape in which the negative refracting power becomes weaker from the center to the periphery on the image side lens surface. 4. The zoom lens according to claim 1, wherein the material of the negative lens of the third group is plastic. 5. The zoom lens system according to claim 1, wherein the second lens unit moves in a non-linear manner so as to draw a convex surface toward the object side during zooming.
Zoom lens.