JPH04102815A - Zoom lens - Google Patents

Abstract

PURPOSE:To compensate aberrational variation excellently by providing 1st - 4th lens groups in order from an object side and providing the 3rd lens group with a biconvex lens and a negative meniscus lens and the 4th lens group with a negative meniscus lens, a biconvex lens, and a positive lens. CONSTITUTION:The 1st lens group I with positive refracting power, the 2nd lens group II with negative refracting power, the 3rd lens group III with positive refracting power, and the 4th lens group IV with positive refracting power are arranged in order from the object side. Those 1st and 3rd lens groups I and III are fixed and the 2nd lens group II is moved in one direction to vary the power. The 3rd lens group III is formed of the biconvex lens and the negative meniscus lens which has a large-curvature concave surface on the object side. The 4th lens group IV is formed of the negative meniscus lens, biconvex lens, and the positive lens and aberrational variation in focusing from a short- distance body to an infinite-distance body, specially, a curvature of field and astigmatism are compensated. Thus, the zoom lens which is thus formed meets requirements shown by inequalities (1) - (4).

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a zoom lens with an F number of 1.8 to 2.
The present invention relates to a small and lightweight zoom lens suitable for photographic cameras, video cameras, etc., which has good optical performance over the entire range of high magnification, such as a zoom ratio of about 0 and a zoom ratio of about 8.

[Conventional technology]

2. Description of the Related Art There is a so-called four-group zoom lens, which has been used in still cameras, video cameras, and the like, and has a relatively high zoom ratio and a large aperture ratio. This 4-group zoom lens consists of, in order from the object side, the first lens group for focusing, and the second lens group for zooming.
．． It consists of a lens group, a third lens group for correcting image plane fluctuations due to zooming, and a fourth lens group for balancing the focal length and aberration correction of the entire system. A four-group zoom lens has a configuration in which a total of three lens groups are moved: two lens groups for zooming and one lens group for focusing. For this reason, the lens barrel tends to be relatively complicated.

Furthermore, when focusing on a short-distance object, the first lens group is extended toward the object side, so if an attempt is made to secure a sufficient amount of off-axis light, the diameter of the front lens tends to increase.

For this reason, various zoom lenses have been proposed that utilize a so-called rear focus system in which focusing is achieved by moving lens groups other than the first lens group.

[Problem that the invention seeks to solve]

By the way, the above-mentioned publication discloses a zoom lens with a zoom ratio of about 6 to 8 times, but it uses a relatively large number of lenses, and if you try to reduce the number of lenses from an 8 times zoom lens, it will be infinite. 4th for distant to close objects
As the amount of movement of the lens group increases, fluctuations in aberrations due to focusing tend to increase, making it difficult to correct fluctuations in aberrations.

[Problem that the invention is trying to solve]

In consideration of this problem, the present invention has achieved an aperture ratio of approximately F1.8 to F2.0, with a smaller optical effective diameter and shorter overall length (despite the fact that the optical effective diameter is smaller and the overall length is shorter in order to promote miniaturization with a high zoom ratio). We propose a small and lightweight mirror focus type zoom lens that satisfactorily corrects aberration fluctuations due to high magnification and focusing.

[Means to solve the problem]

The zoom lens of the present invention has positive refractive power J in order from the object side.
It has four lens groups: a first lens group with a negative refractive power, a second lens group with a negative refractive power, a third lens group with a positive refractive power, and a fourth lens group with a positive refractive power. The third lens group is fixed, the second lens group is moved in one direction to change the magnification, and the fourth lens group is moved so as to correct image plane fluctuation due to the change in magnification, and the fourth lens group is moved. In a zoom lens that focuses by moving the lens, the third lens group has two lenses: a biconvex lens and a negative meniscus lens with a strongly concave surface facing the object side, and the fourth lens group has a biconvex lens with a negative meniscus lens facing the object side. It has three lenses: a negative meniscus lens with a strongly concave surface facing toward the object, a biconvex lens, and a positive lens with a strongly convex surface facing the object side.

〔Example〕

FIG. 1 is a sectional view of a lens according to Numerical Example 1 of the present invention.

In the figure, I is the first group with positive refractive power, ■ is the second group with negative refractive power that moves monotonically toward the image plane when changing the magnification from the wide-angle end to the telephoto end, and ■ is the second group with positive refractive power. The third group (■) moves along the optical axis with a convex trajectory toward the object from the wide-angle end to the telephoto end, and focuses at the same time to maintain the image plane, which changes as the magnification changes, at a constant position. The fourth group SP having positive refractive power is a fixed aperture disposed in the third group. Note that the first and third lens groups are fixed during zooming and focusing.

In this embodiment, the first lens group is always fixed without being extended during focusing to prevent the lens diameter from increasing due to being extended forward. And when focusing, the first
By moving the fourth lens group, which is part of the variable power system, instead of the lens group, the overall number of movable lens groups is reduced, and the mechanism is simplified. In addition, by providing the fourth lens group with both zooming and focusing functions, and by moving the space between the third and fourth lens groups, we aim to effectively utilize the space within the lens system. The overall length of the lens has been shortened.

Furthermore, the third lens group is composed of a biconvex lens and a negative meniscus lens with a strongly concave surface facing the object side, thereby effectively correcting spherical aberration and coma aberration. and the fourth
The lens group consists of a negative meniscus lens with a strongly concave surface facing the image side, a biconvex lens, and a positive lens with a strongly convex surface facing the object side, thereby reducing aberration fluctuations caused by focusing from close objects to objects at infinity, especially Field curvature and astigmatism are corrected.

The zoom lens that is the object of the present invention is achieved by the above configuration, but in order to correct aberrations, it is preferable to set the radius of curvature of the j-th lens surface of the i-th lens group to R1,j
i When the lens thickness or air gap is Di, and the focal length at the L wide-angle end is fw, 1.75<R3,1/R3,3
<-1,10(1)0.7 <R,l,3/R4,5
<1.0 (2)0,07<D
3.2/f w < 0, 12
(3) 0.01<D, 12/fw<0.03
(4) Titrate under the following conditions.

Conditional expression (1) is the first lens in the third lens group counting from the object side.
This relates to the ratio of the radius of curvature of the third lens surface to the radius of curvature of the third lens surface, and is mainly a condition for appropriately correcting spherical aberration. When the lower limit is exceeded, the spherical aberration becomes under, and when the upper limit is exceeded, the spherical aberration becomes over.

Conditional expression (2) applies to the third lens group counting from the object side in the fourth lens group.
This relates to the ratio of the radius of curvature of the fifth lens surface to the radius of curvature of the fifth lens surface, and is a condition for well-balanced (correcting) field curvature and coma aberration.

If the lower limit value is exceeded, the curvature of field becomes under, and more introverted coma aberration occurs, and if the upper limit is exceeded, the curvature of field becomes excessive, and more outward coma aberration occurs, and this It becomes difficult to make a good correction. Further, the fluctuation of astigmatism due to focusing from a close object to an infinite object becomes large.

Conditional expression (3) is a condition for mainly correcting spherical aberration and astigmatism in a well-balanced manner. If the lower limit value is exceeded, astigmatism becomes thick, and if the two limit values are exceeded, higher-order spherical aberration occurs.

Conditional expression (4) is a condition mainly for correcting coma aberration and distortion aberration. If the lower limit value is exceeded, external coma aberration will occur and it will be difficult to properly correct this aberration, and if the upper limit value is exceeded, barrel-shaped distortion will occur at the wide-angle end, which is not good.

Next, numerical examples of the present invention will be shown. In numerical examples R
1 is the radius of curvature of the i-th lens surface in order from the object side, D
l is the thickness and air distance of the i-th lens from the object side, Ni
and ν1 are the refractive index and Abbe number of the glass of the first lens, respectively, in order from the object side. R2□.

R23 is a face plate, a filter, etc.

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

H size) ≧ One generation size D■ To ■zz
z z zz z z z dimension
Size CD (4 q〕, Ha size ba) For A OL-+ ≧ ≧ ≧ Prisoner's 〇- 1 z ) Co C + D size 08 sun] Heto ■ 10 ■ 0 [ F] ω N 09 Dimension i Curse - To 〇 - O Tsu ■ The meridional image plane, ΔS is the sagittal image plane, d is the d-line, g is the g-line, and SP is the aperture.

〔Effect of the invention〕

According to the present invention, a compact device suitable for photographic cameras, video cameras, etc., which has an F number of about 1.8 to 2.0 and a variable power ratio of about 8, which has good optical performance over the entire range of high zoom ratios. A lightweight zoom lens can be achieved.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a lens according to Numerical Example 1 of the present invention, and FIGS. 2 and 3 are aberration diagrams of Numerical Examples 1 and 2 of the present invention, respectively. In the aberration diagrams, (A) is at the wide-angle end, (B) is at the middle, and (C) is at the wide-angle end.
is an aberration diagram at the telephoto end.

Claims (2)

[Claims] (1) In order from the object side, the first lens group has a positive refractive power, the second lens group has a negative refractive power, the third lens group has a positive refractive power, and the fourth lens group has a positive refractive power. It has a lens group, the first and third lens groups are fixed, the second lens group is moved in one direction to change the magnification, and the fourth lens group is used to correct image plane fluctuations due to the change in magnification. In the zoom lens, the third lens group includes two lenses: a biconvex lens and a negative meniscus lens with a strongly concave surface facing the object side. The fourth lens group includes three lenses: a negative meniscus lens with a strongly concave surface facing the image side, a biconvex lens, and a positive lens with a strongly convex surface facing the object side. lens. (2) The radius of curvature of the j-th lens surface of the i-th lens group is Ri,j, the lens thickness or air gap is Di,j,
When the focal length at the wide-angle end is fw, -1.75<R_3
＿． _1/R_3_. _3<-1.10(1)0.7<
R_4_. _3/R_4_. _5<1.0(2)0.0
7<D_3_. _2/fw<0.12(3)0.01<
D_4_. The zoom lens according to claim 1, wherein the zoom lens satisfies the following condition: _2/fw<0.03 (4).