JPH0360408B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a small zoom lens, and particularly to a small zoom lens that does not have a large back focus and is suitable for use in a lens shutter type camera. Recently, as lens-shutter cameras have become more multifunctional, there has been a demand for cameras of this type that are also highly portable to be able to use zoom lenses. Most of them are designed for single-lens reflex cameras with mirrors, and because they are designed to have a long back focus, they have problems with compactness.
This leads to an increase in the size of the entire camera. The present invention aims to provide a compact zoom lens that has a simple structure, does not have a large back focus, and is suitable for a highly portable camera such as a lens shutter type camera. In conventional compact zoom lenses, especially zoom lenses that have two lens groups, a positive group and a negative group, from the object side, the positive and negative groups are separated at the wide-angle side, resulting in a so-called telephoto lens configuration, which shortens the overall length. However, at the telephoto end, the positive and negative groups become closer together, weakening this effect and increasing the overall length. In this invention, the negative group of the two-group zoom lens as described above is divided into two lens groups,
The negative group on the object side is the second lens group, and the negative group on the image plane side is the third lens group, forming a three-group zoom lens having a positive, negative, and negative configuration. On the wide-angle side, by bringing the second and third lens groups closer together, the same compactness as before can be maintained, while on the telephoto side, the first lens group with positive refractive power of the third lens group By placing it away from the lens group, the telephoto lens configuration is maintained.
The overall length of the lens can be made shorter than that of the conventional two-group configuration. The lack of focal length on the telephoto side due to such a configuration could be compensated for by bringing the second lens group, which is a negative group, closer to the first lens group, which is a positive group. As a result, the zoom lens of the present invention has a first
The amount of movement of the lens group can be kept small, and the length of the entire lens system can be maintained at approximately 1.1 to 1.15 times the focal length of the entire system even at the telephoto end. It is. More specifically, in order from the object side, the lens group consists of a first lens group with positive refractive power, a second lens group with negative refractive power, and a third lens group with negative refractive power.
A zoom lens having a group structure, the first lens group consists of at least two positive lenses and at least one negative lens, the second lens group consists of at least one negative lens, and the third lens group consists of at least one negative lens. By changing the distance between the first lens group and the second lens group, and the distance between the second lens group and the third lens group, it is possible to correct the movement of the image plane due to zooming. This is what is being done. In this zoom lens, the first lens group is a combination of a positive lens and a negative lens, which eliminates axial and lateral chromatic aberrations, spherical aberrations, coma aberrations, and variations in field curvature that occur with zooming. It's for a reason. In such a zoom lens system consisting of three positive, negative, and negative groups, the focal length of the first lens group is f ₁ , the focal length of the second lens group is f ₂ , and the focal length of the third lens group is
f ₃ , the distance between the rear principal point position of the first lens group and the front principal point position of the second lens group is t ₁ , and the distance between the rear principal point position of the second lens group and the front principal point position of the third lens group. _{When the distance of _ _ _ _ _ _ _ _} )
...It is expressed as (1). In the zoom lens of this invention, t ₂ is small at the wide-angle end, and the back focus f _B is also small, so the focal length f _w at the wide-angle end is almost equal to the focal length f ₁ of the first lens group. . f _W = f ₁ ...(2) On the other hand, since t ₁ is small at the telephoto end, the focal length f _T at the telephoto end is expressed as f _T = f ₁ if t ₂ at the telephoto end is expressed as t _2T . f ₂ f ₃ / f ₁ f ₂ + (f ₃ −t _2T ) (f ₁ + f ₂ )……(
3) becomes. Furthermore, if we assume that f ₁ <<|f ₂ |, then f _T ≈f ₁ f ₃ /f ₁ +f ₃ −t _2T (4). Therefore, the magnification ratio M is approximately as follows: M=f _T /f _M =f ₃ /f ₁ +f ₃ -t _2T (5). It is desirable that such a zoom lens satisfy the following conditions. 0.3f _W ＜t _2T ＜0.8f _W ……(6) 0.8f _W ＜f ₁ ＜1.5f _W ……(7) f _W ＜｜f ₃ ｜＜2f _W ……(8) Condition (6) is If the lower limit is exceeded, the third
The lens group is too close to the first lens group and the second lens group, causing an increase in the total length of the lens system. If the upper limit is exceeded, the denominator of equation (5) will be increased, making it impossible to obtain a sufficient zoom ratio. If condition (7) exceeds the lower limit, in order to achieve the specified focal length f _W at the wide-angle end, the second and third lens groups will need to move closer to the first lens group at the wide-angle end, and the lens will move toward the telephoto side. The first step to perform the scaling of
It is no longer possible to secure the distance between the lens group and the second lens group. If the upper limit is exceeded, the total length of the entire lens system at the wide-angle end becomes long, which is contrary to the purpose of the present invention. If condition (8) exceeds the lower limit, excessive curvature of field will be significant for angles of view exceeding 55°, and it will be difficult to correct it by selecting other conditions. On the other hand, if the upper limit is exceeded, the amount of movement of the third lens group becomes large in order to obtain a sufficient variable power ratio, which deviates from the purpose of the present invention, which is to obtain a compact lens. Examples of the present invention will be shown below.

【table】

【table】 [Brief explanation of drawings]

FIG. 1 and FIG. 2 are explanatory diagrams of movement paths of each lens group in a first embodiment and a second embodiment of the present invention, respectively. Figures 3 and 4 show the first embodiment and the second embodiment, respectively.
A sectional view of an example. FIGS. 5 and 6 are aberration curve diagrams of the first embodiment and the second embodiment, respectively.

Claims (1)

[Claims] 1 A zoom lens with a three-group configuration consisting of, in order from the object side, a first lens group with positive refractive power, a second lens group with negative refractive power, and a third lens group with negative refractive power. The lens group consists of at least two positive lenses and at least one negative lens, the second lens group consists of at least one negative lens, the third lens group consists of at least one negative lens, and the first lens group consists of at least one negative lens. A compact zoom characterized by changing the distance between the lens group and the second lens group and the distance between the second lens group and the third lens group to change the magnification and correct the movement of the image plane accompanying the change in magnification. lens.