JPH0248086B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is a high-performance variable magnification reading lens system that has a magnification range (on the reduction side) of about 1/7 to 1/20 times (it can also be used as an enlargement photographing lens system for enlargement, etc. if the object and image plane are reversed). ), which provides a lens system that has a large variable power ratio of 2 times or more, is compact, and has high performance at all magnifications. Conventionally, with regard to variable magnification lenses, there is Japanese Patent Application Laid-Open No. 57-73715 that uses a so-called zoom lens with a constant object-to-image distance, but the variable magnification ratio is as small as about 1.6x, and the lens system is large. It was several times larger than a fixed focal length lens system. There are also many fixed focal length lens systems known, but they can only be said to have high performance at a certain magnification, or at a magnification very close to that. The present invention has an intermediate configuration between a zoom lens and a fixed focal length lens, and magnification is mainly performed by changing the object-to-image distance in the same way as a fixed focal length lens, but by changing a part of the distance within the lens system. This enabled us to achieve high performance over the entire range of magnifications used, with a variable power ratio of 2x or more, and when changing the distance between some parts of the lens system, it is possible to increase the focal length on the high magnification side. Therefore, the amount of change in the distance between objects and images is reduced. In actual reading devices such as facsimile and 0CR, the object and image plane are fixed, so as a means to change the distance between the object and image, it is generally known that a mirror is placed in the middle and the position of the mirror is moved. However, it is effective in reducing the amount of movement of this mirror, and is very advantageous in terms of compactness of the device. First, to explain the present invention in detail, from the object side, it is composed of a front group with a negative focal length and a rear group with a positive focal length, and when changing from a low magnification to a high magnification, the distance between the front and rear groups is reduced. (1) 0<f _nax /f _nio -1/K<0.1 (2) 0.2<f _nio /｜F _I ｜＜0.7 Where, f _nio : Minimum focal length of the entire system on the low magnification side f _nax : Maximum value of the focal length of the entire system on the high magnification side K: Magnification ratio of the used magnification f _I : This is a high-performance variable magnification reading lens system that satisfies various conditions regarding the focal length of the front group. In addition, in the above lens system, the front group consists of a negative meniscus lens with a convex surface facing the object side and a positive meniscus lens from the object side, (3) 1.3<f _I /f ₁ <2.1 (4) 0.01<d ₂ ／｜f _I ｜＜0.1 (5) 0.1＜r ₃ ／｜f _I ｜＜0.5 However, f ₁ : Focal length of negative meniscus lens d ₂ : Air distance r between negative meniscus lens and positive meniscus lens ₃ : Constructed to satisfy the object-side radius of curvature condition of a positive meniscus lens. The rear group is similar to the conventional type of fixed focal length lens system. In the present invention, the rear group includes 6 groups and 8
A modified Gaussian type or orthometer type is used. Furthermore, the front group is fixed with respect to the image plane during zooming. This is desirable because it simplifies the structure of the mirror frame. Next, each of the above conditions will be explained. Condition (1) is related to the increment in the change ratio of the focal length and the ratio of the variable magnification ratio; if the lower limit is exceeded, the focal length becomes smaller on the high magnification side and the amount of change in the object-image distance increases. As mentioned above, this is contrary to the need to make the reading device more compact. Also, if the upper limit is exceeded,
It has the advantage of reducing the amount of change in the object-to-image distance and approaching a zoom lens system, but it is difficult to maintain high performance over the entire magnification range of 2x or more, and in addition, the lens system itself becomes larger. Become. Condition (2) relates to the power of the front group; if the lower limit is exceeded, it is advantageous in terms of aberration correction, but the amount of change in the focal length of the entire system due to change in the distance between the front and rear groups becomes small, resulting in an increase in the size of the lens system. If the upper limit is exceeded, the power of the front and rear groups will become stronger, which is advantageous for the compactness of the lens system, but it will not be possible to maintain high performance at all magnifications used above 2x. Since the reading device must have high performance across the entire image plane at all magnifications, the power of the front group is quite small compared to a typical zoom lens for photography. Within the previous group given by this condition (2),
The following conditions (3) to (5) exist. Conditions (3) to (5) are related to the power arrangement in the front group, and if the lower limit of condition (3) is exceeded, it is advantageous in terms of aberration correction, but it increases d ₂ and increases the size. If it exceeds , the radius of curvature _r2 of the second surface of the negative meniscus lens becomes smaller, and the radius of curvature _r3 of the positive meniscus lens also becomes smaller accordingly, resulting in an increase in changes in spherical aberration and coma aberration during zooming. invite. Condition (4) is also related to condition (3), but when the lower limit is exceeded, the power of both the negative and positive meniscus lenses in the front group increases, and the upper limit of condition (1) and the power of condition (5) become stronger. It violates the lower limit and is not appropriate. On the other hand, exceeding the upper limit is advantageous in correcting aberrations, but the lens system becomes large, which is not appropriate. As stated in condition (3), when the lower limit of condition (5) is exceeded, changes in spherical aberration and comatic aberration increase during zooming, and when the upper limit is exceeded, r ₂ must also be increased. As a result, the power of the negative meniscus lens becomes smaller, leading to an increase in d ₂ and a larger size. Examples 1 and 2 of the present invention are shown below. Here, r is the radius of curvature, d is the lens thickness or air gap, N is the refractive index of the d-line, υ is the Atsube number, F is the aperture ratio to the ∞ object, f is the focal length of the entire system, and y is the image height half. The angle of view, m is the lateral magnification, U is the distance between objects and images, and f _B is the back focus.

【table】

【table】 [Brief explanation of drawings]

1 and 3 are lens system configuration diagrams on the low magnification side corresponding to Examples 1 and 2, respectively. Figure 2a,
b, c, FIG. 4 a, b, c are respectively Example 1,
2, in which a shows the aberration diagrams on the low magnification side, b shows the aberration diagrams on the intermediate magnification side, and c shows the aberration diagrams on the high magnification side.

Claims (1)

[Claims] 1. From the object side, a front group having a negative focal length;
It consists of a rear group with a positive focal length, and when changing from low magnification to high magnification, the distance between the front and rear groups is reduced to increase the focal length and reduce the object-image distance. (1) 0<f _nax /f _nio -1/K<0.1 (2) 0.2<fmin/｜f _I |<0.7 where, f _nio : The focal length of the entire system on the low magnification side. Minimum value f _nax : Maximum value of the focal length of the entire system on the high magnification side K: Variable magnification ratio of the used magnification f _I : A high-performance variable magnification reading lens system that satisfies various conditions for the focal length of the front group. 2 The front group consists of a negative meniscus lens and a positive meniscus lens with the convex surface facing the object side, (3) 1.3<f _I /f ₁ <2.1 (4) 0.01<d ₂ / | f _I | ＜0.1 (5) 0.1＜r ₃ /｜f _I ｜＜0.5 However, f ₁ : Focal length of negative meniscus lens d ₂ : Air distance between negative meniscus lens and positive meniscus lens r ₃ : Focal distance of positive meniscus lens A high-performance variable magnification reading lens system according to claim 1, which satisfies the condition of the radius of curvature on the object side. 3. A high-performance variable magnification reading lens system according to claim 1, wherein the front group is fixed when the image plane is taken as a reference during variable magnification.