JPH0750252B2 - Compact zoom lens - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a zoom lens used for a small video camera or the like, and more particularly to a compact zoom lens having a relatively variable lens configuration with a zoom ratio of about 3 times.

(Prior Art) It is composed of four lens components having positive, negative, negative, and positive refractive powers in order from the object side, and zooming is performed by moving the second lens component and the third lens component along the optical axis. Further, a zoom lens of a type in which focusing is performed by moving the first lens component along the optical axis is widely known. Such a configuration is overwhelming especially in a zoom lens having a variable power ratio of about 6 times, which is often used for small video cameras.

Recently, with the downsizing and cost reduction of the video camera body, there has been a strong demand for a zoom lens having a zoom ratio of about 3 times, which is conventionally used for a high zoom ratio zoom lens. If it is attempted to adopt the above-mentioned configuration as it is, there is a limit to the reduction in the number of lens components despite the reduction in the zoom ratio, and it is difficult to make it compact.

A lens having a relatively small number of lenses is disclosed in JP-A-57-122414, which has an F number of 4.5 and a half angle of view.
It is a telephoto zoom lens of about 5.7 ° to 14 ° and is not suitable for a small video camera. In particular, with the widening of the angle, there has been a problem that the height at which the light flux heading to the outermost periphery of the screen at the short focus end passes through the first lens component gradually increases, and the diameter of the front lens increases. Further, the back focus is too short with respect to the focal length at the short focus end, and there is a drawback that a space for inserting a low-pass filter such as a crystal plate or a face plate of an image pickup device cannot be sufficiently obtained.

An example of a zoom lens having a relatively small number can be seen in U.S. Pat. No. 3,614,207, but the angle of view at the short focus end is 16
I was still dissatisfied with the angle of -19 °. Further, it has been clarified that when a compact zoom lens is to be obtained by increasing the refractive power of each component constituting the zoom lens, the aberration change at the time of focusing by the rotation of the front lens becomes large.

(Problems to be Solved by the Invention) The present invention provides a compact zoom lens having a zoom ratio of about 3 times, a half angle of view of 7.3 to 22 °, and an F number of about 1.6, which is mainly used for a small video camera. It's about to get.

(Means for Solving Problems) The zoom lens of the present invention has a positive refractive power that moves back and forth on the optical axis for focusing in order from the object side, as shown in the sectional view of FIG. A first lens component having a negative refractive power, a second lens component having a negative refractive power, a third lens component having a negative refractive power, and a fixed fourth lens component having a positive refractive power for imaging. , A zoom lens that performs zooming by moving the second lens component and the third lens component back and forth on the optical axis, wherein the first lens component is composed of a positive doublet and a positive single lens, The second lens component is composed of a negative single lens, and the composite focal length of the first lens component is f ₁ and the composite focal length of the second lens component is f _2. 1.75 <| f ₁ / f ₂ | <3 It is characterized by satisfying the condition of.

At this time, it is desirable that the aperture stop be on the object side of the fourth lens component.

(Operation) In the zoom lens of the present invention, the number of lenses constituting the lens system is reduced by forming the second lens component having a negative refractive power with a single negative single lens. Further, in order to make the entire system compact, it is general to strengthen the refractive power of each lens component, but if the refractive power of the first lens component is strengthened, the aberration change due to focusing becomes large. In the present invention, the first lens component has the three-lens configuration including the positive doublet and the positive single lens in order to sufficiently suppress such aberration change.

As shown in the above conditional expression, the absolute value of the ratio of the refractive power of the second lens component to the refractive power of the first lens component is set to a relatively large value, but the lower limit of this expression is the compactness of the front lens and the back lens. If the refracting power of the second lens component becomes weaker than this condition under the condition of increasing the focus, in order to keep the total lens length compact, the negative refracting power of the third lens component must be increased, and The height of the light flux that forms an image on the outermost periphery of the screen at the focal point will cut off the first lens component more. Further, since the combined refracting power of the first lens component and the second lens component becomes weak at the short focus end, and the effect of the retro focus is weakened, it is necessary to obtain a sufficient back focus. Becomes complicated.

On the contrary, the upper limit of this expression shows the limit of the refractive power of the second lens component, and if the condition is exceeded, the change in the amount of chromatic aberration generated in the second lens component due to zooming becomes significant. Specifically, the chromatic aberration of magnification increases at the short focus end in the direction of reducing the image.

Further, in the present invention, the aperture stop is arranged closer to the object side than the fourth lens component, and more specifically to the third lens component and the fourth lens component.
It is placed between the lens components. Considering the range where the light flux to be imaged at the outermost part of the screen intersects the diaphragm surface, if the intersection of the optical axis and the diaphragm surface is not included in this range, the peripheral part of the screen will be vignetted in a small diaphragm state. . Therefore, the height at which the light flux at the periphery of the screen cuts the first lens component largely depends on the position of the aperture stop, and tends to become smaller as the position of the aperture stop is closer to the object side. In a relatively wide-angle zoom lens as in the present invention, not the axial light flux on the long focus side, but the light flux at the outermost periphery of the screen at a close distance near the short focus end determines the front lens diameter. It can be seen that the appropriate setting of (1) greatly affects the compactness of the optical system. For this,
When the aperture stop is arranged on the object side of the fourth lens component as in the zoom lens of the present invention, at least one lens in the fourth lens component is present on the object side of the stop, as is often seen in the past. As compared with the case of performing the above, the light flux around the screen can be made lower than the height at which the first lens component is cut, and the front lens diameter can be reduced.

(Examples) Examples of the zoom lens according to the present invention will be described below.

In an embodiment, the third lens component consists of a negative doublet and the fourth lens component comprises at least two positive lenses and at least one negative lens, the most object side and most image side of the fourth lens component. The lens consists of a positive lens or a doublet.

In the table below, r, d, n, and νd are the radius of curvature of the surface, the surface spacing, the refractive index, and the Abbé number, respectively, and f ₁ , f ₂ ,
f ₃ and f ₄ represent the combined focal lengths of the respective lens components. The variable distance between the first lens component and the second lens component is a,
The variable distance between the second lens component and the third lens component is b,
The variable distance between the lens component and the fourth lens component is c. Indicated by ∞ in the table is the cover glass.

(Effects of the Invention) As can be seen from the above-mentioned embodiments and drawings, the present invention
With a small number of lenses, without increasing the front lens diameter,
With a half angle of view of 7.3 to 22 ° and an F number of about 1.6, it is possible to obtain a compact zoom lens with well-balanced aberration correction over the entire zoom range while having a sufficient back focus for use as a small video camera. It was

[Brief description of drawings]

1 to 3 are cross-sectional views of Examples 1 to 3, respectively, and FIGS. 4 to 6 are aberration curve diagrams of Examples 1 to 3, respectively.

Claims (1)

[Claims] 1. A first lens component having a positive refracting power, which moves back and forth on the optical axis for Focusings, a second lens component having a negative refracting power, and a negative refracting power, in order from the object side. The third lens component has a fourth lens component having a positive refracting power and fixed for image formation, and is changed by moving the second lens component and the third lens component back and forth on the optical axis. A zoom lens for performing a magnification, in which a first lens component is composed of a positive doublet and a positive single lens, a second lens component is composed of a negative single lens, and a composite focal length of the first lens component is f _1. A zoom lens characterized by satisfying the condition of 1.75 <| f ₁ / f ₂ | <3, where f ₂ is the combined focal length of the _first and second lens components.