JPS61183613A - Zoom lens - Google Patents

Abstract

PURPOSE:To reduce an outside diameter of a lens barrel by providing a diaphragm for determining an F-number, on the back of the final surface of the second lens group, and moving this diaphragm to an object side, when zooming is executed from a wide angle to a telephone. CONSTITUTION:The second lens group 2 of a positive refractive power is provided on an image side of the first lens group 1 of a negative refractive power, and zooming is attached by drawing, for instance, a locus which is developed and drawn as Z1, Z2. In this case, a diaphragm 3 for determining an F-number is placed immediately behind the lens group 2, and it is moved as one body with the lens group 2, or independently to an object side at the time of zooming from a wide angle to a telephone. At this time, an axial light L is subjected to a divergent action by the lens group 1 and made incident on the lens 2 in a high axial height, converged strongly therein and emitted from the lens group 2. Accordingly, when the diaphragm 3 is placed immediately behind the lens group 2, an open aperture diameter can be reduced comparing with the case when it is provided on the front part of the lens group 2 or immediately before it. In this way, the diameter of the diaphragm mechanism is reduced, the outside diameter of a lens barrel is decreased, and the mechanical structure is simplified.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a zoom lens, and in particular, to miniaturizing the external appearance of the lens barrel.
Related to zoom lenses.

(Description of Prior Art) The dimensions of a lens barrel are affected by the configuration of the lens system housed inside. For example, the outer diameter of a zoom lens barrel depends on the open diameter of the aperture stop, which determines the lower number. Often fixed. The mechanical structure of the aperture is provided in a case that houses the aperture blades, and the aperture structure including the blade case is considerably larger than the open aperture diameter, and is usually about two to three times the open aperture diameter.

The outer diameter near the tip of the lens barrel of a zoom lens is often determined by the outer diameter of the lens itself, but whether or not the outer diameter of the lens barrel can be made thinner on the camera body side depends on the maximum aperture diameter. Determined by size.

In recent years, the introduction of electronic engineering technology has promoted the miniaturization of camera bodies, but since the lens barrel is made up of a glass lens and mechanism, it is difficult to make it smaller. It also has the disadvantage that it may worsen and make it difficult to grasp the lens barrel.

On the other hand, a first lens group with a negative refractive power and a second lens group with a positive refractive power are provided in order from the object side, and zooming is performed by moving the first lens group and the second lens group independently of each other on the optical axis. In reverse telephoto zoom lenses with two or three group configurations, the aperture stop is often placed on the object side within the second lens group or between the first and second lens groups.
However, in the case of a reverse telephoto zoom lens, the axial light beam is once diverged at the negative first lens group and then enters the second lens group, so the open aperture diameter at that position is It becomes larger than the open luminous flux diameter.

Therefore, especially when the open aperture at the telephoto end is increased, it becomes difficult to make the lens barrel thinner. To avoid this phenomenon, there are many lenses that have a smaller aperture at the telephoto end (the lower number is larger and darker) than at the wide-angle end, but this is undesirable as it tends to degrade lens performance.

Note that many reverse telephoto zoom lenses have an aperture behind the second lens group, but this aperture has a constant aperture diameter and is designed to block unnecessary off-axis light beams. It does not affect the axial luminous flux.

(Objective) A first object of the present invention is to provide a zoom lens whose outer diameter can be reduced.

A second purpose is to reduce the diameter of the first lens group of the reverse telephoto lens in order to make the entire lens barrel smaller in outer diameter.

Furthermore, the third purpose is to shorten the overall length.

In order to achieve the above object, the present invention provides a lens that performs zooming by independently moving a first lens group having a negative refractive power and a second lens group having a positive refractive power disposed on the image side in the optical axis direction. A diaphragm for determining the F number is provided behind the final surface of the second lens group, and the diaphragm is moved toward the object side when zooming from wide-angle to telephoto.

Further features will become clear from the description below.

(Description of an embodiment) Fig. 1 shows an embodiment of the present invention, in which 1 is a first lens group with negative refractive power, 2 is a second lens group with positive refractive power, for example, Zl and 22. Achieve zooming by drawing a trajectory drawn by unfolding.

Note that both lens groups 1.2 each consist of a plurality of lenses.

Reference numeral 3 denotes an aperture that determines the lower number according to the present invention, and is disposed immediately after the second lens group 2, and moves toward the object side either integrally or independently with the second lens group 2 when zooming from wide-angle to telephoto.

Ll is one of the axial rays, which undergoes a divergent action in the first lens group 1 and enters the second lens group 28 at a high axial height, where it is strongly converged and exits the second lens group 2. Therefore, aperture 3
If it is arranged immediately after the second lens group 2, the open aperture diameter can be reduced compared to the case where it is arranged in front of the second lens group 2, or just before the side portion.

This has the effect of reducing the diameter of the aperture mechanism and reducing the outer diameter of the lens barrel, but in addition, since the aperture moves closer to the camera body, the communication mechanism with the camera body for driving the aperture becomes smaller in the second lens group 2. It also has the advantage that it does not intersect with the drive mechanism, making the mechanical structure easier.

Although the outer diameter of the lens barrel is reduced by the above, it is desirable that the diameter of the front lens element is further reduced. To solve this problem, two configurations are possible, and they can also be installed in parallel.

Figure 2 shows an example of reducing the diameter of the front lens.

A third lens group 4 is provided on the image side of the aperture 3 to bear negative refractive power. This makes it possible to strengthen the refractive power from the first lens group to the second lens group, so a predetermined zoom ratio can be obtained even if the amount of movement of the first and second lens groups is reduced. , the total length of the lens can be shortened. Therefore, the length from the diaphragm to the foremost lens surface is reduced, and an increase in the diameter of the front lens element can be prevented.

The third lens group has a meniscus shape in which a negative lens and a positive lens are cemented in order from the object side, with a concave surface facing the object side, and it is preferable that the refractive index of the negative lens is higher than the refractive index of the positive lens.

On the other hand, Figures 3A and 3B depict the most common shape of the first lens group of a conventional inverted telephoto zoom lens. In Figure 3A, the first lens is a negative meniscus lens that is convex toward the object side. A large air gap must be maintained between the first lens and the second lens, and in the case of the type shown in Figure B where a positive lens is placed in front, the radius of curvature of the rear surface of the second lens is extremely small. A large air gap must be provided between the second and third lenses. Figure 4 shows the behavior of the light flux of a conventional zoom lens, where A is the negative first lens group, B is the positive second lens group, C is the negative third lens group, and D is the aperture. . Further, Ll is an on-axis light beam, and L2 is an off-axis light beam.As can be seen from the optical path of the off-axis light beam L2, the shape of the first lens in the first lens group A causes the diameter of this lens to increase.

FIG. 5 shows the lens shape according to an embodiment of the present invention. FIG.
It is preferable to introduce an aspherical surface on the image side surface of the second lens or the object side surface of the second lens.The significance of this aspherical surface will be described later.

FIG. 5B shows a configuration suitable for the case where the refractive power of the first lens group is further increased, in which a biconcave lens, a negative lens, and a positive meniscus lens with a convex surface facing the object side are arranged in order from the object side. The most advantageous shape is for the central negative lens to also be biconcave, but the object-side surface can also be flat or slightly convex. In addition, it is better to make the image side surface of the first lens, the image side surface of the second lens, or the object side surface of the third lens an aspheric surface. However, it is also possible to share the amount of aspherical surface among a plurality of lens surfaces.

Figure 6 is an example in which the negative third lens group described in Figure 2 and the lens shape described in Figure 5 are applied together.As shown in Figure 0, the diameter of the foremost lens that takes in the most off-axis light flux. has been reduced, and as is clear from a comparison with FIG. 4, the total length of the lens has been shortened. As can be seen from the lens data and various aberration diagrams described later, the optical performance is maintained despite the shortened overall length.

Next, as mentioned above, at least one aspherical surface is provided in the first lens group, and as the aspherical surface moves away from the optical axis, when it is concave, the negative refractive power becomes weaker, and when it is convex, the negative refractive power becomes weaker. The shape is such that the refractive power is strong. When such an aspherical shape is used for the surfaces of the negative lens and the positive lens that face each other, the distance between the lenses can be reduced without the lenses colliding with each other around the periphery of the lenses. In other words, reducing the overall center thickness of the first lens group S7 contributes to preventing an increase in the diameter of the front lens. In addition, since it is aspherical, performance can be corrected well with a small number of lenses, and the shape of the aspherical surface prevents comb-shaped distortion at the wide-angle end, which tends to occur especially when the refractive power of the first lens group becomes strong. Effective for correction.

Furthermore, since the second lens group also has a strong refractive power, it is possible to improve the performance by providing an aspherical surface.

In this case, it is preferable to use a shape in which the convex surface is aspheric and the positive refractive power becomes weaker toward the periphery, but this is not the case since the target aberrations differ depending on the target lens performance.

It is possible to further actively promote the present invention as described above, but this may require cooperation with the camera body. As a method of downsizing the camera body, it is advantageous to use a half mirror or a fixed prism as the quick return mirror because the mirror movable space can be reduced. In this case, even a lens with a short back focus can be attached, so that the refractive power of the first lens group and the second lens group can be strengthened, making it possible to achieve extreme miniaturization. The same applies to rangefinder type cameras.

Numerical examples will be described below, where Ri is the radius of curvature of each lens surface, D is the distance between lens surfaces, N is the refractive index, and ■ is the Abbe number.

Numerical Example 1 is shown in Figures 7 and 10, Numerical Example 2 is shown in Figure 8.
11 and Numerical Example 3 correspond to FIG. 9 and FIG. 12.

(Effects) The present invention described above is extremely excellent in that the outer diameter of the lens barrel can be reduced. Furthermore, since no diaphragm is provided in the second lens group, the lens barrel can be constructed in one piece, which is effective in holding the lens with high precision. Furthermore, the simplification of the lens barrel structure is often beneficial in automating lens assembly.

[Brief explanation of drawings]

FIG. 1 is an optical sectional view showing an embodiment of the present invention. Fig. 2 is an optical cross-sectional view showing another embodiment, Fig. 3 is a cross-sectional view of a conventional first lens group shape, Fig. 4 is an optical path diagram of a conventional zoom lens, and Fig. 5 is a first lens group shape. 6 is an optical path diagram of the example, FIG. 7 is a lens sectional view of numerical example 1, FIG. 8 is a lens sectional view of numerical example 2, and FIG. 9 is a numerical example. A cross-sectional view of the lens of Example 3, and FIGS. 10, 11, and 12 are various aberration curve diagrams. In the figure, 1 and 1' are the first lens group, 2 and 2' are the second lens group, and 3 is the aperture that determines the F number. 4 is the third lens group. bow twice

Claims (8)

[Claims] (1) In a lens that performs zooming by independently moving a first lens group with negative refractive power and a second lens group with positive refractive power disposed on the image side in the optical axis direction, the final lens group of the second lens group A zoom lens characterized in that a diaphragm for determining a lower number is provided immediately after the lens surface, and the diaphragm is moved toward the object side when zooming from wide-angle to telephoto. (2) The zoom lens according to claim 1, wherein the diaphragm moves together with the second lens group during zooming. (3) In a lens that performs zooming by independently moving a first lens group with negative refractive power and a second lens group with positive refractive power disposed on the image side in the optical axis direction, the final lens group of the second lens group A diaphragm that determines the lower number is provided immediately after the lens surface, and when zooming from wide angle to telephoto, the diaphragm is moved toward the object side, while a lens group with negative refractive power is placed on the image side of the diaphragm. A zoom lens. (4) The zoom lens according to claim 3, wherein the lens group disposed on the image side of the aperture is fixed during zooming. (5) The zoom lens according to claim 3, wherein the lens group disposed on the image side of the diaphragm has a meniscus shape with a concave surface facing the object side, which is formed by joining a negative lens and a positive lens in order from the object side. (6) The zoom lens according to claim 5, wherein the refractive index of the negative lens is higher than the refractive index of the positive lens. (7) In a lens that performs zooming by independently moving a first lens group with a negative refractive power and a second lens group with a positive refractive power disposed on the image side in the optical axis direction, the final lens group of the second lens group An aperture that determines the lower number is provided immediately after the lens surface, and when zooming from wide-angle to telephoto, the aperture is moved toward the object side, while the first lens group has a biconcave lens closest to the object side and a biconcave lens closest to the image side. , a zoom lens comprising a positive meniscus lens with a convex surface facing an object. (8) The first lens group has at least one aspherical surface, and when the aspherical surface is concave, the negative refractive power becomes weaker as it moves away from the optical axis, and when the aspherical surface is convex, The zoom lens according to claim 7, wherein the zoom lens has a shape in which the positive refractive power becomes stronger as the distance from the optical axis increases.