JPS6191614A - Shortening method of focal length of zoom lens - Google Patents

Abstract

PURPOSE:To obtain a focal length shorter than that at a wide-angle end by moving the 1st and the 2nd lens groups in the optical-axis direction, and moving the 4th lens group in the optical-axis direction at a different speed. CONSTITUTION:The 1st, the 3rd, and the 4th lens groups having positive refracting power and the 2nd lens group having negative refracting power are arranged successively from the object side. When the lens is zoomed to the normal telephoto side, a group L1 is moved to the object side Z1, a group L2 is moved to an image side Z2, and a group L3 is put in reciprocal motion Z3 to the image side while a group L4 is fixed. When the focal length is shortened, the groups L1 and L2 provide negative refracting power as one body and move from the wide-angle end of a variable focal length range to the object side as shown by A1, the group L3 is fixed, and the group L4 moves to the image side as shown by A2. Therefore, high performance is maintained continuously in a zoom range and needs to be obtained only at the end terminal of a focal length expansion range, so the power of each group needs not be increased and aberration compensation is facilitated, thereby maintaining the high performance.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a zoom lens, and particularly to a conventional variable focal length lens.
This invention relates to a method for obtaining even shorter focal lengths across the range.

(Prior Art) Various methods have been known in the past to obtain different focal lengths beyond the normal variable focal length range.

For example, a lens system with positive refractive power is inserted between the photographing lens and the image plane to reduce the focal length. There is a so-called rear converter system, or a method in which an afocal converter is installed in front of the camera to reduce or expand the focal length. The former has the disadvantage that the lens configuration is complicated and expensive because it is necessary to remove aberrations using the rear converter alone. The latter allows for either wide-angle or telephoto lenses, but has the drawback of increasing the lens diameter. Also, since both are converter-type, it is necessary to always maintain a lens system other than the I5 chromatic lens.
It is not easy to take pictures instantly and is not portable.

There is also an idea to simply set the zoom magnification to a high magnification, but in this case, even though the power of each group is strengthened, it is necessary to prevent the overall length from increasing, so a low magnification is used throughout the zoom range. The optical performance deteriorates compared to when.

(Objective) The object of the present invention is to obtain a focal length that exceeds the existing variable focal length range, and at the same time to provide excellent instant shooting performance and good portability.

One way to achieve this is to provide, in order from the object side, a first lens group with positive refractive power, a second lens group with negative refractive power, a third lens group with negative refractive power, and a fourth lens group with positive refractive power, 1st
From the wide-angle side of the variable focal length range of a zoom lens that changes the focal length by changing the distance between each lens from the lens group to the fourth lens group, move the first and second lens groups together toward the optical axis. At the same time, the fourth lens group is moved in the optical axis direction at a speed different from those of the fourth lens group to obtain a focal length shorter than that at the wide-angle end.

Also, at this time, the first lens is set so that the focus is always at a constant position within the normal variable focal length range. The 2nd degree third lens group must be moved, but the focal length is f41! In the iI sub-range, it is sufficient that the object is in focus at the shortest focal length.
It doesn't matter if it's out of focus in the middle castle, so
It is not necessary that the image performance is good in the intermediate region, and it is sufficient that the image performance is good only at the end.

(Embodiment) FIG. 1 shows an embodiment of the present invention. In FIG. 1, Ll is the first lens group, which has a positive refractive power and can be moved back and forth for focusing. L2 is the second lens group and has negative refractive power;
3 is the third lens group with positive refractive powereL4 is the fourth lens group, which is an imaging lens with positive refractive power 0 In the figure, the straight or curved arrow indicates the movement of the lens group in the optical axis direction. is expanded over time, and zl, z2. z3 is used for normal zooming, and A1 and A2
corresponds to the movement to make the focal length shorter than the wide-angle end. In other words, in the zoom lens of this example, the positive first lens group L1 is moved toward the object side (Z
l) At the same time, move the negative second lens group L2 toward the image side (
Z2)L, the positive third lens group 3 performs a reciprocating motion (Z3) toward the image side, and the positive fourth lens group L4 is fixed.

On the other hand, when shortening the focal length, the positive first lens group L1
And the negative second lens group L2 moves from the wide-angle end of the variable focal length # range to the object side! (At), the positive third lens group L3 is fixed, and the positive fourth lens group L4 moves toward the image side (A2).

In other words, the entire system is positive in the normal zooming range.

It is a combination of negative, positive, and positive refractive powers. However, when the focal length is shortened, the ft5l lens group L1 and the second lens group L2 together have a negative refractive power, so the refractive power is negative.

This is a combination of positive and positive refractive powers, the middle lens group is fixed, the lens group closer to the object side moves toward the object side, and the lens group closer to the image side moves toward the image side.

Note that the movement of the lens group for shortening does not necessarily have to start from the wide-angle side.

In this way, in the present invention, the focus plane is not always constant from the minimum focal length to the maximum focal length, and high performance is maintained continuously in the normal zoom range, and high performance is maintained only at the end in the extended point distance range. Therefore, there is no need to increase the power of each group compared to a high-magnification zoom using the full-range zoom method, and therefore aberration correction becomes easy and high performance can be maintained. Furthermore, since there is no need to keep the focal plane constant in the middle range of the focal length expansion range, the empty space can be used at the wide-angle end without having to provide a special space for moving the lens group to expand the focal pressure range. You can. Therefore, the basic total length of the lens is approximately equal to the total length normally determined by the specifications of the zooming range, and it is possible to prevent the total length from increasing due to shortening of the focal length.

Although the object of the present invention is achieved with the configuration described above, it is desirable to satisfy the following conditions as a guideline when actually designing a lens.

0.6<f p/ f vr<0.9 However, the shortest focal length when the focal length is shortened is fp, and the focal length at the wide-angle end of the existing variable focal length range is fw.

If it is above the upper limit, the effect of shortening the focal length will be diminished, and if it is below the lower limit, the focal ratio will be reduced! The effect of shortening a becomes remarkable, but since the first lens group and the :tS2 lens group move toward the object side when the focal pressure # is shortened, the front lens moves away from the aperture.

The amount of movement increases, and the increase in nfj ball diameter reduces the amount of rice produced by 31 volumes. At this time, it is more suitable to use a zoom system in which the total length (from the first surface to the film surface) is short at the so-called wide-angle end in which the first lens group moves on the optical axis in the normal zooming range, and the total length is long at the telephoto end. In other words, since the overall length is short at the wide-angle end, there is plenty of room in the diameter of the front lens, and the first lens and second lens group can be moved more toward the object side when the focal length is shortened.

In the lens data described later, with the wide-angle end as a reference, the first lens group moves linearly toward the object side, the second lens group moves linearly toward the image plane, and the third lens group moves convexly back and forth toward the image plane. doing. The fourth lens group is a relay lens.

At this normal zooming range, the focal pressure is 14-29
7. FNo. 1:5.7, and by moving the first lens group and second lens group together from the wide-angle end toward the object side and the fourth lens group toward the image side, the focal pressure @75
, FNo., 1: 5.7. Attached aberration diagram, gSZ diagram, and tJS3 diagram.

In Fig. 4, f=297, f=165, f=11
FIG. 5 shows aberrations when f=75 (shortest focal length).

0〉Pumi a〉Krushimi
〉 Mi a 〉 Mi ; ＾ba 1 In this example, f=
Despite being significantly shortened to 75 mm, as can be seen from the various aberrations in FIG. 5, the aberrations are kept within a range that poses no problem for practical use. And since it doesn't require a wide-angle converter, it fully satisfies the demands for instant shooting and portability.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of a lens showing an embodiment of the present invention, fJS
Figures 2, 3, 4, and 5 are longitudinal aberration diagrams, respectively. In the figure, Ll is the first lens group, L2 is the second lens group, and L3
is the third lens group, L4 is the fourth lens group, 6m is the meridional image plane, and ΔS is the sagittal image plane. Out+11 Canon Co., Ltd.? Art craftsman Hachi Kanei

Claims (1)

[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 negative refractive power, and the fourth lens group has a positive refractive power.
In a zoom lens that includes lens groups and changes the focal length by changing the distance between the lenses from the first lens group to the fourth lens group, the first lens group and the second lens group are moved in the optical axis direction, and A method for shortening the focal length of a zoom lens, the method comprising: obtaining a focal length shorter than the wide-angle focal length by moving the fourth lens group in the optical axis direction at a speed different from that of the fourth lens group. (2) The combined refractive power of the first lens and the second lens group is negative, and these are moved together toward the object side, and the fourth lens group is moved toward the image side. How to shorten the focal length of a zoom lens. (3) The third lens group has positive refractive power, and when the first lens group and the second lens group move together toward the object side and the fourth lens group moves toward the image side, the third lens group 2. The method for shortening the focal length of a zoom lens according to claim 1, wherein the group is fixed. (4) The wide-angle end focal length of the variable focal length range is fw,
If the focal length when the focal length is shortest is fp, then
The method for shortening the focal length of a zoom lens according to claim 1, wherein the zoom lens satisfies 0.6<fp/fw<0.9.