JP2890323B2 - Zoom lens - Google Patents

Description

Description: BACKGROUND OF THE INVENTION (Industrial Application Field) The present invention relates to a zoom lens used for a single-lens reflex camera or the like, in particular, a zoom lens having an angle of view from quasi-wide angle to quasi-telephoto and having a zoom ratio of 2 or more About.

(Prior Art) As a zoom lens for a single-lens reflex camera having a focal length at the wide-angle end shorter than the screen diagonal length and a zoom ratio of about 2 times, a first group of negative refractive power and a positive refractive power of It is known that a two-component zoom lens composed of the second lens group is general and can obtain a compact and favorable aberration correction state with a simple configuration.

However, when trying to obtain a zoom ratio of 2 times or more with a two-component zoom lens, when trying to maintain compactness,
The power of each lens unit becomes strong, making it difficult to correct aberrations, and at the wide-angle end, the back focus required for mirror-up becomes insufficient. Conversely, if a sufficient back focus is to be obtained, it is necessary to increase the distance between the first lens unit and the second lens unit at the wide angle end, which leads to an increase in the overall length of the lens and an increase in the diameter of the front lens.

(Problems to be solved) The present invention has a focal length at a wide-angle end shorter than a screen diagonal length, has a zoom ratio of about 2.2 to 2.5 times, and has a sufficient back focus for a single-lens reflex camera, Another object of the present invention is to obtain a compact zoom lens having good imaging performance.

(Means for Solving the Problems) In order to achieve the above object, a zoom lens according to the present invention includes, in order from the object side, a first group having a negative refractive power, a second group having a positive refractive power, and a positive lens having a positive refractive power. The zoom lens system includes a third lens unit. During zooming from the wide angle end to the telephoto end, the distance between the first lens unit and the second lens unit decreases,
The first group moves, the second group and the third group move toward the object side, and the first group moves in order from the object side to the first lens having a negative refractive power so that the distance between the group and the third group increases. It is characterized by comprising a second lens and a third lens having a positive refractive power and satisfying the following conditions.

2.0 <f ₃ / f _W <5.0 where f ₃ is the focal length of the third lens unit, and f _W is the focal length of the entire system at the wide-angle end.

The third lens unit moves toward the object side during focusing from infinity to a short distance.

The third group of the zoom lens according to the present invention includes a positive lens element, a negative lens element, and a positive lens element in order from the object side, and preferably satisfies the following condition.

▲ ▼ <1.65 1.75 <▲ ▼ 0.3 <R _3F / f _W <0.9 where ▲ ▼: Average refractive index of positive lens element in the third group ▲ ▼: Negative lens element in the third group Average value of refractive index R _3F : radius of curvature of the lens surface closest to the object in the third lens unit (operation) In the zoom lens of the present invention, the rear unit of the conventional two-component zoom lens is formed by a relatively strong positive refraction. Divided into a second group having power and a third group having relatively weak positive refractive power.
The zoom ratio is enlarged while maintaining the required back focus at the wide-angle end by increasing the distance between the group and the third group on the wide-angle side and increasing the distance on the telephoto side. However, even at the telephoto end, it is a necessary condition that the combined refractive power of the first and second units is positive.

This will be described with reference to the drawings. First, FIGS. 1A and 1B each show a conventional two-component zoom lens in which the rear group is tentatively divided into a second group and a third group. It is a schematic diagram at an end and a telephoto end. Therefore, the distance between the second group and the third group does not change. FIGS. 2A and 2B are schematic diagrams of the three-component zoom lens of the present invention at the wide-angle end and the telephoto end, respectively, assuming that the refractive power of each group is the same as in FIG. I have. In this zoom lens, the arrangement at the wide-angle end is the same as seen in FIGS. 1A and 2A, but the arrangement at the telephoto end is the second and third groups in FIG. The distance between the first group and the second group
As long as the combined refractive power with the group is positive, the focal length of the entire system is longer than that of FIG. 1 (b). Therefore, in the zoom lens of the present invention, the focal length region is expanded to the telephoto side.

As described above, in the zoom lens of the present invention, the distance between the second group and the third group is small on the wide-angle side and large on the telephoto side. This allows the zoom ratio to be increased and the third lens unit to be moved from the image side to the object side at a wide angle side and moved to a small distance at the telephoto side, thereby effectively utilizing this distance to perform focusing from infinity to a short distance. Can be done.

The conditional expression is for obtaining the above effect by giving an appropriate refractive power to the third lens unit. 2 below the lower limit
If an attempt is made to obtain a zoom ratio of 2 times or more, f ₃ <f _T. Therefore, at the telephoto end, the combined refractive power of the first lens unit and the second lens unit becomes negative, which is inconsistent with the gist of the present invention. On the other hand, if the upper limit is deviated, the amount of movement of the third lens unit for focusing becomes excessively large, which causes disadvantages such as mechanical problems.

The conditional expressions and the conditions relating to the refractive index of the lens material in the third lens unit. If these conditions are not met, the Petzval sum becomes small and it is difficult to obtain a good image surface.

The conditional expression is a condition for giving a relatively strong positive refractive power to the lens surface closest to the object in the third lens unit and obtaining a good image surface, especially on the telephoto side. If the lower limit is deviated, the image plane becomes largely under at the telephoto end. Conversely, if the value is outside the upper limit, the function of lowering the image surface is reduced, and as a result, the image surface tends to be over.

(Embodiment) Hereinafter, as an embodiment of the present invention, a zoom lens satisfying all the above-described conditions will be described.

In addition, as for each symbol in the table, R is the radius of curvature of each refraction surface, and D is
Is the refractive surface spacing, N _d is the refractive index of the lens material, v _d is the Abbe number, f is the focal length of the entire lens system, 2ω is the angle of view, F
F-number, f _B is the back focus, U is shows the object distance.

Example 1 f = 36.09 to 83.27 F = 3.6 to 4.6 2ω = 64.0 ゜ to 29.0 ゜ No. RD N _d ν _d 1 57.503 1.40 1.72000 50.2 2 23.358 8.50 3 −144.869 1.20 1.72000 50.2 4 50.882 1.55 5 38.882 5.00 1.80 100 35.0 6 726.113 Variable 7 26.918 4.80 1.77 250 49.6 8 −442.119 0.80 9 −108.614 1.00 1.80518 25.4 10 75.347 0.20 11 24.605 3.40 1.72000 50.2 12 71.951 2.633 13 57.119 1.22 1.78590 44.2 14 15.945 2.60 15 43.593 3.00 1.61405 55.0 16 −758.20 4. 45.8 18 287.425 0.60 19 −1788.051 1.00 1.80610 40.9 20 15.872 1.50 21 33.748 2.70 1.56883 56.3 22 −207.296 FIG. 3 shows a lens sectional view of this embodiment, and FIGS. 4 to 9 show aberration curves.

Example 2 f = 39.05 to 83.24 F = 3.6 to 4.6 2ω = 60.0 ゜ to 29.2 ゜ No. RD N _d ν _d 1 54.806 1.40 1.72000 50.2 2 27.051 6.80 3 −280.824 1.20 1.72000 50.2 4 35.076 3.52 5 37.861 5.50 1.80 100 35.0 6 276.427 Variable 7 26.936 5.10 1.71300 53.9 8 −60.527 1.00 1.80518 25.4 9 391.507 1.10 10 19.378 3.00 1.72000 50.2 11 36.556 3.31 12 91.938 1.00 1.78590 44.2 13 14.579 2.60 14 61.590 3.00 1.61405 55.0 15 −515.623 Variable 16 17.419 3.00 1.54814 45.8 17 145 18 2280.796 1.00 1.80610 40.9 19 19.358 2.00 20 38.477 3.50 1.51633 64.1 21 −86.408 FIG. 10 shows a lens sectional view of this embodiment, and FIGS. 11 to 16 show aberration curves.

(Effect of the Invention) As shown in the embodiments, the zoom lens according to the present invention has a zoom ratio of 2 times or more and a sufficient back focus and brightness for a single-lens reflex camera, while having a simple configuration. As can be seen, good performance can be obtained regardless of the subject distance.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 and FIG. 2 are schematic views for explaining the structure of the present invention, FIG. 3 is a sectional view of a lens of Example 1, and FIGS. FIG. 10 is a lens cross-sectional view of the second embodiment, and FIGS. 11 to 16 are aberration curve diagrams of the second embodiment.

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁶ , DB name) G02B 9/00-17/08 G02B 21/02-21/04 G02B 25/00-25/04

Claims (2)

(57) [Claims] 1. A first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a positive refractive power. The first lens unit and the third lens unit are zoomed from the wide-angle end to the telephoto end in order from the object side. The first group moves, the second and third groups move toward the object side, and the first group moves in order from the object side so that the distance between the second group and the distance between the second group and the third group increases. A zoom lens comprising a first lens and a second lens having a negative refractive power and a third lens having a positive refractive power, and satisfying the following condition: 2.0 <f ₃ / f _w <5.0 where f ₃ is the focal length of the third group, f _w is the focal length at the wide angle end of the entire system. 2. The zoom lens according to claim 1, wherein the third lens unit moves toward the object side during focusing from infinity to a short distance.