JP3302063B2 - Rear focus compact zoom lens - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rear-focusing type zoom lens, and more particularly to a small-sized rear-focusing type zoom lens having good optical performance and used for a photographic camera or the like.

[0002]

2. Description of the Related Art Conventionally, negative, positive and negative 3
In a zoom lens composed of groups, a lens adopting a so-called rear focus type in which focusing is performed by moving a lens group other than the first lens group has been proposed in, for example, JP-A-64-74521. I have.

[0003]

SUMMARY OF THE INVENTION Japanese Patent Application Laid-Open No. 64-7452
No. 1 discloses a zoom lens having, in order from the object side, a negative lens, a negative lens, and a negative lens.
Focusing is performed by moving at least one of the lens groups.When focusing is performed by moving the second lens group, the amount of extension when focusing near the telephoto end is extremely large. It is only possible to obtain a short distance in such a range that the two lens groups do not interfere with the first lens group, and this is a major obstacle in meeting the need for a shorter shooting distance, which will become more demanding in the future. I have. In order to reduce the short distance, the distance between the first lens unit and the second lens unit must be intentionally increased, which is against the downsizing of the lens.

When focusing is performed by moving the third lens group, in the case of the embodiment disclosed in JP-A-64-74521, focusing is performed by moving the second lens group up to the intermediate focal length. Focusing is performed by moving the third lens group from the distance to the telephoto end, which further complicates the mechanical structure. Further, if focusing is performed by moving the third lens group in the entire focal length range, the third lens group moves to the image plane side during focusing. Is greatly reduced, leading to a shortage of the peripheral light amount, or a major obstacle to shortening the shortest photographing distance.

[0005] When focusing is performed by moving the second lens group and the third lens group integrally, generally the second lens group and the third lens group are used.
Since the combined refractive power (power) of the lens group and the third lens group becomes weaker from the wide-angle end to the telephoto end, the amount of extension at the time of focusing near the telephoto end increases.
The same inconvenience as in the case where focusing is performed by moving the second lens group described above occurs.

The present invention has been made in view of such a problem, and has as its object the order of negative, positive and negative three in order from the object side.
It is an object of the present invention to provide a compact zoom lens that is a group of zoom lenses that stabilizes optical performance at the time of short-distance focusing in a rear focus system and that shortens the shortest shooting distance, which has been difficult to achieve conventionally.

[0007]

According to the present invention, there is provided a small zoom lens of a rear focus type according to the present invention, in order from the object side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and A zoom lens configured by a third lens group having a negative refractive power and performing zooming from the wide-angle end to the telephoto end by changing the distance between the lens groups, wherein the third lens group has a positive refractive power. The front lens group includes a front lens group and a rear lens group having a negative refractive power. The front lens group having the positive refractive power and the second lens group are simultaneously moved toward the object side to perform focusing. .

[0008]

The reason and operation of the above configuration will be described below. The present invention is intended to reduce the shortest photographing distance, which is expected to increase in the future, with a primary focus on suppressing aberration fluctuation due to focusing on a short distance in a small zoom lens. is there.

As a method of realizing this, a so-called rear focus type is introduced, and the degree of aberration fluctuation is reduced with a simple lens configuration, and the shortest photographing distance is intended to be shortened.

Accordingly, in the present invention, as shown in FIGS. 1 and 2, a first lens group G1 having a negative refractive power, a second lens group G2 having a positive refractive power, and a third lens group G3 having a negative refractive power. In a three-unit zoom lens having a power arrangement consisting of: a second lens unit G2 and a third lens unit G3 serving as a focusing lens unit, and a front unit and a second unit having a positive refractive power of the third lens unit G3.
Focusing is performed by simultaneously extending the lens group G2 to the object side. In addition, FIG.
FIG. 2 is a sectional view of a lens at a wide-angle end (a) and a telephoto end (b) of Examples 1 and 2 to be described later.

At this time, the on-axis air gap between the second lens group G2 and the front group having the positive refractive power of the third lens group G3 may be variable (FIG. 2), but the mechanical structure is limited. From the problem,
During focusing, it is more desirable to feed out integrally (FIG. 1) without changing the air interval on the axis.

Also, a second focusing lens group,
The combined refractive power (power) φ _FT at the telephoto end of the lens group G2 and the front group having the positive refractive power of the third lens group G3 is preferably used in the following range. 1.0 <φ _FT / φ _T2,3 <3.0 (1) where φ _T2,3 is the combined refractive power (power) of the second lens group G2 and the third lens group G3 at the telephoto end. It is.

In conditional expression (1), if the upper limit of 3.0 is exceeded, it will be disadvantageous for aberration correction, and it will be difficult to ensure satisfactory optical performance from infinity to short distance. On the other hand, if the lower limit of 1.0 is exceeded, the amount of extension at the time of focusing becomes extremely large, and the short distance performance is greatly deteriorated, making it difficult to shorten the shortest photographing distance.

Further, it is desirable to satisfy the following conditional expression. 1.0 <φ _FT / φ ₂ <3.0 (2) where φ ₂ is the refractive power of the second lens group G2.

In conditional expression (2), if the upper limit of 3.0 is exceeded, it is disadvantageous in aberration correction, and it becomes difficult to ensure satisfactory optical performance from infinity to short distance. On the other hand, if the lower limit of 1.0 is exceeded, the amount of extension at the time of focusing becomes extremely large, and the short distance performance is greatly deteriorated, making it difficult to shorten the shortest photographing distance.

In a later-described embodiment of the present invention, the third
The front group having positive refracting power of the lens group G3 is composed of a single lens. Even if this front group is a cemented lens or a lens group including a plurality of lenses, the conditional expression (1) ) And (2), there is no problem at all.
However, as a result, the overall length of the lens system becomes large, which is not desirable. The front group of the third lens group G3 can sufficiently exhibit its effect with a single-lens configuration. It is desirable that the lens component having a positive refractive power at this time has a meniscus shape with the convex surface facing the image surface side.

In general, in a zoom lens employing a rear focus type, the amount of focusing movement becomes large near the telephoto end, and there is a major problem in how aberration fluctuations are suppressed. However, the focusing method of the present invention is required. Thereby, the focusing movement amount can be reduced, the short-range aberration fluctuation is suppressed, and the shortest shooting distance can be shortened.

The focusing movement amounts in the first and second embodiments described below are shown below. However, the numerical value of the second embodiment is the focusing movement amount when the front group of the second lens group G2 and the third lens group G3 is integrally extended. In the above table, W represents the wide-angle end, S represents the intermediate focal length, and T represents the telephoto end.

[0019]

Next, Embodiments 1 and 2 of a rear focus type small zoom lens according to the present invention will be described. The lens data of each embodiment will be described later.
The zoom lens has a wide angle of view of 8.9 to 48.5 mm.
As shown in the lens cross-sectional views at the wide-angle end (a) and the telephoto end (b) in FIG. 1, the zoom lens is a three-unit zoom lens including a first lens unit G1 to a third lens unit G3. By simultaneously extending the positive lens component and the second lens group G2 disposed on the object side toward the object side, it is possible to focus on a short-distance object point.

The first lens group G1 is composed of a negative meniscus lens having a convex surface facing the object side and a positive meniscus lens having a convex surface facing the object side. The second lens group G2 is a biconvex lens. , Stop, negative meniscus lens having a convex surface facing the image surface side, and a bi-convex lens. The third lens group G3 has a positive meniscus lens having a convex surface facing the image surface and a convex surface facing the image surface. It consists of two negative meniscus lenses. Further, aspherical surfaces are used for the rear surface of the negative meniscus lens of the first lens group G1, the front surface of the first biconvex lens of the second lens group G2, and the rear surface of the second biconvex lens.

At the wide-angle end, intermediate focal length, and telephoto end of this embodiment, spherical aberration (a), astigmatism (b), chromatic aberration of magnification (c), and distortion at an object point at infinity and at a short distance of -1 m The aberration (d) is shown in FIG. 3 for comparison. As can be seen from FIGS. 1 and 3, the optical system is small in size, has small aberration fluctuation from infinity to short distance, and has stable and good optical performance.

Embodiment 2 has a focal length of 24.8 to 43.6.
mm wide-angle zoom lens. As shown in the sectional views of the lens at the wide-angle end (a) and the telephoto end (b) in FIG. 2, the lens system group configuration is the same as that of the first embodiment. By simultaneously moving the positive lens component disposed on the object side and the second lens group G2 toward the object side while changing the distance between them, it is possible to focus on a short-distance object point.

The first lens group G1 is composed of a biconcave lens and a biconvex lens, and the second lens group G2
Comprises a biconvex lens, a stop, a biconcave lens, and a positive meniscus lens having a convex surface facing the image surface side. The third lens group G3 includes a positive meniscus lens having a convex surface facing the image surface side and a biconcave lens. Consists of sheets. Aspheric surfaces are used for the front surface of the biconvex lens of the second lens group G2 and the rear surface of the positive meniscus lens.

FIG. 4 shows an aberration diagram similar to FIG. 2 of this embodiment. As is clear from FIG. 2 and FIG.
The aberration fluctuation is small, and the optical performance is stable and good.

Next, lens data of each embodiment will be shown.
In the following, the symbols are the above, f is the focal length of the entire system, F _NO is the F number, ω is the half angle of view, and f _B is the back focus (from the last lens surface to the image surface at the object point at infinity). , R ₁ , r ₂ ... Are the radii of curvature of the respective lens surfaces, d ₁ ,
d ₂ ... the spacing between the lens surfaces, n _d1, n _d2 ... d-line refractive index of each lens, ν _d1, ν _d2 ... is the Abbe number of each lens. The aspherical shape is represented by the following equation, where x is the optical axis direction and y is the direction orthogonal to the optical axis. ^{x = y 2 / {r +} (r 2 -y 2) 1/2} + A 4 y 4 + A 6 y
⁶ + A ₈ y ⁸ + A ₁₀ where r is the radius of curvature on the optical axis, and A ₄ , A ₆ , A ₈ , and A ₁₀ are aspherical coefficients. Note that the values in parentheses in the zoom interval table of the second embodiment are air intervals at a short distance (-1 m).

[0026] Example 1 f = 28.9 ~ 37.1 ~ 48.5 F NO = 4.12 ~ 4.88 ~ 5.77 ω = 36.8 ~ 30.2 ~ 24.0 ° f B = 6.999~ 18.925~ 29.708 r 1 = 128.6408 d 1 = 1.800 n d1 = 1.54739 ν _d1 = 53.55 r ₂ = 15.1270 _{(aspherical) d 2 = 3.252 r 3 =} 15.7875 d 3 = 3.100 n d2 = 1.75520 ν d2 = 27.51 r 4 = 21.6156 d 4 = ( variable) r ₅ = 12.5130 (aspherical) d _{5 = 3.018 n d3 = 1.54771 ν} d3 = 62.83 r 6 = -154.4681 d 6 = 0.984 r 7 = ∞ ( _{stop) d 7 = 1.500 r 8 =} -13.7278 d 8 = 1.500 n d4 = 1.75520 ν d4 = 27.51 r 9 _{= -202.2773 d 9 = 1.431 r 10} = 113.2344 d 10 = 2.530 n d5 = 1.63854 ν d5 = 55.38 r 11 = -16.4556 ( aspherical) d ₁₁ = _{(variable) r 12 = -28.5866 d 12 =} 2.600 n d6 = _{1.78470 ν d6 = 26.22 r 13 =} -20.8379 d 13 = 3.807 r 14 = -16.4909 d 14 = 1.480 n d7 = 1.51821 ν d7 = 65.04 r 15 = -181.2999 Aspheric coefficient Second surface A ₄ = 0.29016 × 10 ^-5 A ₆ = 0.69734 × 10 ^-7 A ₈ = -0.54647 × 10 ^-9 A ₁₀ = 0.20686 × 10 ^-11 Fifth surface A ₄ = 0.41762 × 10 ^-4 A ₆ = 0.46842 × 10 ^-6 A ₈ = 0.23804 × 10 ^-7 A ₁₀ = -0.16870 × 10 ^-9 Surface 11 A ₄ = 0.10243 × 10 ^-3 A ₆ = 0.23797 × 10 ^-6 A ₈ = 0.20462 × 10 ^{_{-7 A 10 = -0.12628 × 10 -9}} φ FT / φ T2,3 = 1.222 φ FT / φ 2 = 1.100
.

[0027] Example 2 f = 24.8 ~ 32.8 ~ 43.6 F NO = 4.12 ~ 4.88 ~ 5.77 ω = 42.3 ~ 33.4 ~ 26.4 ° f B = 10.791~ 21.843~ 32.975 r 1 = -1043.6612 d 1 = 1.800 n d1 = 1.74320 ν _d1 = 49.31 r ₂ = 20.3709 d ₂ = 4.187 r ₃ = 38.5333 d ₃ = 3.845 nd ₂ = 1.69895 ν _d2 = 30.12 r ₄ = -803.9040 d ₄ = (variable) r ₅ = 18.1552 (aspheric) d ₅ = _{2.500 n d3 = 1.64250 ν d3 =} 58.37 r 6 = -53.2947 d 6 = 1.000 r 7 = ∞ ( _{stop) d 7 = 2.056 r 8 =} -32.1913 d 8 = 2.000 n d4 = 1.75211 ν d4 = 25.05 r 9 = 31.2373 _{d 9 = 1.221 r 10 = -65.0619} d 10 = 3.443 n d5 = 1.78650 ν d5 = 50.00 r 11 = -13.9312 ( aspherical) d ₁₁ = _{(variable) r 12 = -43.4298 d 12 =} 3.921 n d6 = 1.78472 ν _{d6 = 25.71 r 13 = -22.1705 d} 13 = 2.077 r 14 = -29.6034 d 14 = 1.800 n d7 = 1.86300 ν d7 = 41.53 r 15 = 58.4928 Aspherical coefficients fifth surface ^{_{A 4 = -0.30499 × 10 -4 A}} 6 = -0.36324 × 10 -6 A 8 = 0.10292 × 10 -7 11th surface ^{_{A 4 = 0.42477 × 10 -4 A}} 6 = 0.27039 × 10 - ⁶ A ₈ = 0.37434 × 10 ^-8 φ _FT / φ _T2,3 = 1.625 φ _FT / φ ₂ = 1.209
.

[0028]

As described above, according to the small zoom lens of the rear focus type according to the present invention, a small zoom lens having a small aberration fluctuation when focusing on a short distance and a shortest photographing distance is achieved. Can be achieved.

In particular, by adopting the focusing method of the present invention, a wide-angle zoom lens can be formed with a minimum necessary number of lenses, and a lens system having stable performance even at a short distance can be realized. In addition, it has a significant meaning in that the overall size of the lens system can be reduced.

[Brief description of the drawings]

FIG. 1 is a sectional view of a small zoom lens according to a first embodiment of the present invention at a wide-angle end (a) and at a telephoto end (b).

FIG. 2 is a lens sectional view similar to FIG. 1 of Example 2.

FIG. 3A shows spherical aberration (a), astigmatism (b), chromatic aberration of magnification (c) at an object point at infinity and at a short distance of −1 m at a wide-angle end, an intermediate focal length, and a telephoto end in Example 1. It is an aberration figure which shows distortion aberration (d) by contrast.

FIG. 3B shows spherical aberration (a), astigmatism (b), chromatic aberration of magnification (c) at an object point at infinity and at a short distance of −1 m at the wide-angle end, an intermediate focal length, and a telephoto end in Example 1. It is an aberration figure which shows distortion aberration (d) by contrast.

FIG. 4A is an aberration diagram similar to FIG. 2 of the second embodiment.

FIG. 4B is an aberration diagram similar to FIG. 2 of the second embodiment.

[Explanation of symbols]

 G1: first lens group G2: second lens group G3: third lens group

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G02B 15/22

Claims (6)

(57) [Claims] 1. A lens system comprising, in order from an object side, 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 negative refractive power. In the zoom lens which changes the magnification from the wide-angle end to the telephoto end by changing the position, the third lens group includes a front group having a positive refractive power and a rear group having a negative refractive power. A small zoom lens of a rear focus type, wherein focusing is performed by simultaneously moving a group and the second lens group toward the object side. 2. The rear focus type small zoom lens according to claim 1, wherein during focusing, the front group of the second lens group and the front group of the third lens group having a positive refractive power move integrally. 3. The combined refractive power φ _{FT of} the second lens group and the front group of the third lens group having a positive refractive power at the telephoto end satisfies the following conditional expression (1). The compact zoom lens of the rear focus type according to claim 1 or 2. 1.0 <φ _FT / φ _T2,3 <3.0 (1) where φ _T2,3 is a combined refractive power of the second lens unit and the third lens _unit at the telephoto end. 4. The rear focus type small zoom lens according to claim 3, wherein the following conditional expression (2) is satisfied. 1.0 <φ _FT / φ ₂ <3.0 (2) where φ ₂ is the refractive power of the second lens group. 5. The rear focus type small zoom lens according to claim 4, wherein the front group having a positive refractive power of the third lens group is composed of one lens. 6. The rear focus type small zoom lens according to claim 5, wherein the front group having a positive refractive power of the third lens group has a meniscus shape having a convex surface facing the image plane side.