JPH0512685B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a compact zoom lens, and particularly to a zoom lens with a short overall length suitable for lens shutter cameras and the like. [Prior art] As a zoom lens that has a field angle range that includes the standard field angle (about 2ω = 47°) and has a variable power ratio of about 2x, it has a negative refractive power front. A zoom lens of the type that is composed of two groups, a rear group and a rear group with positive refractive power, and performs magnification by changing the air gap between the two groups is well known. Since this type of zoom lens has a retrofocus lens system configuration, the back focus is long, and when used for a single-lens reflex camera, it has the advantage of securing a space for arranging a mirror. However, since the back focus is long, the total length of the lens system (distance from the first surface of the lens to the image plane) becomes long, and the diameter of the front lens also becomes large, making it impossible to make the lens system compact. On the other hand, as a zoom lens that has been miniaturized to the extent that it can be incorporated into a lens-shutter camera, there is a known zoom lens that includes a front group with a positive refractive power and a rear group with a negative refractive power. This lens system is characterized by having a telephoto type configuration, which makes it possible to shorten the overall length. A conventional example of this type is JP-A-Sho.
No. 57-201213 (USP4682860), JP-A-60-191216
The number etc. are known. However, these conventional examples
The zoom ratio is small at around 1.5, and each aberration is not corrected in a well-balanced manner. Conventional zoom lenses of this type with a variable power ratio of up to 2 include the lens systems described in Japanese Patent Application Laid-Open Nos. 170816-1982 and 15115-1980; The total length of the lens system is long. Furthermore, these conventional examples have the drawback that aberrations are not sufficiently corrected. [Problems to be Solved by the Invention] The present invention provides a zoom lens that includes a standard angle of view, has a variable power ratio of about 2, and has well-corrected aberrations. In addition to adopting a lens type composed of a rear group with a refractive power of
The object of the present invention is to provide a compact zoom lens that satisfies the following requirements (a) to (c). (a) Reduction in the total length of the lens system (b) Reduction in the outer diameter of the lens (c) Reduction in the amount of movement of the movable group [Means for solving the problem] The zoom lens of the present invention has a positive refractive power. It consists of a group and a rear group with negative refractive power, and magnification is changed by changing the distance between the two groups.The front group is a positive meniscus lens with a convex surface facing the object side in order from the object side, and a negative lens. , one or more positive lenses, and the rear group includes, in order from the object side, a positive lens, a biconcave lens, and a negative meniscus lens with a convex surface facing the image side. The lens system of the present invention is characterized by having the lens configuration as described above and satisfying the following condition (1). (1) −1<(r _a +r _b )/(r _a −r _b )<0.2 where r _a and r _b are the radius of curvature of the object-side surface and image-side surface of the biconcave lens in the rear group, respectively. be. The most suitable lens configurations for the front group of the lens system of the present invention include, in order from the object side, a positive lens, a biconcave lens, one positive lens, a positive lens, a biconcave lens, and two positive lenses. ing. The above condition (1) defines the shape of the biconcave lens in the rear group. If the upper limit of this condition (1) is exceeded, the refractive power of the object-side surface of this lens becomes too weak, making it impossible to sufficiently correct the field curvature. If the lower limit is exceeded, this lens will not be a biconcave lens but will have a meniscus shape, so the distance f _C from the final lens surface to the image plane will become shorter.In other words, if the above lens becomes a meniscus shape, the rear principal point will move toward the object side. As a result, the final lens surface becomes closer to the image plane and f _C becomes shorter. Next, the zoom lens of the present invention described above satisfies the following condition (2).
It is desirable to satisfy the following. (2) 1.15<β〓 _W <1.7 where β〓 _W is the imaging magnification of the rear group at the wide end. In a zoom lens of a type such as the lens system of the present invention, the distance f _B from the image-side principal point of the rear group to the image plane can be shortened, so the total length of the lens system can be shortened. However, since the rear group is close to the film surface, the lens diameter of the rear group becomes large. It is assumed here that the aperture matches the front group. In order to prevent this increase in the lens diameter of the rear group, f _C should be made longer. However, since the image-side principal point of the rear group usually exists within the lens system, f _C < f _B , which requires a larger f _B than the target value of f _C , which goes against the shortening of the total length of the lens system. It turns out. Therefore, in order to make the lens system compact, it is important to set f _C and f _B in a well-balanced manner so that the requirements for shortening the overall length of the lens system and reducing the diameter of the rear group are compatible. be. In the present invention, a biconcave lens is placed in the rear group to bring the image-side principal point of the rear group closer to the image side, thereby reducing the difference between f _C and f _B , thereby ensuring back focus and reducing the overall length of the lens system. This is a well-balanced combination of shortening and shortening. The following equations are derived from FIG. L _W = ew + f _B = f〓−(1−β〓 _W ) ² /β〓 _W・f〓 (i) f _B =f〓(1−β〓 _W ) (ii) Δ ₁ =f〓(1− Z) (β〓 _W −1/Z・β〓 _W ) (iii) Δ〓＝f〓(1−Z)β〓 _W (iv) Here, Z is the magnification ratio, and f〓 and f〓 are respectively The focal length of the group and the rear group, L _W is the total length of the lens system at the wide-angle end, ew is the distance between the principal points of the front group and the rear group at the wide-angle end, Δ〓, Δ〓 are the distance from the wide-angle end of the front group and the rear group, respectively This is the amount of movement to the telephoto end. Since f〓<0, in order to make f _B >0 in order to prevent mechanical interference between the lens system and the film surface,
From equation (), β〓 _B > 1 must be satisfied. And in order to make f _B large, we must make f〓- or β〓 _W large. As is clear from equations () and (), if f = - becomes large, Δ ₁ and Δ ₂ also become large, which means that the amount of movement of the movable group becomes large, making it impossible to make the lens system compact. On the other hand, if β〓 _W is made too large, it becomes impossible to satisfactorily correct aberrations with the limited number of lenses in the lens system. From the above, it is preferable that β〓 _W satisfies the above-mentioned condition (2). Exceeding the upper limit of condition (2) is preferable for increasing the back focus f _B , but it becomes impossible to satisfactorily correct aberrations. Furthermore, if the lower limit of condition (2) is exceeded, the back focus f _B cannot be increased. It is more desirable if the upper limit in condition (2) is 1.45. In other words, it is more desirable to satisfy the following conditions. 1.15<β〓 _W <1.45 In the zoom lens of the present invention, the front group consists of, in order from the object side, a positive meniscus lens with a convex surface facing the object side, a negative lens, a positive lens with a convex surface facing the image side, and a positive lens with a convex surface facing the image side. It is desirable that the lens be composed of a positive lens with a convex surface and that the following conditions (3) to (6) be satisfied. (3) 0.6＜f ₁ ／f _W ＜0.87 (4) 0.8＜−f〓−／f ₁ ＜1.6 (5) −2.4＜(r ₄ −r ₅ )／(r ₄ +r ₅ ) (6) 0.1 <d ₅ /f ₃ <0.6 where f _W is the focal length of the entire system at the wide-angle end, r ₄ is the radius of curvature of the image side surface of the negative lens in the front group, and r ₅ is the radius of curvature of the image side surface of the negative lens in the front group. The radius of curvature of the object-side surface of a positive lens with a convex surface, d ₅ is the thickness of the positive lens, and f ₃ is the focal length of the positive lens. In a lens system with the above configuration, various aberrations, especially spherical aberration, are suppressed by the action of the air lens between the positive meniscus lens and the negative lens in the front group, and the positive lens with its convex surface facing the image side suppresses various aberrations, especially spherical aberration. By balancing the wall thickness and refractive power, the image-side principal point of the front group is brought closer to the image side, ensuring a variable magnification ratio and an air gap between both groups at the telephoto end. Furthermore, the variable power ratio is ensured by balancing the refractive powers of each lens, suppressing the occurrence of off-axis aberrations, and making the outer diameter of the negative meniscus lens small. Furthermore, the zoom lens system of the present invention satisfies the above-mentioned conditions in order to satisfactorily correct aberrations over the entire zoom range.
It was made to satisfy (3) to (6). Conditions (3) and (4) define the refractive power distribution of the zoom lens of the present invention, and are provided in order to make the lens system compact and to achieve a variable power ratio of approximately 2 or more than 2. It is. If the upper limit of condition (3) is exceeded, the rear group will come into contact with or intersect with the image plane, making it difficult to secure back focus and increasing the overall length of the lens system, which goes against the aim of compactness. Condition (3)
If the lower limit of is exceeded, the total length of the lens system will be shortened, but the refractive power of each group will be strong as described above, making it difficult to ensure good performance all the way to the periphery. Furthermore, the refractive power of each lens in each group becomes strong, making it difficult to correct aberrations. If the upper limit of condition (4) is exceeded, the outer diameter of the lens in the rear group becomes large and it becomes difficult to secure a zoom ratio of about 2 or more than 2. When the lower limit of condition (4) is exceeded, positive distortion on the wide-angle side increases. Condition (5) relates to the shape of the air lens between the negative lens in the front group and the positive lens with its convex surface facing the image side. If the lower limit of condition (5) is exceeded, higher order aberrations of spherical aberration will largely occur in the positive direction. This harmful higher-order spherical aberration occurs significantly at the telephoto end. It is preferable to set an upper limit to this condition (5) and satisfy the following condition (5'). (5') −2.4<(r ₄ −r ₅ )/(r ₄ +r ₅ )<5 It is preferable that the upper limit is not exceeded under the above conditions because it becomes easy to correct spherical aberrations well. Furthermore, it is more preferable to set this upper limit value to 0.1. That is, when it exceeds 0.1, higher-order spherical aberrations occur largely in the negative direction. Condition (5') concerns the shape of the air lens between the negative lens in the front group and the next positive lens. In the present invention, the front group is composed of a triplet type consisting of a positive lens, a negative lens, and a positive lens, but the performance of the negative lens and the following positive lens is highly sensitive to manufacturing errors, resulting in high cost. This condition (5') was established to reduce the sensitivity of these lenses to manufacturing errors and to facilitate processing and assembly of parts and to achieve low costs. If this condition (5') is not satisfied, the negative lens and the subsequent positive lens become more sensitive to manufacturing errors, making it difficult to process and assemble parts, which is not preferable. Condition (6) defines the refractive power distribution in the front group, and is a condition that is strongly related to the position of the rear principal point of the front group. If the upper limit of condition (6) is exceeded, the refractive power of each lens in the front group becomes strong, making it difficult to obtain a flat image surface. If the lower limit of condition (6) is exceeded, the rear principal point position of the front group approaches the object side, and it is necessary to secure an air gap between the front group and the rear group at the telephoto end, or to increase the zoom ratio of about 2 or more than 2. It becomes difficult to secure. It is also desirable that the following condition (7) be satisfied regarding the refractive power distribution of the lenses in the rear group. (7) 0.7< _-f7 -/-f〓-<1.4 where _f7 is the focal length of the negative meniscus lens in the rear group. When the upper limit of condition (7) is exceeded, it becomes difficult to flatten the meridian image plane over the entire zoom range, and when the lower limit of condition (7) is exceeded, positive distortion at the wide-angle end increases. Here, it is more desirable to set the upper limit of condition (7) to 1.2. This is particularly preferable for flattening the image plane. Furthermore, in order to maintain a good balance of chromatic aberrations, it is desirable to satisfy the following conditions (7) and (8). (8) ν ₅ >30 (9) ν ₇ >45 where ν ₅ is the Abbe number of the positive lens, and ν ₇ is the Abbe number of the negative meniscus lens. Condition (8) is for maintaining good chromatic aberration of magnification at the wide-angle end, and if the lower limit of this condition is exceeded, a large negative chromatic aberration of magnification for the g-line at the wide-angle end occurs. Condition (9) is to maintain a balance between axial chromatic aberration and lateral chromatic aberration. If the lower limit of this condition is exceeded, the axial chromatic aberration of the g-line at the wide-angle end will be large and will occur in the positive direction, and the telephoto The lateral chromatic aberration of the g-line magnification on the side is large and occurs in the positive direction, which hinders the achievement of good imaging performance. In the zoom lens of the present invention, it is desirable to satisfy the following condition (2') instead of condition (2) and further satisfy the following condition (10). (2') 1.2<β〓 _W <1.5 (10) 0.6<−f〓−/f _W <0.95, f〓<0 Next, each of the above conditions will be explained. When β〓 _W becomes large beyond the upper limit of condition (2'), the power of the front group becomes strong at the same time, and aberrations cannot be sufficiently corrected with a small number of lenses as in the lens system of the present invention. If β〓 _W becomes small beyond the lower limit of condition (2'), the total length of the lens system becomes long, which is contrary to the purpose of the present invention, as can be seen from equation (). If -f〓- becomes longer than the upper limit of condition (10), the amount of movement of each group determined by equations () and () becomes large, which goes against the purpose of the present invention. Condition(10)
If the lower limit of f is exceeded and f is shortened, as can be seen from equation (), the necessary back focus cannot be secured, and furthermore, the number of lenses in the rear group cannot sufficiently correct aberrations. Furthermore, in order to better correct the aberrations of the lens system, it is effective to use an aspheric surface in the front group.

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However, r ₁ , r ₂ , ..., is the radius of curvature of each lens surface, d ₁ , d ₂ , ..., is the thickness and distance between each lens, and n ₁ , n ₂ , ..., is the radius of curvature of each lens. refractive index,
ν ₁ , ν ₂ , . . . are the Atsube numbers of each lens. Among the above examples, Example 1 has the lens configuration shown in FIG. 2, and the aberration conditions at the wide-angle end, intermediate focal length, and telephoto end are as shown in FIGS. 14, 15, and 16, respectively. . Embodiment 2 has the lens configuration shown in FIG. 3, and the aberration conditions at the wide-angle end, intermediate focal length, and telephoto end are as shown in FIGS. 17, 18, and 19, respectively. The aberration conditions at the wide-angle end, intermediate focal length, and telephoto end with the lens configuration shown in Figure 4 are as shown in Figures 20, 21, and 22, respectively. In Example 4, the lens configuration shown in Figure 5 was used at the wide-angle end. , the aberration situation at the intermediate focal length and telephoto end is shown in Figures 23 and 24.
As shown in FIG. Example 5 has the configuration shown in FIG. 6, and the eighth surface _r8 is an aspherical surface. In this example, the wide-angle end, intermediate focal length,
The aberration situation at the telephoto end is as shown in FIGS. 26, 27, and 28, respectively. Example 6 has the configuration shown in FIG. 7, and the eighth surface _r8 is an aspherical surface. The aberration situations in each state of this embodiment are shown in FIGS. 29, 30, and 31, respectively. Example 7 has the lens configuration shown in FIG. 8, and the eighth surface _r8 is an aspherical surface. The aberration situations in each state of this example are as shown in FIGS. 32, 33, and 34, respectively. Example 8 has the lens configuration shown in FIG. 9, and the sixth surface _r6 is an aspherical surface. The aberration situation in this example is shown in Figure 35.
As shown in FIGS. 36 and 37. Example 9 has the lens configuration shown in FIG. 10, and the 11th surface r ₁₁
is an aspherical surface. The aberration situations in each state of this embodiment are as shown in FIGS. 38, 39, and 40, respectively. Example 10 has the configuration shown in FIG. 11, and the aberration conditions at the wide-angle end, intermediate focal length, and telephoto end of this example are as shown in FIGS. 41, 42, and 43, respectively. Example 11 has the lens configuration shown in FIG. 12, and the eighth surface r ₈
and the 11th surface r ₁₁ is an aspheric surface. The aberration conditions in each state of this example are shown in FIGS. 44, 45, and 46, respectively.
As shown in the figure. Example 12 has the lens configuration shown in FIG. 13, with the eighth surface _r8 and the eleventh surface _r11 being aspherical. The aberration situation in this example is as shown in FIGS. 47, 48, and 49. The aspheric surface used in these examples has an optical axis direction of z, a direction perpendicular to the optical axis of y, and a paraxial radius of curvature of R.
It is expressed by the following formula. In addition, the conical coefficient k, aspherical coefficient A, B,
Values of C, D, and E are shown in the data. [Effects of the Invention] The zoom lens of the present invention secures the necessary amount of back focus despite its large zoom ratio of 2, and is compact with a short overall length of the lens system and a small amount of group movement during zooming. Performance is good.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the basic configuration of the zoom lens of the present invention and the movement status of each group, and FIGS. 2 to 13 are cross-sectional views of Examples 1 to 12 of the present invention, respectively.
14 to 16 are aberration curve diagrams of Example 1, FIG. 17 to 19 are aberration curve diagrams of Example 2, and FIGS. 20 to 22 are aberration curve diagrams of Example 3. , FIGS. 23 to 25 are aberration curve diagrams of Example 4, FIGS. 26 to 28 are aberration curve diagrams of Example 5, and FIGS. 29 to 31 are aberration curve diagrams of Example 6. 32 to 34 are Example 7
, FIGS. 35 to 37 are aberration curve diagrams of Example 8, FIGS. 38 to 40 are aberration curve diagrams of Example 9, and FIGS. 41 to 43 are aberration curve diagrams of Example 8. 10, FIGS. 44 to 46 are aberration curve diagrams of Example 11, and FIGS. 47 to 49 are aberration curve diagrams of Example 12.

Claims (1)

[Claims] 1. A front group having positive refractive power and including, in order from the object side, a positive meniscus lens with a convex surface facing the object side, a negative lens, and one or more positive lenses; It has a negative refractive power and is composed of, in order from the object side, a positive lens, a biconcave lens, and a negative meniscus lens with a convex surface facing the image side. The magnification is changed by changing the interval of
A compact zoom lens that satisfies the following conditions. (1) −1＜(r _a +r _b )／(r _a −r _b )＜0.2 (2) 1.15＜β〓 _W ＜1.7 (3) 0.6＜f〓／f _W ＜0.87 (10) 0.6＜｜ f〓｜/f _W <0.95, f〓<0 However, r _a and r _b are the polar radii of the object-side surface and image-side surface of the biconcave lens in the rear group, respectively, and β〓 _W is the wide-angle end The magnification of the rear group at , f _W is the focal length of the entire system at the wide-angle end, f〓 is the focal length of the front group, and f〓 is the focal length of the rear group.