JPH0476087B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a compact zoom lens with a small back focus, which is suitable for, for example, a lens shutter type camera. [Prior Art] Most conventional zoom lenses for still cameras are aimed at single-lens reflex cameras, and are designed to have a certain degree of backfocus in order to take up the space necessary for the operation of the quick return mirror. For example, the device described in JP-A-58-111013 is extremely compact for single-lens reflex cameras, but its total length exceeds 100 mm even at the shortest focal length, and it is used in lens-shutter cameras. As a result, portability, which is a feature of lens-shutter cameras, is significantly hindered. From this point of view, several zoom lenses have been proposed for lens-shutter type cameras, as described in, for example, Japanese Patent Laid-Open No. 57-201213 and Japanese Patent Laid-Open No. 58-184916. [Problems to be Solved by the Invention] However, the zoom lens described in the above-mentioned Japanese Patent Application Laid-open No. 57-201213 has a variable focal length range of 40 mm to 60 mm, and the zoom lens has a focal length variable range of 40 mm to 60 mm, and the zoom lens has a focal length of 40 mm to 60 mm, and
It consists of a group lens and a second lens group with negative power, and the lens distance between the two groups is changed to change the magnification, and the total length at the wide-angle end is about the same as the focal length, making it highly portable. However, the zoom ratio is small at 1.5x, and the performance is also insufficient. In addition, the zoom lens disclosed in Japanese Patent Application Laid-Open No. 183916/1983 has a variable focal length range of 40 mm to 68 mm.
Then, from the object side, the first group lens with positive power, the second group lens with positive power, and the third group lens with negative power are arranged in order, and the magnification is changed while moving the three groups relative to each other, until the wide-angle end is reached. It is compact with a total length of 60.4 mm, and is also satisfactory in terms of performance. However, the outer diameter of the third group lens reaches about the diagonal of the film, and it moves by 26mm during zooming, so space is required on the camera side for this movement. Furthermore, at the wide-angle end, the back focus is extremely short at 1.16mm, and is very close to the film surface. Therefore, it becomes difficult to secure a space for arranging the film feeding member, and it becomes necessary to process the lens into a rectangular shape. Furthermore, since the final surface of the lens is very close to the image forming surface, the influence of dust adhering to that surface cannot be ignored. The present invention has been made in view of the above-mentioned shortcomings of the prior art.It shortens the overall length to improve portability, secures enough back focus to accommodate film feeding members, and moreover It is an object of the present invention to provide a zoom lens that can obtain a variable power ratio close to that of a magnification. [Means for Solving the Problems] In order to achieve the above object, the present invention basically consists of a first group lens 1 having a positive power, a first group lens 1 having a negative power, and A second group lens 2 with positive power and a third group lens 3 with positive power are arranged. When changing the magnification from the wide-angle side shown in FIG. 1 and the second lens group 2 while narrowing the distance between them and moving them toward the object side. In addition, the first
Reference numeral 4 in the figure indicates an imaging surface (film surface). In a zoom lens configured in this way, the third
Since the group lens 3 is given positive refractive power, the first
It is possible to strengthen the refractive power of the second group lens 2 without making the refractive power of the group lens 1 excessive, and therefore it is possible to increase the variable power ratio. Also,
By using the imaging magnification of the third group lens 3 in a positive range, a telephoto type refractive power arrangement is always achieved during zooming, which is advantageous in shortening the overall length.
Furthermore, the third group lens 3 is designed to perform different aberration correction actions depending on the zoom position, so that astigmatism and distortion can be particularly well corrected over the entire range of zooming. The first group lens 1 is configured by arranging, in order from the object side, a front group lens with a negative refractive power and a rear group lens with a positive refractive power. With this configuration, the rear principal point of the first group lens 1 is closer to the image side, so the movement range of the second group lens 2 for variable magnification can be increased, which is useful for achieving high variable magnification. It will be advantageous. In particular, between the refractive power ψ ₀ of the entire first group lens 1, the refractive power ψ _a of the front group lens, and the refractive power ψ _b of the rear group lens, -0.5<ψ _a /ψ ₀ <0 (1 ) 1<ψ _b /ψ ₀ <1.5 (2) It is convenient to satisfy the following relationship in order to increase the magnification ratio. Note that if the lower limit of conditional expression (1) and the upper limit of conditional expression (2) above are exceeded, the rear principal point of the first group lens 1 will be too close to the image side, resulting in a long overall length, which will hinder compactness. Become so. Furthermore, if the upper limit of conditional expression (1) and the lower limit of conditional expression (2) are exceeded, the rear principal point of the first group lens 1 will be too close to the object side, increasing the movement space of the second group lens 2. Therefore, in order to achieve a high zoom ratio, the refractive power of the second group lens 2 must be made excessively strong, making it difficult to correct aberrations during zooming. Examples of the present invention will be described below. [Example] Figures 2 to 13 show an example in which the first lens group 1 disposed on the object side has a two-group configuration of a front group 1a and a rear group 1b, and the aberrations at three zooming positions are shown. Shown with characteristics. In addition, in the spherical aberration diagram, the solid line indicates the aberration due to the d-line, the dashed line indicates the aberration due to the g-line, and the dashed line indicates the amount of violation of the sine condition.The solid line in the astigmatism diagram indicates the aberration in the sagittal plane, and the broken line indicates the aberration It shows aberrations within the meridional plane. In each of these examples, the variable range of the focal length is 36.0 mm to 68.0 mm, and the variable range of the F number is 2.88 to 5.44. For each example,
If the radius of curvature of the i-th surface counting from the object side is R _i , the axial distance from the next surface is D _i , the refractive index of the d-line is N _i , and the Atsube number is ν _i , then The data of the first to third embodiments shown in FIGS. 4 and 6 are as follows.

[Table] The variable distances D ₁₁ and D ₁₇ are as follows at focal lengths of 36.0 mm, 50.0 mm, and 68.0 mm. =36.0mm 50.0mm 68.0mm D ₁₁ 5.376 2.759 1.0 D ₁₇ 1.0 12.83 28.235 Also, the refractive power ψ a of the front group lens 1a _{and the refractive power ψ b of} the rear group lens 1b with respect to the refractive power ψ 0 of the entire first group lens ₁ The ratio is ψ _a /ψ ₀ = −0.077 ψ _b /ψ ₀ = 1.05, and the back focus _b is 7.0.

【table】

[Table] The variable interplanar distances D ₁₀ and D ₁₆ are as follows at focal lengths of 36.0 mm, 50.0 mm, and 68.0 mm. =36.0mm 50.0mm 68.0mm D ₁₀ 4.918 2.575 1.0 D ₁₆ 1.0 12.158 26.722 Also, the refractive power ψ a of the front group lens 1a _{and the refractive power ψ b of} the rear group lens 1b with respect to the refractive power ψ 0 of the entire first group lens ₁ The ratio of is ψ _a /ψ ₀ = −0.124 ψ _b /ψ ₀ = 1.094, and the back focus _b is 7.0.

【table】

[Table] The variable interplanar distances D ₁₀ and D ₁₆ are as follows at focal lengths of 36.0 mm, 50.0 mm, and 68.0 mm. =36.0mm 50.0mm 68.0mm D ₁₀ 7.388 3.574 1.0 D ₁₆ 1.0 14.05 31.1 Also, the refractive power ψ a of the front group lens 1a _{and the refractive power ψ b of} the rear group lens 1b with respect to the refractive power ψ 0 of the entire first group lens ₁ The ratio is ψ _a /ψ ₀ = −0.23 ψ _b /ψ ₀ = 1.196, and the back focus _b is 7.0. In the first embodiment described above, spherical aberration is well corrected by the air lens with negative power between the first lens 5 and the second lens 6 included in the first lens group 1. . In addition, by making the distance between the first lens 5 and the second lens 6 variable,
It is also possible to obtain a soft focus effect by increasing only spherical aberration without affecting other aberrations. In the second embodiment, the first lens 7 and the second lens 8 are
In the third embodiment, in order to reduce costs, the first lens 7 in the second embodiment is
and the second lens 8 are configured as one single lens 9, and the final lens 10 of the first lens group 1 is separated to correct spherical aberration. The fourth and fifth embodiments shown in FIGS. 8 and 10 are examples in which an aspherical surface is introduced. These aspherical shapes are It is expressed as In the above equation, △Z is the amount of displacement from the apex of the aspherical surface, Y is the height from the optical axis, R _* is the paraxial radius of curvature, k is the conic constant, and A _o is the nth-order aspherical coefficient. The data of the fourth and fifth examples are as follows. Note that with respect to the radius of curvature R, surfaces marked with "*" are aspherical surfaces.

【table】

[Table] The values of the aspheric coefficients k, A ₄ , and A ₆ for surface numbers i=6 and 13 are as follows. i k A ₄ A _{6 6} 0 0.463×10 ^-4 0.641×10 ^-7 13 −0.281 0.196×10 ^-4 0 Also, the variable interplanar distances D ₁₀ and D ₁₄ have focal lengths of 36.0 mm, 50.0 mm, At 68.0mm, it becomes as follows. =36.0mm 50.0mm 68.0mm D ₁₀ 8.31 3.95 1.0 D ₁₄ 1.0 14.18 31.46 Also, the refractive power ψ _a of the front group lens 1a and the refractive power ψ _b of the rear group lens 1b with respect to the refractive power ψ 0 of the entire first group lens ₁ The ratio is ψ _a /ψ ₀ = −0.268 ψ _b /ψ ₀ = 1.300, and the back focus _b is 7.36.

[Table] Aspheric coefficient k for surface number i = 6, 10, 11,
The values of A ₄ , A ₆ , and A ₈ are as follows. i k A ₄ 6 0 0.592×10 ^-4 10 0 0.516×10 ^-4 11 −0.0578 0.660×10 ^-4 A ₆ A ₈ 0.111×10 ^-6 0.731×10 ^-10 0 0 0.231×10 ^-7 0 Also, The variable distances D ₈ and D ₁₂ are as follows at focal lengths of 36.0 mm, 50.0 mm, and 68.0 mm. =36.0mm 50.0mm 68.0mm D ₈ 9.666 4.504 1.0 D ₁₂ 1.0 14.72 32.68 Also, the refractive power ψ a of the front group lens 1a _{and the refractive power ψ b of} the rear group lens 1b with respect to the refractive power ψ 0 of the entire first group lens ₁ The ratio is ψ _a /ψ ₀ = −0.361 ψ _b /ψ ₀ = 1.385, and the back focus _b is 7.0. In the fourth embodiment, the first
Instead of correcting spherical aberration by an air lens with negative power between the lens and the second lens, spherical aberration is effectively corrected by making the rear surface of the third lens 12 aspherical. are doing. Furthermore, by making the concave lens 13 of the second group lens 2 aspherical, the number of concave lenses used is reduced, the axial thickness of the second group lens 2 is reduced, and the movement space of the second group lens 2 is increased. I'm trying to get it. This makes it possible to shorten the overall length while weakening the power of each group. Further, in the fifth embodiment, the convex lens 14 of the second lens group 2 is also provided with an aspherical surface to improve performance. In the first to fifth embodiments described so far,
Since the diaphragm 20 is installed inside the first lens group 1, the entire diaphragm mechanism must be moved during focusing. In this regard, in the sixth embodiment described later, by providing an aperture between the first group lens 1 and the second group lens 2, focusing can be performed by moving only the front lens group. become. For this reason, there is an advantage that the driving force can be reduced, especially in a device that performs focusing using an autofocus mechanism.

【table】

〔Effect of the invention〕

As explained above, according to the zoom lens of the present invention, positive, negative, and positive power distribution is performed in order from the object side in a lens system composed of three groups, and the power distribution between the first group lens and the second group lens is performed. By moving these toward the object side while narrowing the distance, the magnification is changed from wide-angle to telephoto. As a result, it is possible to obtain a zoom lens that is shorter in overall length and has a higher magnification ratio than conventional zoom lenses proposed for lens-shutter cameras. In addition, the back focus can be maintained to a certain extent and the film surface is not extremely close to the film surface, which is very convenient for incorporating into a camera.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the basic configuration of a zoom lens according to the present invention. FIG. 2 is a sectional view showing a first embodiment of the present invention. FIG. 3 is an aberration diagram of the first embodiment. FIG. 4 is a sectional view showing a second embodiment of the present invention. FIG. 5 is an aberration diagram of the second embodiment. FIG. 6 is a sectional view showing a third embodiment of the present invention. FIG. 7 is an aberration diagram of the third embodiment. FIG. 8 is a sectional view showing a fourth embodiment of the present invention. FIG. 9 is an aberration diagram of the fourth embodiment. FIG. 10 is a sectional view showing a fifth embodiment of the present invention. FIG. 11 is an aberration diagram of the fifth embodiment. FIG. 12 is a sectional view showing a sixth embodiment of the present invention. FIG. 13 is an aberration diagram of the sixth embodiment. 1...First group lens, 1a...Front group lens, 1
b... Rear group lens, 2... Second group lens, 3...
Third group lens, 4...imaging surface (film surface), 2
0...Aperture.

Claims (1)

[Claims] 1. A first group lens having a positive refractive power, a second group lens having a negative refractive power, and a third group lens having a positive refractive power are arranged in order from the object side, By narrowing the distance between the first group lens and the second group lens and moving them toward the object side, zooming from the wide-angle side to the telephoto side is performed, and during zooming, the third group lens The lens is always used with a positive imaging magnification, and when the first group lens is bisected at the point where the air gap is maximum, the front group lens has a negative refractive power ψ _{a and} a positive refractive power in order from the object side. force ψ _{b
and a rear group} lens _having A zoom lens featuring