JPS6230604B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a photographic lens. Conventionally, in a photographic lens having a standard angle of view, in order to shorten the length from the first surface of the photographic lens to the image plane and thereby reduce the size of the photographic lens, there is a method described in Japanese Patent Publication No. 10831-1971. It is known that a configuration in which the front lens group has strong refractive power and the final lens in the rear lens group includes a meniscus lens with negative refractive power is effective. On the other hand, for example, in a camera with an automatic focusing mechanism, in order to simplify the camera structure and perform the focusing operation mechanically or electrically, the lens group that is extended during focusing is as light and small as possible, and the aperture shutter mechanism is also focused. It is preferable to remain immobile. Conventionally, this type of photographic lens is a Tetsusar-type lens with an aperture placed at the rear of the lens, and when focusing, the entire lens system is stopped or only the first lens on the object side is extended, making the photographic lens more compact. In addition, efforts have been made to reduce the weight of the steering lens. However, if the aperture is placed after the final lens, the off-axis principal ray will not pass through the periphery of the negative lens surface of the final lens, so in order to make the photographic lens more compact, the lens In a lens configuration in which strong refractive power is given to the front group and a negative meniscus lens is arranged as the final lens in the rear lens group, it becomes difficult to satisfactorily correct astigmatism, distortion, etc. Additionally, in a configuration in which a diaphragm is placed between the lenses and all or part of the lens group in front of the diaphragm is extended for focusing, spherical aberration tends to be under-corrected as the object distance approaches, especially as mentioned above. If strong refractive power is given to the lens group in front of the diaphragm in order to make the lens more compact, spherical aberration will be significantly undercorrected. An object of the present invention is to provide a compact and high-performance photographic lens, and furthermore, focusing is performed by focusing all of the lens groups arranged in front of the diaphragm.
To provide an extremely compact photographic lens in which aberrations are well corrected with little variation in aberrations even due to differences in object distance. In the numerical examples described later, an aberration-corrected photographic lens with an F number of 2.8 and a photographic field angle of 58 degrees is provided. The photographic lens of the present invention is characterized by, in order from the object side, a positive meniscus first lens with a convex surface facing the object side, a biconvex second lens, a biconcave third lens,
It consists of a positive fourth lens, a positive meniscus fifth lens with a convex surface facing the image side, and a negative meniscus sixth lens with a convex surface facing the image side, and the second and third lenses are bonded together. The lens surface on the object side of the sixth lens is an aspherical surface, focusing is performed by extending the fourth lens from the first lens, and an aperture is arranged between the fourth lens and the fifth lens. are being taken. However, this lens can also be used for focusing using the entire system. The above-mentioned lens configuration is characterized in terms of aberration correction because negative spherical aberration is generated from the lens group in front of the focusing aperture.
This negative spherical aberration is corrected within the lens group, thereby preventing the spherical aberration from changing even when the lens group is moved out due to focus. For this purpose, the positive refractive power is shared between the first surface of the first lens and the second lens in the lens group to suppress the occurrence of spherical aberration as much as possible, and the second and third lenses are bonded together. The surface has a strong correction effect for spherical aberration. Then, if it is possible to arrange a positive fourth lens immediately after the third lens to correct the strong inward coma aberration that occurs on the bonded surface, and to suppress fluctuations in coma and astigmatism due to focusing to a small value. It's tight. However, in the above lens configuration, since a strong positive refractive power is given to the lens group in front of the aperture, positive distortion occurs and it becomes difficult to correct this distortion. Therefore, a fifth positive meniscus lens with a convex surface facing the image side is arranged in the fixed lens group behind the aperture to correct distortion, and a sixth lens with a negative meniscus with a convex surface facing the image side is arranged. By arranging the lens and making its first surface aspherical, distortion can be corrected even better. In addition, by making the first surface of the sixth lens aspherical, it is possible to better correct higher-order aberrations, especially peripheral astigmatism, which is better when configured only with spherical lenses. This makes it possible to satisfactorily correct unavoidable negative astigmatism over the entire screen. By molding the sixth lens with so-called acrylic resin, the degree of freedom in shaping the aspherical surface is increased and it is possible to manufacture the lens at low cost. The present invention has the above-mentioned characteristics and has achieved the initial objectives, but in order to further improve the performance, it is preferable to provide the following conditions. The focal length of the photographing lens is f, the radius of curvature of the i-th lens surface counting from the object side is Ri, the thickness and air gap of the i-th lens counting from the object side is Di, the i-th lens surface counting from the object side The refractive index of the glass of the lens is Ni, the aspherical shape has the apex of the surface as the origin, R is the paraxial apex radius of curvature, the traveling direction of light on the optical axis is the X axis, and the direction perpendicular to the X axis is the Y axis. When, and when A, B, C, D, and E are each aspheric coefficient, (1) −1.2f<R ₄ <−0.5f (2) 0.2<N ₃ −N ₂ <0.3 (3) -1.6f< _R7 <-0.7f (4) 0.06f< _D7 <0.12f (5) 5<f|2・A+1/ _R10 |<6.5. Next, the numerical ranges of the above conditions will be explained.
Conditions (1) and (2) are conditions for correcting the spherical aberration of the focusing lens group, and if the lower limit of conditions (1) and (2) is exceeded, the second and third lenses The spherical aberration correction effect on the R4 surface of the bonding becomes weaker, and as a result, the fluctuation of spherical aberration due to the difference in subject distance increases, and the upper limit value exceeds the upper limit value because it suppresses the occurrence of inward coma aberration. This makes it difficult to correct inward coma aberration. Condition (3) is a condition to reduce fluctuations in astigmatism due to differences in subject distance; if the upper limit of condition (3) is exceeded, negative astigmatism will occur at the periphery of the screen when focusing on a close object. If the value exceeds the lower limit, positive astigmatism will become excessive in the middle of the screen when focusing on a close object, which is undesirable. Condition (4) is a condition for adjusting the amount of distortion correction and the size of the sixth lens, which is the final lens.If the lower limit of condition (4) is exceeded, it is difficult to correct positive distortion. If the upper limit is exceeded, the effective diameter of the sixth lens becomes large, leading to an increase in the size of the lens, and the tangent to the surface of the first surface of the sixth lens near the maximum diameter becomes parallel to the optical axis. The lens has an inclination close to , that is, close to horizontal, making it extremely difficult to manufacture the lens. Condition (5) is a condition regarding the apex refractive power of the R10 surface of the first surface of the sixth lens, and if the upper limit of condition (5) is exceeded, negative astigmatism in the middle of the screen cannot be corrected. Moreover, if the lower limit value is exceeded, it becomes difficult to correct distortion aberration. The present invention achieves an F number of 2.8 and an angle of view by satisfying the above lens configuration and various conditions.
This makes it possible to achieve an extremely compact photographic lens with well-corrected aberrations of 58 degrees. In Numerical Example 1, the cross-sectional view of the lens is shown in the first
Figure 2 shows various aberrations when the object is at infinity.
Figure 3 shows various aberrations when the object is 26f from the image plane, where f is the focal length. In Numerical Example 2, the cross-sectional view of the lens is shown in the fourth example.
Figure 5 shows various aberrations when the object is at infinity.
Figure 6 shows various aberrations when the object is 26f from the image plane. In Numerical Example 3, the cross-sectional view of the lens is shown in the seventh figure.
Figure 8 shows various aberrations when the object is at infinity.
Figure 9 shows various aberrations when the object is 26f from the image plane. Next, numerical examples of the present invention will be shown. In the numerical examples, Ri is the radius of curvature of the i-th lens surface from the object side, Di is the thickness and air gap of the i-th lens from the object side, and Ni and νi are the i-th lens surface from the object side, respectively. The refractive index and Abbe number of the glass, A, B, C, D, and E are the aspherical coefficients in the formula expressing the aspherical shape described above. In Numerical Example 1 and Numerical Example 3, the aspherical shape was expressed only by an aspherical coefficient, and in Numerical Example 2, the aspherical shape was expressed by a paraxial apex radius of curvature and an aspherical coefficient. Further, Table 1 shows the third-order aberration coefficients of Numerical Example 1.

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【table】 [Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a lens according to Numerical Example 1 of the present invention, FIGS. 2 and 3 are various aberration diagrams of Numerical Example 1 of the present invention, and FIG. 4 is a cross-sectional view of a lens according to Numerical Example 2 of the present invention. Figures 5 and 6 are numerical example 2 of the present invention.
FIG. 7 is a sectional view of a lens according to Numerical Example 3 of the present invention, and FIGS. 8 and 9 are various aberration diagrams of Numerical Example 3 of the present invention. In the figure, S is a sagittal image plane, and M is a meridional image plane.

Claims (1)

[Claims] 1. In order from the object side, a positive meniscus first lens with a convex surface facing the object side, a biconvex second lens, a biconcave third lens, a positive fourth lens, and a positive fourth lens with a convex surface facing the object side. It consists of a positive meniscus fifth lens with a convex surface facing and a negative meniscus sixth lens with a convex surface facing the image plane side, the second lens and the third lens are bonded together, and the sixth lens The lens surface on the object side is aspheric, the focal length of the entire system is f, and the radius of curvature of the i-th lens surface counting from the object side is
Ri, the thickness and air gap of the i-th lens counting from the object side, Di, the refractive index of the glass of the i-th lens counting from the object side, Ni, the aspherical shape with the apex of the surface as the origin, and R as the approximate Axis apex radius of curvature, when the direction of light travels on the optical axis is the X axis, and the direction perpendicular to the X axis is the Y axis, When A, B, C, D, and E are each aspheric coefficients, −1.2f<R ₄ <−0.5f 0.2<N ₃ −N ₂ <0.3 −1.6f<R ₇ <−0.7 A compact device that satisfies the following conditions: f 0.06f<D ₇ <0.12f 5<f|2・A+1/R ₁₀ |<6.5 and performs focusing by extending the first lens to the fourth lens as one unit. Photography lens.