JPS6132655B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention has an angle of view of about 75 degrees and an F number of
The objective is to provide a compact wide-angle photographic lens that is bright at F1.4 and has extremely excellent performance. Conventionally, many wide-angle photographic lenses for single-lens reflex cameras with an angle of view of about 75° have been known, mainly retrofocus types, but those with a large aperture ratio with an F number of F1.4 have been known. There are extremely few examples. The reason for this is that with a large aperture ratio, it is not only necessary to correct spherical aberration, but also to solve a number of problems such as securing sufficient peripheral illumination and correcting the increasing coma flare, while also making the lens system compact. The point is that it is extremely difficult. In particular, coma flare not only reduces the contrast of the image, but also causes drawbacks such as a significant loss of image quality when shooting a point light source or a light source close to it, such as when shooting night scenes, so it should be adequately corrected. , the conventional examples were not satisfactory. The present invention has focused on the above points, and by introducing an aspherical lens, the coma flare can be sufficiently corrected, and even after securing a sufficient amount of peripheral light, remaining various aberrations can be favorably removed, and a large amount can be achieved. We have achieved a wide-angle photographic lens with extremely good performance considering its aperture. The present invention will be explained in detail below. The present invention has a four-lens configuration in which, in order from the object side, the first lens is a positive lens, the second and third lenses are both negative meniscus lenses, and the fourth lens is a positive meniscus lens, and has a negative refractive power as a whole. a fifth lens and a sixth lens, both of which are positive lenses.
lens, negative lens 7th lens, negative lens 8th lens
Consists of a second group that has positive refractive power as a whole and consists of six elements: a bonded lens with a positive 9th lens, and a positive 10th lens, and both groups are arranged with an aperture in between. A lens system in which the image side surface of either the ninth lens or the tenth lens is an aspherical surface, and the y-axis is aligned in the optical axis direction with the apex (pole) of the aspherical surface as the origin. , when the x-axis is taken perpendicular to the optical axis and the equation of an aspherical surface is generally set as y=f(x), then 2 of f(x)
Extremely good performance with a large aperture ratio characterized by the fact that there is one place where the sign of the order derivative f″(x)=d ² y/dx ² changes from negative to positive near the end of the effective diameter. It is a wide-angle photographic lens. Possible purposes for introducing an aspherical lens include correction of spherical aberration, correction of distortion aberration, correction of coma aberration, etc. In general, retrofocus lenses have negative refractive power. A large amount of negative distortion occurs in the front group, and this negative distortion tends to worsen as the entire system is made more compact.Therefore, in the conventional example of using an aspherical lens,
There are many devices whose main purpose is to correct this distortion aberration. In contrast, in the present invention, distortion is corrected by first making the first lens a positive lens, and furthermore, by making the second lens group symmetrical, the occurrence of distortion is suppressed as much as possible. The optical system is fundamentally resistant to residual distortion. In addition, the second and third lenses are both negative meniscus lenses, giving a concentric structure to the incident light beam, suppressing the occurrence of various aberrations, and providing sufficient negative refractive power. As a meniscus lens, spherical aberration and chromatic aberration are corrected, and the first group as a whole has a strong negative refractive power, yet has a structure that makes it difficult for various aberrations to occur. In addition, in the second group, the fifth lens, the sixth lens and the second
Two positive lenses are arranged to provide strong positive refractive power, and the seventh lens is used as a negative lens to correct spherical aberration and astigmatism. The 8th negative lens and the 9th positive lens are used as a bonded lens to correct astigmatism, field curvature, and chromatic aberration, and finally the 10th positive lens is placed to correct spherical aberration and coma aberration. We are trying to Furthermore, as described above, the entire second group has a symmetrical structure to suppress the occurrence of distortion. As described above, the first and second groups have a structure that can provide strong refractive power while minimizing the occurrence of various aberrations, and have created the basis for a compact and high-performance optical system. One feature is that the image side surface of either the ninth lens or the tenth lens is an aspherical surface. One of the major purposes of using an aspherical surface in the present invention is to correct coma flare, which occurs particularly large in wide-angle lenses with large aperture ratios. As mentioned above, since the optical system structure is basically such that various aberrations are difficult to occur, aspheric surfaces can be used very effectively to suppress coma flare. Since aspherical surfaces are generally difficult to manufacture and expensive, it is desirable to use them effectively with a small number of surfaces. In the present invention, the ninth positive lens is the first or second positive lens counting from the image side.
It has been found that using an aspheric surface on the image side surface of the lens or the tenth lens is most effective in removing coma flare. In addition, when the apex (pole) of the aspheric surface is the origin, the y axis is in the optical axis direction, and the x axis is perpendicular to the optical axis, and the equation of the aspheric surface is generally y = f (x), f
The quadratic function of (x) f″(x)=d ² y/dx The sign of ² is x, f(x), f″(x) in Examples 1 and 2
The condition that there is one place near the end of the effective diameter where the value changes from negative to positive, as disclosed in the table above, is related to the shape of the aspheric surface. In other words, the aspherical curve has an inflection point within the effective diameter, but using such an aspherical shape is extremely effective in eliminating coma flare, as well as The burden of aberration correction can be appropriately shared with other elements other than the aspherical surface constituting the system. With the above configuration, it is possible to achieve a wide-angle photographic lens with extremely good performance at a large aperture ratio.
By further adding conditions to the present invention, it is possible to realize a large aperture ratio photographic lens that is even more compact and has excellent performance. That is, (1) 1.2f<｜f ₁₂₃ |<2.0f, f ₁₂₃ <0 (2) 0.4f<D ₆ <0.9f (3) 1.0f<f<1.6f (4) 2f<L<3f (5) ) 5f＜f ₁ <12f (6) N _P ＞1.7 Here, f ₁₂₃ is the composite focal length from the first lens to the third lens, f is the focal length of the entire system, and D ₆ is the third and fourth lenses. f is the focal length of the second group, L is the total length of the lens, f ₁ is the focal length of the first lens, and N _P is the average value of the refractive index of the fifth lens and the sixth lens. It is to satisfy the following. Conditions (1) to (4) are to ensure the necessary back focus to avoid mechanical interference with the mirror inside the camera body as a lens for single-lens reflex cameras, and to keep the entire lens system compact and maintain high performance. This is a condition for doing so. First, condition (1) relates to the refractive power of the lens group from the first lens to the third lens. If the lower limit of condition (1) is exceeded, the refractive power will increase and it will be advantageous for compactness, but various aberrations such as negative spherical aberration, coma aberration, and distortion will occur, making it no longer possible to achieve good performance. It becomes difficult to maintain. If the upper limit of condition (1) is exceeded, the refractive power decreases, which is advantageous in terms of aberration correction, but increases the overall length, which goes against the intention of compactness. Condition (2) relates to the air gap between the third lens and the fourth lens. If the lower limit of condition (2) is exceeded, the air interval becomes too short and it becomes difficult to secure the necessary back focus. Also, if the upper limit of condition (2) is exceeded,
It becomes difficult for the fourth lens to correct negative spherical aberration, chromatic aberration, etc. generated in the lens groups from the first lens to the third lens group. Condition (3) relates to the refractive power of the second group. The second group has the role of converging the light beam diverged by the first group, and the light beam with a large aperture ratio passes through it, so in order to minimize the occurrence of various aberrations and make the entire system compact, its refractive power is required. The key point is to decide. If the lower limit of condition (3) is exceeded, the refractive power of the second group becomes too large, causing positive spherical aberration, curvature of field, and chromatic aberration.
Maintaining good performance becomes difficult. If the upper limit of condition (3) is exceeded, the refractive power of the second group becomes too loose, which is advantageous in terms of aberration correction, but compactness cannot be achieved. Condition (4) relates to the total length of the lens. If the lower limit of condition (4) is exceeded, the refractive power of each lens group becomes strong, various aberrations occur, and good aberration correction cannot be obtained. Moreover, if the upper limit of condition (4) is exceeded, it becomes necessary to increase the diameter of the front lens in order to secure the necessary amount of peripheral light, which goes against the intention of compactness. Condition (5) relates to the refractive power of the first lens. Condition (5)
If the upper limit of It becomes difficult to correct aberrations. If the lower limit of condition (5) is exceeded, the refractive power of the first lens becomes too strong, but since the first group as a whole has negative refractive power, the first lens, which has positive refractive power, becomes too strong. In this case, the refractive power of the second and third lenses, which are negative lenses, must also be correspondingly strong, which is disadvantageous in terms of aberration correction. Condition (6) relates to the refractive index of the fifth lens and the sixth lens. This condition is necessary to correct spherical aberration, which tends to worsen in the second group, and to improve Petzval's sum to eliminate field curvature and astigmatism. Below, data of examples of the present invention will be described. Here, f is the focal length, R is the radius of curvature of the lens, D is the lens thickness or distance between the lenses, N is the refractive index, and ν is the Abbe number. Example 1

【table】

【table】 Example 2

【table】

【table】 [Brief explanation of the drawing]

Figures 1 and 3 are lens diagrams of Examples 1 and 2, respectively, Figures 2 and 4 are aberration diagrams of Examples 1 and 2, respectively, and Figures 5 and 6 are lens diagrams of Examples 1 and 2, respectively. FIG.

Claims (1)

[Claims] 1. In order from the object side, the first lens is a positive lens, the second and third lenses are both negative meniscus lenses, and the fourth lens is a positive meniscus lens.
The first group has a negative refractive power as a whole, the fifth and sixth lenses are both positive lenses, the seventh lens is a negative lens, the eighth lens is a negative lens, and the ninth lens is a positive lens. It is a lens system consisting of a 6-element lens, a 10th lens which is a positive lens, and a 2nd group having positive refractive power as a whole, and both groups are arranged with an aperture in between. , the image-side surface of either the ninth lens or the tenth lens is an aspherical surface, and the apex (pole) of the aspherical surface is the y-axis in the optical axis direction, and the optical axis is perpendicular to the aspherical surface. When the equation of an aspheric surface is generally taken as y = f (x) with the x axis taken, the sign of the second derivative of f (x) f'' (x) = d ² y/dx ² is within the effective diameter. A large aperture ratio wide-angle photographic lens using an aspherical surface, characterized in that there is one place near the edge of which changes from negative to positive, and configured to satisfy the following conditions. ( 1) 1.2f＜｜f ₁₂₃ ｜＜2.0f, f ₁₂₃ ＜0 (2) 0.4f＜D ₆ ＜0.9f (3) 1.0f＜f＜1.6f (4) 2f＜L＜3f (5) 5f <f ₁ <12f (6) N _P >1.7 Here, f ₁₂₃ is the combined focal length from the first lens to the third lens, f is the focal length of the entire system, and D ₆ is the combined focal length of the third and fourth lenses. air spacing, f is the second
The focal length of the group, L is the total length of the lens, f ₁ is the focal length of the first lens, and N _P is the average value of the refractive index of the fifth lens and the sixth lens.