JPH0426443B2 - - Google Patents

Description

[Detailed description of the invention]

Purpose of the Invention (Field of Industrial Application) This invention relates to a wide-angle photographic lens, especially a telephoto lens with a telephoto ratio of 0.95.
Regarding the following compact lenses. (Prior art) A wide-angle lens with a small telephoto ratio, consisting of a front group consisting of a convex, concave, and convex structure and a rear group strongly curved toward the image side, has an angle of view of 32 degrees or less, and is suitable for use in, for example, 35 mm lens shutter cameras. When used for this purpose, the focal length was approximately 35 mm. In addition, the telephoto ratio is about 1, and even when an aspherical surface is introduced, it is more than 0.95, and when used in a 35mm version camera, the distance from the front edge of the lens to the film surface is 30mm.
It was difficult to reduce the thickness to less than mm. For example, the lens of JP-A No. 55-73014 has a telephoto ratio of approximately 1 and the angle of view is approximately 31.5 degrees, while the lens of JP-A No. 56-94317 has an angle of view of approximately 30 degrees. Unexamined Japanese Patent Publication No. 58-85410 can be seen as a compact structure.
The F number is dark at 4.5 to 5.6, and the angle of view is 30.7 degrees. JP-A-56-59217 has an aspheric lens in the rear lens group, but it has an angle of view of 31.5 degrees and a telephoto ratio of 0.95 or more. (Problems to be solved by this invention) This invention has an F number of 4.0, which is bright enough to be widely used as a photographic lens, and an angle of view of 35°, which is unprecedented in wide angle of view. Moreover, the telephoto ratio is around 0.94, and when used in a 35mm camera, the objective is to create a very short lens with a length of less than 30mm from the front of the lens to the film surface. Structure of the Invention (Means for Solving Problems) The lens of this application has a four-group structure in which the front group has a positive/negative/positive structure and the rear group is a negative meniscus lens strongly curved toward the image side. Yes, f: composite focal length of the entire system f ₄ : focal length of the 4th group (rear group) lens N ₂ : refractive index of the 2nd group lens 0.45<f/|f ₄ |<2.5, f ₄ <0...(1) 1.7<N ₂ ...(2), and at least one of the refractive surfaces of the first group lens to the third group lens, and one surface of the fourth group lens or Both surfaces are made aspherical, and the degree of asphericity is determined by the ^paraxial curvature _. 0.13f)+ΔX ₃ (0.1f)+ΔX ₅
(0.1f) −ΔX ₂ (0.13f) −ΔX ₄ (0.1f) −ΔX ₆ (0.1f) <−0
.1
×10 ^-4 f ……(3) However, the aspherical shape is If it is expressed as C ^* = 3 + 2A ₂ (Paraxial curvature) φ: Represented by the height from the optical axis. More preferably, 1.15<f/f _1.2.3 <2.0...(4) f _1.2.3 : If the composite focal length of the front group is satisfied and an aspherical surface is introduced on each surface, the object side of the first lens should be −0.5×10 ^-2 <ΔX ₁ (0.13f)/f<−0.1×10 ^-4
...(5) When introduced on the image side of the first lens, 0.1×10 ^-4 <ΔX ₂ (0.13f)/f<0.1×10 ^-1 ...(6) When introduced on the object side of the second lens - 0.5×10 ^-2 <ΔX ₃ (0.1f)/f<-0.1×10 ^-4
...(7) On the image side of the second lens 0.1×10 ^-4 <ΔX ₄ (0.1f)/f<0.5×10 ^-2 ...(8) On the object side of the third lens -0.5×10 ^-2 <ΔX ₅ (0.1f)/f<0.1×10 ^-4
...(9) When the image side of the third lens is 0.1×10 ^-4 <ΔX ₆ (0.1f)/f<0.5×10 ^-2 ...(10) The object side of the fourth lens is 0.1×10 ^-1 <ΔX ₇ (0.13f)/f<0.3 ...(11) The image side satisfies the following conditions: 0.3×10 ^-3 <ΔX ₃ (0.14f)/f<0.3×10 ^-1 ...(12) is desirable. And for the entire fourth group lens, 0.1×10 ^-3 f<ΔX ₇ (0.13f)−ΔX ₈ (0.14f)<0.3f
...(13) is satisfied. (Function) As mentioned above, in a wide-angle lens where the front group has a positive-negative-positive configuration and the rear group is a negative meniscus lens strongly curved toward the image side, if you try to further reduce the telephoto ratio, It is necessary to strengthen the positive refractive power of the front group and the negative refractive power of the rear group. Condition (1) relates to the negative refractive power of the fourth lens, which is the rear group, and if the lower limit is exceeded, the movement of the image-side principal point toward the front of the lens becomes small, making it impossible to make the lens compact. However, when the upper limit is exceeded, the curvature of field becomes overcorrected, and the occurrence of extroverted coma and pincushion distortion becomes significant. Condition (2) is to keep the Petzval sum appropriate; in general, compact lenses tend to have too small a Petzval sum, so the refractive index of the second lens, which is a negative lens, is set to be large according to the range of this condition. By doing so, the curvature of field can be kept within an appropriate range. Condition (4) is intended to strengthen the refractive power of the front group and reduce the telephoto ratio; if the lower limit is exceeded, the effect cannot be obtained, and if the upper limit is exceeded, spherical aberration is insufficiently corrected and pincushion distortion occurs. Aberrations and significant extroverted coma occur, and correction becomes difficult even when the surface is made aspherical. As mentioned above, if the refractive power of the front group is strengthened to reduce the telephoto ratio, spherical aberration will be insufficiently corrected, extroverted coma flare will occur, field curvature will be insufficiently corrected, and distortion will be strong pincushion type. It has a tendency to be. Furthermore, increasing the refractive power of the rear group causes extroverted coma flare and increases pincushion distortion. Therefore, the reason why an aspherical surface shape is introduced to correct this is because the refractive power of the positive/negative/positive lens in the front group is too strong and cannot be sufficiently corrected by the concave lens in the rear group. Therefore, as you go outward from the optical axis, a surface with convex power has a less curvature and better aspherical deformation than a genera with paraxial curvature, and conversely, a surface with concave power has a better aspherical deformation as you move away from the optical axis. It is preferable to have an aspherical surface whose curvature becomes stronger toward the outside than the parent sphere due to paraxial curvature. Conditions (5) to (10) are related to such aspheric deformation, and condition (5) corrects the spherical aberration that was insufficiently corrected.
Extroverted coma and pincushion distortion are corrected, and field curvature is also corrected, tilting to the over side. Condition (6) indicates that the refractive surface is deformed from the mother sphere toward the image side off the optical axis, and corrects insufficient correction of spherical aberration and curvature of field. Condition (7), like condition (5), deforms toward the object side off the optical axis, thereby reducing extroverted coma occurring in the front group, and also reducing spherical aberration and pincushion distortion. Condition (8) is a modification similar to condition (6), and is mainly effective for correcting spherical aberration and curvature of field. The deformation direction and effect of the aspheric surface in condition (10) are the same as in condition (5). Condition (10) means that off the optical axis, the paraxial surface is deformed toward the image side, reducing extroverted coma, and spherical aberration and pincushion distortion. Among the limit values of each of the above conditions, the lower limit of the positive value and the upper limit of the negative value are the limits at which the aspherical effect occurs, and the upper limit of the positive value and the lower limit of the negative value are the limits where the aspheric deformation becomes too large and correction is the limit that does not become excessive. As is clear from the optical path diagram shown in FIG. 1, as the angle of view of the lens becomes wider, part A of the peripheral light beam is incident on the outside of the axis of the convex, concave, and convex lenses at a large angle, and aberrations occur significantly in this part. This is based on conditions (5) to (10) mentioned above.
This can be corrected by introducing an aspheric surface. On the other hand, the B portion of the light beam passes through a high off-axis position of the fourth lens, and this portion causes significant aberrations. The conditions for making the surface aspherical for this purpose are conditions (11) to (13). Conditions (11) and (12) are displacements in the direction in which both surfaces of the fourth group lens have a gentler curvature off-axis than the mother sphere,
This is to flatten coma aberration when the angle of view is 28 degrees or more. Condition (11) allows the aspherical displacement of the side surface of the object to be large, which makes it possible to flatten coma aberration, but at the same time, the curvature of field becomes insufficiently corrected and pincushion distortion becomes large. However, by making the image side surface aspherical as in condition (12), field curvature and pincushion distortion can also be corrected. The above limits apply to one plane in the front group and two planes in the rear group.
This is a case where the surfaces are made aspherical, but of course the present invention is not limited to this, and four or more surfaces in the front and rear groups may be made aspherical. In such a case, since it is sufficient that the sum of the effects of asphericalization produces the above-mentioned effects, the directions of individual aspherical displacements may deviate from each of the above-mentioned conditions. Condition (3) is a restriction regarding the total amount of aspherical displacement in the front group in such a case, and condition (13) is a restriction regarding the total amount of displacement in the rear group. The upper limit of condition (3) and the lower limit of condition (13) are the limits at which a correction effect can be expected, and conversely, the lower and upper limits are limits at which the displacement does not become excessive and an excessive aspherical effect does not occur. (Example) Examples of the lens system of the present invention will be shown below. In the table, R is the radius of curvature, D is the interval between refracting surfaces, N is the refractive index, ν is Abbe's number, W is the angle of view, f _B is the back focus, K is the conic constant, and A is the aspheric coefficient.

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[Table] Effects of the Invention As seen in the above embodiments, the lens of the present invention has a simple structure but has sufficient brightness as an F4 photographic lens, has an angle of view of approximately 35°, and has a telephoto ratio. The lens is extremely compact, with an aberration of less than 0.95, and as shown in each aberration curve diagram, we were able to obtain an extremely superior wide-angle lens with well-balanced aberration correction.

[Brief explanation of drawings]

FIG. 1 is an optical path diagram of the lens of the present invention, and in FIGS. 2 to 6, a is a sectional view of Examples 1 to 5, respectively, and b is an aberration curve diagram thereof.

Claims (1)

[Claims] 1. A four-group configuration consisting of a front group consisting of a positive lens, a negative lens, and a biconvex lens from the object side, and a rear group that is a negative meniscus lens strongly curved toward the image side, and 0.45<f /｜f ₄ | < 2.5, f ₄ < 0 1.7 < N ₂ , and one or more surfaces of the front group and both surfaces of the rear group are aspheric, and the paraxial curvature makes the matrix aspheric. The amount of displacement of the refracting surface due to the front group is -0.5×10 ^-2 f<ΔX ₁ (0.13f) +ΔX ₃ (0.1f) +ΔX ₅ (0.1f)−ΔX ₂ (0.13f)−ΔX ₄ (0.1f) −ΔX ₆ (0.1f)＜−0.1×10 ^-4 f Regarding the object side of the rear group 0.1×10 ^-1 f＜ΔX ₇ (0.13f)＜0.3f Regarding the image side 0.3×10 ^-3 f＜ΔX ₈ ( A wide-angle photographic lens that satisfies the following: 0.14f)<0.3×10 ^-1 f. However, f : Focal length of the entire lens system f ₄ : Focal length of the rear group negative meniscus lens N ₂ : Refractive index of the second lens C ^* =C+2A ₂ 2 Regarding the rear group, the first claim is characterized in that 0.1×10 ^-3 f<ΔX ₇ (0.13f)−ΔX ₈ (0.14f)<0.3f is satisfied.
Wide-angle photographic lens.