JPH0412446B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a wide-angle lens, and in detail, it has an aperture ratio of 1:2.8, a half-field angle of about 30°, and an aperture efficiency.
Aberrations for near 100% and close objects,
In particular, it relates to a wide-angle lens used for facsimile, OCR, etc., which has well-corrected distortion aberration of about 1% or less. Ordinary wide-angle lenses are designed to be used in cameras that use photosensitive materials (film, dry plates, etc.), so they generally have an aperture efficiency of at most 60 when wide open.
It was about %. However, facsimile and
Lenses used for OCR etc. need to have as much light as possible at the periphery of the screen, so they are required to have an aperture efficiency of nearly 100%.
%, the off-axis light beam becomes large, making it difficult to correct comatic aberration. Also, as this type of lens, for example, there is Japanese Patent Application Laid-Open No. 54-49132,
The distortion aberration was about 2%, which was not yet sufficiently corrected. The present invention satisfactorily corrects the coma aberration with an aperture ratio of 1:2.8, a half angle of view of about 30°, and an aperture efficiency of nearly 100%, and also satisfactorily corrects distortion to about 1% or less, covering the entire screen. The objective is to provide a wide-angle lens used for facsimile, OCR, etc. with good imaging performance. First, to explain the lens configuration of the present invention, in order from the object side, there is a first lens that is a negative-negative meniscus lens, a second lens that is a positive lens, a third lens that is a negative lens, a fourth lens that is a negative lens, and a positive lens. 5th of
In a lens consisting of 6 elements in 5 groups, consisting of a cemented lens and a 6th lens of a meniscus lens, the first lens has its convex surface facing the object side, the second lens has its convex surface with a strong curvature facing the object side, and the 6th lens is a meniscus lens. This wide-angle lens is characterized in that the third lens has a concave surface with a strong curvature facing the image side, and the sixth lens has a concave surface facing the image side, and is configured to satisfy the following conditions. (1) 1.3f＜｜f ₁ ｜＜1.9f, f ₁ ＜0 (2) 0.32f＜r ₂ ＜0.40f (3) 0.59f＜r ₃ ＜0.80f (4) 0.60f＜r ₁₀ ＜0.75 f 1.80＜r ₁₁ /d ₁₀ ＜3.05 (5) 0.38f＜d _3〜5 ＜0.57f (6) n ₂ , n ₃ ＞1.65 45＞(ν ₂ +ν ₃ )／2＞28 However, f is the total The focal length of the system, f ₁ is the focal length of the first lens, r _i is the radius of curvature of the i-th surface from the object side,
d ₁₀ is the thickness of the sixth lens, d _{3 to 5} are the distance from the object side surface of the second lens to the image side surface of the third lens,
n ₂ and n ₃ are the refractive indices of the second lens and the third lens, respectively, and ν ₂ and ν ₃ are the Abbe numbers of the second lens and the third lens, respectively. Next, each of the above conditional expressions will be explained. Condition (1) is an expression regarding the power of the first lens,
This represents the range in which a wide angle of view can be achieved.
Therefore, when the lower limit is exceeded and the negative power becomes strong, a large inward coma flare occurs on the second surface,
On the other hand, if the upper limit is exceeded and the negative power becomes weak, external coma flare will occur and the wide angle of view, which is the objective of the present invention, will not be achieved. In order to secure the amount of peripheral light aimed at by the present invention, it is advantageous to downsize the entire device, and the above conditions are indispensable for this purpose as well. Further, in connection with this, it is naturally desirable that the first lens has a large Abbe number, and in the embodiment, 60 or more is used. Condition (2) concerns the image-side surface r ₂ of the first lens, and in order to widen the angle of view under condition (1), the radius of curvature of the second surface of the negative meniscus lens must naturally bear negative degrees. However, if the radius of curvature is too small, comatic aberration will occur significantly, and correction on other surfaces will be difficult. Therefore, beyond the lower limit r ₂
When the radius of curvature becomes small, it becomes difficult to correct positive spherical aberration and coma flare in the upper rays, and conversely, when it increases beyond the upper limit, correction for negative spherical aberration becomes insufficient and coma flare cannot be corrected either. ,
Imaging performance becomes unsatisfactory. Condition (3) regulates the object-side surface r ₃ of the second lens, and indicates the range in which spherical aberration and coma can be corrected. If the radius of curvature of r ₃ increases beyond the upper limit, Correction of positive spherical aberration becomes insufficient, and coma flare also occurs. Conversely, if the radius of curvature becomes small beyond the lower limit, the spherical aberration will be insufficiently corrected, which is inappropriate. That is, in this lens, most of the positive spherical aberration occurs at the third and ninth surfaces. It is desirable to share the burden, but if the third surface does not bear the burden of correcting the negative spherical aberration generated at the second surface, the residual will have a large adverse effect, so the above conditions are appropriate. Condition (4) regulates the shape of the sixth lens, which is an advantageous lens that can correct astigmatism and distortion with little influence on spherical aberration. Therefore, when the radius of curvature increases beyond the upper limit of the conditional expression 0.60f<r ₁₀ <0.75f, the Petzval sum becomes large and the meridional image plane in particular becomes overcorrected, making it difficult to correct astigmatism. On the other hand, if the radius of curvature becomes smaller than the lower limit, astigmatism will be insufficiently corrected, making it impossible to balance the imaging plane at the periphery, and at the same time increasing negative distortion, which tends to occur with wide-angle lenses, which is disadvantageous. become. In addition, when the conditional expression 1.80 < r ₁₁ / d ₁₀ < 3.05 becomes smaller than the lower limit, that is, when d ₁₀ becomes thick, the image plane becomes under-corrected, the astigmatism difference becomes large, and the balance of the image plane deteriorates. I can't take it anymore. on the other hand,
When r ₁₁ becomes small, the image plane becomes overcorrected and correction becomes difficult. On the other hand, when it increases beyond the upper limit, that is, when d ₁₀ becomes thin or r ₁₁ becomes large, the image plane becomes overcorrected, making it difficult to correct, and negative distortion becomes large, making it difficult to achieve the goal of the present invention. The purpose becomes unattainable. Condition (5) regulates the thickness of the second lens, the thickness of the third lens, and the distance between the second and third lenses, and is extremely advantageous for correcting field curvature. . Therefore, if the value becomes smaller than the lower limit, the image plane will be under-corrected, and conversely, if it becomes larger than the upper limit, the image plane will be over-corrected, and in both cases it is very difficult to balance the image plane. becomes. Condition (6) corrects the chromatic aberration caused by condition (1), and satisfactorily corrects the chromatic aberration in the front group (1st to 3rd lenses) in correspondence with the rear group (4th to 6th lenses). At the same time, in order to prevent the occurrence of coma aberration,
This is because it is necessary to increase the refractive index. That is, in order to reduce the burden on the fourth surface in relation to the radius of curvature of r ₃ limited by the condition (3), n ₂ is
1.65 or more is required, and if n ₂ is smaller than 1.65, the fourth surface will have a small negative radius of curvature, which may make it difficult to maintain aberration balance at the fifth surface and beyond. The same applies to n ₃ , and it is necessary not to reduce the radius of curvature of the sixth surface while maintaining the refractive power of the fifth surface, and n ₃ >1.65 is an essential condition. Condition 45 regarding Atsube number > (ν ₂
+ν ₃ )/2>28 depends on the condition (1), but in a wide-angle lens like this lens, which is also compact, the second and third lenses have a fairly extreme The chromatic aberration is insufficiently corrected with the 2nd lens, and the 3rd lens
It is desirable to overcorrect the lens. As a result, as mentioned above, it is necessary to make a certain amount of correction in the front group in correspondence with the rear group, and these conditions indicate this appropriate range. Examples of the present invention will be shown below. Here, f is the focal length of the entire system, r ₁ , r ₂ ... r ₁₁ is the radius of curvature of each lens surface, d ₁ , d ₂ ... d ₁₀ is the wall thickness and air gap of each lens, n ₁ , n ₂ ...n ₆ is the refractive index of each lens, ν ₁ , ν ₂
... ν ₆ is the Atsube number of each lens.

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【table】 [Brief explanation of drawings]

1, 3, 5, 7, and 9 are cross-sectional views of lenses of Examples 1, 2, 3, 4, and 5 of the present invention, and FIGS. 2, 4, and 6 respectively. 8 and 10 are Examples 1, 2, and 3 of the present invention, respectively.
4 and 5 are aberration diagrams.

Claims (1)

[Claims] 1. In order from the object side, the first negative meniscus lens
5 groups 6 each consisting of a lens, a second lens that is a positive lens, a third lens that is a negative lens, a cemented lens of a fourth lens that is a negative lens and a fifth lens that is a positive lens, and a sixth lens that is a meniscus lens. In a lens composed of two lenses, the first lens has its convex surface facing the object side, and the second lens has a convex surface facing the object side.
The lens is arranged with a convex surface with a strong curvature facing the object side, a third lens with a concave surface with a strong curvature facing the image side, and a sixth lens with a concave surface facing the image side, and each of the following conditions is satisfied. A wide-angle lens characterized by its composition. (1) 1.3f＜｜f ₁ ｜＜1.9f, f ₁ ＜0 (2) 0.32f＜r ₂ ＜0.40f (3) 0.59f＜r ₃ ＜0.80f (4) 0.60f＜r ₁₀ ＜0.75 f 1.80＜r ₁₁ /d ₁₀ ＜3.05 (5) 0.38f＜d _3〜5 ＜0.57f (6) n ₂ , n ₃ ＞1.65 45＞(ν ₂ +ν ₃ )／2＞28 However, f is the total The focal length of the system, f ₁ is the focal length of the first lens, r _i is the radius of curvature of the i-th surface from the object side,
d ₁₀ is the thickness of the sixth lens, d _{3 to 5} are the distance from the object side surface of the second lens to the image side surface of the third lens,
n ₂ and n ₃ are the refractive indices of the second lens and the third lens, respectively, and ν ₂ and ν ₃ are the Abbe numbers of the second lens and the third lens, respectively.