JPH0239764B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a compact, low-cost Gaussian lens that uses glass and has an angle of view of about 46° and an aperture ratio of about 1:1.8 to 1:2.0. In recent years, various Gaussian lenses with a relatively compact shape with an aperture ratio of about 1:1.8 to 1:2.0 with a configuration of 6 elements in 5 groups have been announced. This aberration is corrected by using high-quality glass with a large refractive index for the four positive lenses, which inevitably leads to high costs. On the other hand, lenses using so-called low-cost glass are insufficiently compact even if aberrations are sufficiently corrected. This invention requires a long back focus f _B
While maintaining the total lens thickness Σd is short, the total lens length L including the back focus is equal to the composite focal length
The objective is to provide a lens system that is compact with a magnification of about 1.3 times, uses relatively low-cost glass, and has aberrations well corrected. In order to make the shape more compact, the air gap d ₆ at the center of the lens system, which has the largest distance between lens surfaces, is set as small as possible. Then, a relatively strong radius of curvature is given to the biconcave surfaces of lens surfaces R ₆ and R ₇ that sandwich this air gap, and the Petzval sum is reduced, thereby reducing the divergent spherical aberration and coma aberration. gave a strong radius of curvature to the two convex surfaces R ₁ and R ₃ on the object side, and performed correction by the convergence effect.
Furthermore, such a refractive power distribution is also effective in correcting the positive curvature of the sagittal image plane at the peripheral angle of view, which tends to occur due to the short total lens thickness Σd. Specifically, the lens system based on the above basic concept includes the first group, the second group is a positive meniscus lens with a convex surface facing the object side, the third group is a negative meniscus lens with a convex surface facing the object side, and the fourth group is a negative meniscus lens with a convex surface facing the object side. is a meniscus lens with a concave surface facing the object side, which is a combination of a biconcave lens and a biconvex lens, and the 5th group is a positive lens, consisting of 6 elements in 5 groups. f: Composite focal length of the entire system Σd: Total distance between lens surfaces ni: Refractive index of the i-th lens material from the object side νi: Atsube number of the i-th lens material from the object side Gi: Glass index (=n _i ⁸ × νi/100) Ri: i-th lens surface from the object side Radius of curvature di: Distance between i-th lens surface from object side 0.59f＜Σd＜0.63f ……(1) 0.17f＜d ₆ ＜0.22f ……(2) 26 ＜G ₁ ＜31 …… (3) 26 ＜G ₂ ＜31 ……(4) 26 ＜G ₅ ＜35 ……(5) 26 ＜G ₆ ＜35 ……(6) 0.27f＜R ₆ +｜R ₇ ｜/2＜0.32 f R ₇ <0 ... (7-1) 1.1 < | R ₇ | / R ₆ <1.3 ... (7-2) 0.45f < R ₁ <0.55f ... (8-1) 0.37f < R ₃ <0.42f (8-2) 0.37 <R ₄ /R ₅ <0.47 ...(9) is satisfied. The above condition (1) is quite small for a Gaussian lens with F: 1.8 to F2.0, and keeping this value small is important in making the lens more compact and keeping the peripheral illumination ratio at the required value. It is also advantageous for downsizing in the sense of suppressing the increase in lens diameter that would occur. The upper limit is the limit of this effect, and the lower limit is beyond which it becomes difficult to correct aberrations using low-cost glass materials meeting conditions (3) to (6). Condition (2) sets the air spacing _d6 small so as to suppress the increase in Petzval sum and shorten the total lens thickness Σd.If the upper limit is exceeded under the restrictions of condition (1), the thickness of each lens decreases. becomes too small, leading to an increase in the Petzval sum. On the other hand, if it exceeds the lower limit and becomes small,
Not only does it become difficult to correct divergent spherical aberration and coma aberration, but mechanical conditions such as insertion of a diaphragm also deteriorate. Conditions (3), (4), (5), and (6) are conditions that specify low-cost glass. FIG. 1 shows the distribution range of the refractive index and Abbe number of the optical glasses used for the biconvex lenses in the first, second, and fourth groups and the four positive lenses in the fifth group.
In the distribution of optical glasses, where the vertical axis represents the d-line refractive index n and the horizontal axis represents the Atsube number ν, the range of ordinary glasses generally used for positive lenses, including some lanthanum-based glasses, is (n _d ⁸ ×ν)
Those having almost the same value are equivalent in aberration correction effect. In addition, the cost of glass materials having the same value is about the same, and the cost of glass materials in the range of conditions shown here is relatively low. A material with a high refractive index is more advantageous in terms of aberration correction, and the conditions
If the lower limits of (3)(4)(5)(6) are exceeded, aberration correction becomes difficult, while if the upper limits are exceeded, costs increase. Condition (7-1) of condition (7) is condition (1) to (6)
R ₆ , which has a large influence on the occurrence of aberrations under
A strong curvature was intentionally set for _R7 . If the upper limit is exceeded, the occurrence of spherical aberration, coma flare, etc. will be reduced, but the Petzval sum will become large, making it difficult to correct field curvature, and also making it impossible to maintain the required back focus fB. If the lower limit is exceeded, the occurrence of positively acting high-order spherical aberration divergent coma flare becomes large, and even if other conditions are selected, it becomes difficult to correct the aberrations in a well-balanced manner as a whole. Furthermore, condition (7-2) is such that _R7 , which is greatly affected by the shortness of _d6 among these two concave surfaces, has a larger radius of curvature than _R6 . Under this condition, the divergent effects of the biconcave surfaces are averaged, and higher-order spherical aberrations and unbalanced comatic aberrations can be avoided. If the lower limit is exceeded and _R7 becomes small relative to _R6 , the diverging effect of the _R7 surface becomes too large, the higher-order spherical aberration of the biconcave surface as a whole becomes large, and at the same time the refractive power of the _R6 surface becomes weak. This makes it difficult to obtain the necessary back focus. On the other hand, if the upper limit is exceeded, the divergence effect of the _R7 surface becomes too weak, which is also related to setting a strong convergence effect in the front part of the lens system in the next condition (8), This results in an astringent coma flare. By giving R ₁ and R ₃ a strong convergence effect according to condition (8), we also have the effect of correcting the divergence effect of R ₆ and R ₇ , and the particularly strong effect of R ₁ is sagittal. This is to prevent deterioration of the image plane. The upper limit is the limit value for obtaining each of these effects. When the value exceeds the lower limit, it is possible to prevent the distortion from becoming negative, but it does not allow the generation of higher-order spherical aberrations that act negatively, and causes worsening of chromatic aberration of magnification. Condition (9) is achieved by giving the concave surface R ₄ a relatively stronger radius of curvature than the convex surface R ₅ , that is, by increasing the negative power of the air lens formed between the second group lens and the third group lens. This is an attempt to correct the meridional field curvature, which tends to deteriorate in the negative direction due to the above-mentioned condition (8). If the upper limit is exceeded and the negative power of the air lens becomes weak, the negative curvature of the meridional image surface becomes too large and cannot be corrected under other conditions. On the other hand, if the lower limit is exceeded, the divergence effect becomes too strong, leading to the occurrence of coma flare at intermediate angles of view, and distortion in the negative direction also increases. In addition to the basic conditions mentioned above, it is desirable that the following secondary conditions be satisfied. 13＜ν ₁ +ν ₂ /2−ν ₃ <20 ...(10) 8＜ν ₅ +ν ₆ /2−ν ₄ <18 ...(11) 36＜ν ₁ , ν ₂ , ν ₅ , ν ₆ < 60 ……(12) Conditions (10) and (11) are the conditions for satisfying color correction throughout the entire system, and each lower limit is exceeded and the ν value of the negative lens is larger than the ν value of the positive lens. and color correction becomes impossible. On the other hand, if the upper limit is exceeded and the difference becomes large, the refractive index of the negative lens will inevitably become large, which is related to conditions (3), (4), (5), and (6), which use low-cost glass. This will lead to an increase in the Petzval sum. The lower limit of condition (12) is the limit for making axial and magnification color correction possible in the achromatic role of each positive lens, and the upper limit is condition (3) (4) (5) (6). This is to prevent the refractive index from becoming too small within the range of glass materials defined by . Examples of this invention will be shown below.

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【table】 [Brief explanation of drawings]

Fig. 1 is a glass index distribution diagram of optical glass, Fig. 2 is a cross-sectional view of the lens of the first embodiment, Fig. 3 is its aberration curve diagram, and Fig. 4 is a cross-sectional diagram of the lens of the fourth embodiment. FIG. 5 shows its aberration curve diagram.

Claims (1)

[Claims] 1. The first group and the second group are positive meniscus lenses with a convex surface facing the object side, the third group is a negative meniscus lens with a convex surface facing the object side, and the fourth group is a biconcave lens and a biconvex lens. A meniscus lens with a concave surface facing the object side, consisting of a laminated lens of Refractive index νi of the i-th lens material from the object side: Atsube number Gi of the i-th lens material from the object side: Glass index (=n _i ⁸ × ν _i /100) R _i : Radius of curvature d of the i-th lens surface from the object side _i : When the distance between the i-th lens surfaces is taken from the object side 0.59f＜Σd＜0.63f 0.17f＜d ₆ ＜0.22f 26 ＜G ₁ ＜31 26 ＜G ₂ ＜31 26 ＜G ₅ ＜35 26 ＜G ₆ ＜35 0.27f＜R ₆ +｜R ₇ ｜/2＜0.32f (R ₇ ＜0) 1.1 ＜｜R ₇ ｜／R ₆ ＜1.3 0.45f＜R ₁ ＜0.55f 0.37f＜R ₃ ＜0.42 A Gaussian lens that satisfies the following conditions: f 0.37 < R ₄ / R ₅ < 0.47.