JPS6161365B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a photographic lens suitable for a standard or near standard angle of view for a 35 mm single-lens reflex camera, and in particular to an improvement of a Gaussian lens. Gaussian lenses have been widely used and have undergone various improvements because they are suitable for the standard angle of view of 35mm single-lens reflex cameras. Among these well-known lenses, the most commonly used lens arrangements are: positive meniscus lens, negative meniscus lens with positive and negative lenses bonded together,
It consists of an aperture, a negative meniscus lens with a negative and a positive lens attached together, and one or two positive lenses. On the other hand, in the case of photographic lenses that do not require such high performance, it is known that one side of the laminated lens is composed of a single lens because it is advantageous to make the lens compact. Conventionally, when trying to create a standard lens with an F number of 2 class with a lens configuration of 5 elements in 5 groups (positive, positive, negative, negative, positive), the lens near the aperture was single compared to a standard lens with 6 elements. Due to the nature of the lens, it is difficult to correct axial chromatic aberration, lateral chromatic aberration, chromatic coma, etc., and as it takes some effort to improve these aberrations, spherical aberration and curvature of field also worsen, and the amount of each aberration increases. As a result, it is normal to see a difference in performance. Therefore,
Preventing the deterioration of performance is being carried out by reducing the angle of view or increasing the F-number. The purpose of the present invention is to use a 5-element construction in 5 groups to achieve performance equivalent to a conventional 6-element standard lens system, and to make the lens system extremely compact. We were able to create a high-performance lens with a ratio of 1:2. The present invention includes, in order from the object side, a first positive meniscus lens with its convex surface facing the object side, a second positive meniscus lens with its convex surface facing the object side, a third negative meniscus lens with its convex surface facing the object side, and a third negative meniscus lens with its convex surface facing the image side. A lens system is composed of four negative meniscus lenses and a fifth positive lens whose image side surface has a stronger curvature, and satisfies the following conditions. (1) 1.7<(N ₁ +N ₂ +N ₅ )1/3<1.8 (2) 0.2<|φ ₁ |・f<0.4 (φ ₁ <0) (3) 7<(1/R ₆ +1/| R ₇ |)・f<8.5 (4) 0.5<|φ ₂ |・f<0.63 (φ ₂ <0) (5) 10<ν ₂ −ν ₃ <20 (6) ν ₃ <29, N ₃ > 1.73 (7) ν ₅ ＞49 Here, f is the focal length of the entire system, N ₁ , N ₂ , N ₃ ,
N ₅ is the refractive index for the d-line of the first, second, third, and fifth lenses, respectively; ν ₂ , ν ₃ , and ν ₅ are the Abbe numbers of the second, third, and fifth lenses, respectively; R ₆ ,
_R7 is the radius of curvature of the lens surface on the image side of the third lens and the object side of the fourth lens, respectively, _φ1 is the refractive power (reciprocal of the focal length) of the air lens between the second and third lenses, and _φ2 indicates the refractive power of the fourth lens. Next, the significance of each condition will be explained. (1) is a condition for satisfactorily correcting field curvature by regulating the refractive index of the positive lens in the lens system. Below the lower limit of the conditional formula, the Petzval sum becomes too large and the curvature of field increases; on the other hand, when the upper limit is exceeded, the Petzval sum improves, but the Atsbe number that can be selected from commonly available glass types is (vd)
Since the range of is limited, it becomes difficult to correct chromatic aberration. Therefore, if one were to find a type of glass suitable for correction, it would be extremely expensive and not suitable for general use. Condition (2)
Together with condition (3), these are conditions for correcting spherical aberration, field curvature, coma aberration, sagittal halo, and chromatic aberration in a well-balanced manner. In other words, the condition
In (3), the curvatures of the surfaces R ₆ and R ₇ before and after the aperture are somewhat weakly constrained to improve the comatic aberration at the intermediate angle of view of the sagittal halo, and as a result, the worsened spherical aberration and curvature of field astigmatism are reduced. In condition (2), by keeping the power of the air lens between the second lens and the third lens strong to a certain extent, the correction is performed satisfactorily. Therefore, below the lower limit of condition (2), spherical aberration and astigmatism will be under-corrected, and above the upper limit, the aberrations will be over-corrected, and the balance of chromatic aberration will deteriorate. Furthermore, below the lower limit of condition (3), it becomes impossible to correct spherical aberration, field curvature, and astigmatism, and above the upper limit, the occurrence of sagittal halo and coma aberration at intermediate angles of view becomes extremely significant. It becomes brighter. (4) is a condition for constraining the power of the fourth lens to maintain the required amount of back focus and to properly correct spherical aberration.If it is below the lower limit, it will not be possible to maintain the required amount of back focus, and the In this case, there will not be enough space for mirror-up. Also, lateral chromatic aberration (g-line) is undercorrected,
If the upper limit is exceeded, the back focus will be more than necessary, but if the power of the 4th lens becomes stronger, the power of the 5th lens must also be made stronger, and higher-order aberrations will be extremely large (to keep the focal length constant). In particular, it becomes impossible to properly correct spherical aberration. (5), (6), and (7) are conditions for correcting longitudinal chromatic aberration and lateral chromatic aberration in a well-balanced manner without worsening various aberrations other than chromatic aberration. The structure of the present invention does not have a bonding surface on the lens after the diaphragm, unlike the conventional standard lens with 6 Gaussian lenses.
In order to properly correct the above-mentioned chromatic aberration, the fifth lens must be made of a glass material with as low dispersion as possible, and at least the range of condition (7) is important. Furthermore, for the same purpose, the third lens must be made of a glass material with as high a dispersion as possible, and must also have a high refractive index in order to correct spherical aberration and sagittal halo, thus setting the range of condition (6). Outside this range, the annular spherical aberration increases, the sagittal halo also increases, and high performance becomes impossible. Condition (5) is to restrict the difference in Atsube number between the second lens and the third lens, to maintain the power of the second lens strong to some extent, and to properly correct chromatic aberration and astigmatism. Below this value, axial chromatic aberration (g-line) will be under-corrected, and above the upper limit, astigmatism at intermediate angles of view will be under-corrected, making it difficult to achieve high performance. As a result of the above, by satisfying the conditions of the present invention, as shown in this example, the angle of view is
With specifications of 45.5° and an F number of 1:2, we were able to create a standard lens that is compact and has good performance. FIG. 1 shows the cross-sectional shape of Example 1, and since other examples do not noticeably deform this lens shape, the cross-sectional shapes are not shown. Also, spherical aberration for infinity, sine condition (SC), astigmatism, distortion,
FIGS. 2 to 8 showing lateral chromatic aberration and coma aberration correspond to the first to seventh embodiments in order. Example 1

【table】 Example 2

【table】 Example 3

【table】 Example 4

【table】 Example 5

【table】

【table】 Example 6

【table】 Example 7

【table】

【table】

【table】

[Table] The correspondence between Examples 1 to 7 and numerical conditions is shown below.

【table】 [Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of the lens of Example 1. FIG. 2 to FIG. 8 are aberration curve diagrams of Examples 1 to 7 in sequence. In the figure, R is a lens surface, D is a lens thickness or lens interval, S is a sagittal image surface, and m is a meridional image surface.

Claims (1)

[Claims] 1. In order from the object side, a first positive meniscus lens with its convex surface facing the object side, a second positive meniscus lens with its convex surface facing the object side, a third negative meniscus lens with its convex surface facing the object side, and a third negative meniscus lens with its convex surface facing the image side. 1. A compact photographic lens comprising a fourth negative meniscus lens that faces toward the camera and a fifth positive lens whose image side surface has a stronger curvature, and which satisfies the following conditions. (1) 1.7<(N ₁ +N ₂ +N ₅ )1/3<1.8 (2) 0.2<|φ ₁ |・f<0.4 φ ₁ <0 (3) 7<(1/R ₆ +1/|R ₇ |)・f<8.5 (4) 0.5<|φ ₂ |・f<0.63 φ ₂ <0 (5) 10<ν ₂ −ν ₃ <20 (6) ν ₃ <29, N ₃ >1.73 (7) ν ₅ > 49 However, f is the focal length of the entire system, N ₁ , N ₂ , N ₃ , N ₅
are the d- of the first, second, third, and fifth lenses, respectively.
The refractive index for the line, ν ₂ , ν ₃ , and ν ₅ are the Abbe numbers of the second, third, and fifth lenses, respectively, and R ₆ and R ₇ are the image side of the third lens and the object side of the fourth lens, respectively. The radius of surface curvature, φ ₁ is the refractive power (reciprocal of the focal length) of the air lens between the second and third lenses, and φ ₂ is the refractive power of the fourth lens.