JP3444453B2 - Wide angle lens for camera - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inexpensive and simple camera wide-angle lens suitable for still cameras and electronic still cameras, and more particularly to a camera wide-angle lens comprising two lenses and an aperture stop. .

[0002]

2. Description of the Related Art As a wide-angle lens used in a still camera or an electronic still camera, a type known as a so-called "bitouin diaphragm" in which an aperture stop is sandwiched by two convex lenses is known. There is.

Many of the bite-win diaphragm types have a symmetrical shape, and such a symmetrical shape has the advantages of good image formation and excellent wide-angle function.

[0004]

However, in the case of the above-mentioned Vitowin diaphragm type, there is a problem that the total lens length from the front lens surface of the lens system to the image forming surface becomes long and the assembling property deteriorates. In particular, in today's simple cameras that require thin and compact cameras,
It is necessary to avoid increasing the total lens length as much as possible.

The present invention has been made in view of the above circumstances, and can shorten the total lens length from the front lens surface of the lens system to the image forming surface, thereby making the optical system compact and improving the assembling property. At the same time, it is an object of the present invention to provide a wide-angle lens for a camera which is inexpensive, has a simple structure, and can have good imaging performance.

[0006]

According to the wide-angle lens for a camera of the present invention, the overall length of the lens is shortened by disposing the aperture stop, the first lens L ₁ and the second lens L ₂ in this order from the object side. The two-lens pre-aperture lens configuration is possible, and the four pre-aperture two-lens configuration is configured so as to satisfy the four conditional expressions capable of improving the imaging performance.

That is, the wide-angle lens for a camera according to the present invention comprises a first lens L _{1 made of} a positive or negative meniscus lens having an aperture stop from the object side and a convex surface facing the object side, and a second lens made of a positive lens. L ₂ is arranged in this order and satisfies the following conditional expressions (1) to (4).

(1) −0.10 <f ₂ / f ₁ <0.7 (2) 4.0 <f / R ₁ <7.5 (3) 0 <f / R ₃ <1.6 (4) 0.10 <D / f <0.22 where , F ₁ : the focal length of the first lens L ₁ , f ₂ : the focal length of the second lens L ₂ , f: the focal length of the entire lens system R ₁ : the paraxial radius of curvature R of the object-side surface of the first lens L _{1. 3:} the paraxial curvature radius D of the surface of the second lens L ₂ on the object side: a distance from the surface of the first lens L ₁ on the object side to the surface of the image plane side of the second lens L ₂ Further, the two The lenses L ₁ and L ₂ are preferably made of plastic.

Further, it is preferable that at least one of the lens surfaces of the _two lenses L ₁ and L ₂ is an aspherical surface.

The position of the aperture stop is the first
The case where it coincides with the lens surface position on the object side of the lens L ₁ is also included.

[0011]

According to the above construction, in the lens system including the first lens L _{1 made of} a positive or negative meniscus lens having a convex surface facing the object side, and the second lens L _{2 made of a} positive lens following the first lens L ₁ . 1 lens L _{1 has} an object side lens surface position, or has a front diaphragm having two lens elements having an aperture stop disposed on the object side of the lens surface. This makes it possible to connect the front lens surface of the lens system with a simple structure. By shortening the total length of the lens up to the image plane, the optical system can be made compact and the assemblability can be improved.

The conditional expression (1) relates to the distribution of the refractive powers of the first lens L ₁ and the second lens L ₂ , and by satisfying the conditional expression (1) in which the distribution is appropriate, the total lens length is The length and the imaging performance can be satisfied at the same time.

That is, if the lower limit of conditional expression (1) is exceeded, the refracting power of the second lens L ₂ becomes large, and the spherical aberration, the field curvature, and the distortion are deteriorated.

Even if the conditional expression (1) is not satisfied, by making the lens surface aspheric or curving the film surface, spherical aberration and field curvature can be improved to some extent, but conversely, distortion aberration is further deteriorated. Or, the total lens length becomes long, which is not desirable.

Further, when the upper limit of the conditional expression (1) is exceeded, the total lens length becomes short, but chromatic aberration of magnification and astigmatism become large, and it is impossible to correct even if the film surface is curved.

Next, the above conditional expression (2) and the above conditional expression (3)
Is a conditional expression relating to each lens shape for simultaneously improving the overall lens length and the imaging performance when the conditional expression (1) is satisfied.

If the lower limit of the conditional expression (2) is exceeded, the spherical aberration will be good, but the total lens length will be long and the field curvature will be deteriorated. On the other hand, if the upper limit is exceeded, the total lens length will be short and the field curvature will also be decreased. Although it is good, spherical aberration and coma are deteriorated, which causes a problem.

If the lower limit of the conditional expression (3) is exceeded, the total lens length becomes long, and the rear surface of the second lens L ₂ has a large curvature, which affects spherical aberration. On the contrary, if the upper limit is exceeded, lateral chromatic aberration increases.

Further, the conditional expression (4) relates to the lens length, and if the upper limit is exceeded, the lens outer diameter of the second lens L ₂ becomes too large, and the optical system cannot be compactly assembled. If the lower limit is exceeded, the first lens L ₁ and the second lens
The center thickness of each of the lenses L ₂ may not be sufficient, the distance between the first lens L ₁ and the second lens L ₂ may not be secured, and the field curvature may not be corrected sufficiently.

Further, by forming the _two lenses L ₁ and L ₂ from plastic, the manufacturing cost of the lens system can be reduced.

If at least one of the lens surfaces of the _two lenses L ₁ and L ₂ is an aspherical surface, it becomes possible to simultaneously achieve a wide angle and high performance with a two-lens structure. That is, unlike the wide-angle diaphragm type described in the prior art, a lens system having a front diaphragm like the wide-angle lens of the present invention and in which the shapes of the _two lenses L ₁ and L ₂ are largely different from each other. In this case, since distortion and the like tend to increase as the angle of the optical system becomes wider, it is preferable to solve the problem caused by the asymmetry of the lens system by making the lens surface aspherical.

[0022]

Embodiments of the present invention will be described below with reference to the drawings.

FIGS. 1 to 4 are cross-sectional views of lens systems according to examples in which numerical values are set as shown in Table 1 below. The symbols in Table 1 are defined as follows.

F ₁ : focal length of the first lens L ₁ f ₂ : focal length of the second lens L ₂ f: focal length of the entire lens system R ₁ : paraxial curvature of the object side surface of the first lens L ₁ radius R _3: the paraxial curvature radius D of the surface of the second lens L ₂ on the object side: a distance from the surface of the first lens L ₁ on the object side to the surface of the image plane side of the second lens L ₂ L: lens Total length (distance from the object side surface of the first lens L ₁ to the image plane)

[0025]

[Table 1]

FIG. 1 is a lens cross-sectional view showing a wide-angle lens for a camera according to a first embodiment, including an aperture I, a first lens L ₁ having a negative refracting power and a second lens L ₂ having a positive refracting power.
Are arranged in order from the object side. Here, the first lens L ₁ is a negative meniscus lens having a convex surface facing the object side, and the second lens L ₂ is a biconvex lens having a strong curvature surface facing the film F (image forming surface; the same applies hereinafter). Is.
Further, the focal length f ₁ of the first lens L ₁ is -312.24mm
, And the focal length f ₂ of the second lens L ₂ is 26.85mm
And the combined focal length f of these lens systems is 32.47 mm. Also, the back focus length l is 29.571 mm,
The radius of curvature r of the curved film F is -100.0 mm.
Further, the diaphragm I is located on the object-side surface of the first lens L ₁ (hereinafter referred to as the first surface), and the angle of view 2ω is set to 67.4 °. In the figure, X indicates an optical axis (same in FIGS. 2, 3 and 4).

The two lenses L ₁ and L ₂ are plastic lenses. The same applies to the other examples shown below.

Optical axis X of each lens surface in Example 1
Table 2 below shows the radius of curvature R (mm) in the vicinity, the center thickness of each lens, the air gap d (mm) between each lens, the refractive index N of each lens at the d-line, and the Abbe number ν of each lens.

However, in Table 2, each symbol R,
The numbers attached to d, N, and ν represent the order from the object side.

The object-side surfaces of the lenses L ₁ and L ₂ are aspherical surfaces, and their shapes are aspherical expressions.

[0031]

[Equation 1]

In each coefficient C, K, a ₁ , a ₂ , a
The values of ₃ and a ₄ are represented by those shown in Table 3 below. In the above aspherical expression, X is the distance from the lens surface in the optical axis X direction, and Y is the distance from the optical axis X in the direction perpendicular to the optical axis X. Further, C represents the curvature.

[0033]

[Table 2] R d N ν 5.309 ^{* 1} 1.50 1.49116 57.8 4.654 0.90 198.555 ^{* 2} 1.50 1.49116 57.8

[0034]

[Table 3] Surface 1st surface 3rd surface C 0.188359 0.005036 K 7.44698 3674.719 a ₁ -0.637315 × 10 ^-2 0.781309 × 10 ^-3 a ₂ 0.968608 × 10 ^-4 0.141142 × 10 ^-3 a ₃ -0.571697 × 10 ^-3 -0.548809 x 10 ^-5 a ₄ 0.0 0.0

Next, FIG. 2 is a lens cross-sectional view showing a wide-angle lens for a camera according to a second embodiment. The diaphragm I, the first lens L ₁ having a positive refractive power and the second lens L having a positive refractive power are shown. ₂ are arranged in order from the object side. Here, the first lens L ₁ is a positive meniscus lens having a convex surface facing the object side, and the second lens L ₂ is a biconvex lens having a surface having a strong curvature facing the film F side. Also, the first
The focal length f ₁ of the lens L ₁ is 68.52 mm, the second
The focal length f ₂ of the lens L ₂ is a 34.61Mm, the combined focal length f of the lens system is 25.30Mm. The back focus length 1 is 21.351 mm and the radius of curvature r of the curved film F is -100.0 mm. Further, the diaphragm I is located on the first surface and the angle of view 2ω is set to 74.5 °.

Optical axis X of each lens surface in Example 2
Table 4 below shows the radius of curvature R (mm) in the vicinity, the center thickness of each lens, the air gap d (mm) between each lens, the refractive index N of each lens at the d-line, and the Abbe number ν of each lens.

Table 5 below shows the values of the coefficients C, K, a ₁ , a ₂ , a ₃ and a ₄ in the above aspherical expression for the second embodiment.

The numbers attached to the respective symbols are the same as those in Tables 2 and 3.

[0039]

[Table 4] R d N ν 4.439 1.40 1.49244 57.6 4.580 1.66 37.913 1.60 1.49244 57.6 -30.538 ^{* 1}

[0040]

[Table 5] Surface 4th surface C -0.032746 K 1.00 a ₁ 0.2645396 × 10 ^-3 a ₂ -0.193124 × 10 ^-4 a ₃ 0.129464 × 10 ^-6 a ₄ 0.807664 × 10 ^-9

Next, FIG. 3 is a lens sectional view showing a wide-angle lens for a camera according to a third embodiment. The diaphragm I, the first lens L ₁ having a positive refractive power and the second lens L having a positive refractive power are shown. ₂ are arranged in order from the object side. Here, the first lens L ₁ and the second lens L ₂ are positive meniscus lenses having convex surfaces facing the object side. The focal length f ₁ of the first lens L ₁ is 62.89 mm, the focal length f ₂ of the second lens L ₂ is a 36.44Mm, the combined focal length f of the lens system is 25.00 mm. The back focus length 1 is 20.655 mm and the radius of curvature r of the curved film F is -85.0 mm. Further, the aperture I is arranged at a position 3.0 mm from the first surface toward the object side, and the angle of view 2
ω is set to be 75.2 °.

Optical axis X of each lens surface in Example 3
Table 6 below shows the radius of curvature R (mm) in the vicinity, the center thickness of each lens, the air gap d (mm) between each lens, the refractive index N of each lens at the d-line, and the Abbe number ν of each lens.

The numbers attached to the respective symbols are the same as those in Tables 2 and 3.

Table 7 below shows the values of the coefficients C, K, a ₁ , a ₂ , a ₃ and a ₄ in the above aspherical expression for the third embodiment.

[0045]

[Table 6] R d N ν 4.289 1.30 1.49244 57.6 4.481 1.39 17.550 1.90 1.49244 57.6 767.040 ^{* 1}

[0046]

[Table 7] Surface 4th surface C 0.0013037 K -71158.06 a ₁ 0.7620 47 × 10 ^-3 a ₂ -0.610663 × 10 ^-4 a ₃ 0.316390 × 10 ^-5 a ₄ -0.798321 × 10 ^-7

Next, FIG. 4 is a lens cross-sectional view showing a wide-angle lens for a camera according to the fourth embodiment. The diaphragm I, the first lens L ₁ having a positive refracting power and the second lens L having a positive refracting power are shown. ₂ are arranged in order from the object side. Here, the first lens L ₁ is a positive meniscus lens having a convex surface facing the object side, and the second lens L ₂ is a biconvex lens having a surface having a strong curvature facing the object side. The focal length f ₁ of the first lens L ₁ is a 95.19Mm, the focal length f ₂ of the second lens L ₂ is a 27.92Mm, the combined focal length f of the lens system is 24.00 mm. The back focus length l is 19.737 mm, and the radius of curvature r of the curved film F is -90.0 mm. Further, the diaphragm I is located on the first surface, and the angle of view 2ω is set to 77.5 °.

Optical axis X of each lens surface in Example 4
Table 8 below shows the radius of curvature R (mm) in the vicinity, the center thickness of each lens, the air gap d (mm) between each lens, the refractive index N of each lens at the d-line, and the Abbe number ν of each lens.

Table 9 below shows the values of the respective coefficients C, K, a ₁ , a ₂ , a ₃ and a ₄ in the above aspherical expression for the fourth embodiment.

The numbers attached to the respective symbols are the same as those in Tables 2 and 3.

[0051]

[Table 8] RD N ν _d 5.067 1.82 1.49244 57.6 5.006 ^{* 1} 0.88 15.926 1.98 1.49244 57.6 -96.500 ^{* 2}

[0052]

TABLE 9 faces the second surface (* 1) the fourth surface (* 2) C 0.199760 -0.010363 K -1.191124 -1.585373 × 10 4 a 1 -0.458142 × 10 -3 0.413083 × 10 -3 a 2 0.124943 × 10 - ² -0.143931 x 10 ^-4 a ₃ -0.2010 13 x 10 ^-3 -0.298829 x 10 ^-6 a ₄ 0.626807 x 10 ^-16 -0.792994 x 10 ^-14

FIG. 5 shows aberration diagrams of Example 1 (spherical aberration / sinusoidal condition, field curvature and distortion; the same applies hereinafter).
FIG. 6 is an aberration diagram of Example 2 and FIG. 7 is an aberration diagram of Example 3.
8 is an aberration diagram of the fourth embodiment shown in FIG.

According to the above embodiment, the telephoto ratio is 1.0 to 1.03.
It is possible to obtain an inexpensive wide-angle lens that is compact and has high performance and is practically sufficient with an angle of view of 67 to 78 degrees.

The construction of the wide-angle lens of the present invention is not limited to those of the four embodiments described above,
Various modifications can be made. For example, in the above embodiment, the film F, which is the image forming surface, is curved to improve the field curvature and distortion. However, in the wide-angle lens of the present invention, It is also possible to use the film F arranged in a plane as the image plane.

Of course, the two lenses L ₁ and L ₂ may be made of glass instead of plastic lenses.

The wide-angle lens of the present invention can be applied to various cameras such as an electronic still camera and a still camera, for example, a surveillance camera (CCTV).

[Brief description of drawings]

FIG. 1 is a sectional view showing a wide-angle lens according to Embodiment 1 of the present invention.

FIG. 2 is a sectional view showing a wide-angle lens according to Embodiment 2 of the present invention.

FIG. 3 is a sectional view showing a wide-angle lens according to Embodiment 3 of the present invention.

FIG. 4 is a sectional view showing a wide-angle lens according to Embodiment 4 of the present invention.

FIG. 5 is an aberration diagram of a wide-angle lens according to Example 1 of the present invention.

FIG. 6 is an aberration diagram of a wide-angle lens according to Example 2 of the present invention.

FIG. 7 is an aberration diagram of a wide-angle lens according to Example 3 of the present invention.

FIG. 8 is an aberration diagram of a wide-angle lens according to Example 4 of the present invention.

[Explanation of symbols]

L ₁ First lens L ₂ Second lens I Aperture F Film X Optical axis

Claims (3)

(57) [Claims] 1. An aperture stop is arranged from the object side, and a first lens L _{1 made of} a positive or negative meniscus lens having a convex surface facing the object side and a second lens L ₂ made of a positive lens are arranged in this order. And a wide-angle lens for a camera, which satisfies the following conditional expressions (1) to (4). (1) −0.10 <f ₂ / f ₁ <0.7 (2) 4.0 <f / R ₁ <7.5 (3) 0 <f / R ₃ <1.6 (4) 0.10 <D / f <0.22 where f ₁ : Focal length f ₂ of the first lens L ₁ : focal length f of the second lens L ₂ : focal length R _{1 of the} whole lens system: paraxial radius of curvature R ₃ of the object side surface of the first lens L ₁ : first second lens L paraxial curvature of the surface of the _second object side radius D: distance from the surface of the first lens L ₁ on the object side to the surface of the image plane side of the second lens L ₂ 2. The wide-angle lens for a camera according to claim ₁ , wherein the first and second lenses L ₁ and L ₂ are both plastic lenses. 3. The wide-angle lens for a camera according to claim 1, wherein at least one of the lens surfaces of the first and second lenses L ₁ and L ₂ is an aspherical surface.