JP2001272598A - Taking lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a taking lens where chromatic aberration of magnification and other aberration are excellently compensated, which can be used to an extent that a half viewing angle is about 32.5 deg. and which is suitable to use an imaging element though it is constituted of three lenses. SOLUTION: This taking lens is constituted of a 1st negative meniscus lens whose convex surface faces to an object side, a 2nd positive lens and a 3rd negative meniscus lens whose concave surface faces to an image side from the object side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photographing lens, and more particularly, to a photographing lens suitable for a digital still camera, a personal computer camera, and a video camera using an image pickup device.

[0002]

2. Description of the Related Art Hitherto, a three-lens configuration of an imaging lens suitable for using an imaging element has been disclosed. For example, JP-A-11-52227, JP-A-10-300907, and
No. 0-170819 is known.

Further, as an imaging lens using a diffractive element in addition to a normal lens, Japanese Patent Application Laid-Open Nos. 10-115777, 10-111449, and 1
For example, Japanese Patent Application Laid-Open No. 1-352397 is known.

[0004]

However, the former photographic lens has large chromatic aberration of magnification and a half angle of view of 2 mm.
If it is 5 ° or more, it is difficult to obtain good image quality. Also,
In the latter photographic lens, when achromatism is performed using a diffraction element, there is a problem that flare due to diffracted light of other orders cannot be ignored due to reduction in diffraction efficiency, and it is difficult to obtain a good image at the time of backlight or the like. is there.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and an object of the present invention is to provide a lens having a small number of three lenses, but to have excellent correction of various aberrations including chromatic aberration of magnification, and a half field angle of 32. An object of the present invention is to provide a photographing lens which can be used up to about 0.5 ° and is suitable for using an image pickup device.

[0006]

The above object can be achieved by any of the following means. (1) In order from the object side, a negative meniscus first lens having a convex surface facing the object side, a positive second lens, and a negative meniscus third lens having a concave surface facing the image side. A photographing lens characterized by being constituted.

(2) When the Abbe number of the second lens is ν ₂ and the Abbe number of the third lens is ν ₃ , the following condition is satisfied. Shooting lens.

50 <ν ₂ <70 Expression 20 [ν ₃ <35 Expression 1] ............ formula [2] (3) the focal length of the imaging lens system is f, the focal length of the third lens and the f _3, characterized by satisfying the following condition The imaging lens according to (1) or (2), wherein

−0.5 <f / f ₃ <−0.1 Equation (3) (4) The focal length of the first lens is f ₁ , First
The taking lens according to (1), (2) or (3), wherein the following conditional expression is satisfied, where t1 is the thickness on the axis of the lens.

0.45 <t ₁ / f ₁ <0.8 Expression (4) (5) The radius of curvature of the object side surface of the first lens is r _1. Wherein, when the radius of curvature of the image side surface of the first lens is r ₂ , the following conditional expression is satisfied: (1), (2), (3) or (4) lens.

−0.6 <(r ₂ −r ₁ ) / (r ₂ + r ₁ ) <− 0.3 Equation [5] (6) The first lens, the second lens, and the third lens The photographic lens according to any one of (1) to (5), wherein the lenses are all formed of plastic lenses.

(7) The focal length of the entire photographing lens system is f
And then, when said focal length of the first lens and the f _1, the imaging lens according to any one of (1) to (6), characterized by satisfying the following conditional expression.

−0.7 <f / f ₁ <−0.4 Equation (6) Next, each term will be described in order.

According to the invention described in the above (1), in particular,
Since the first lens is a negative meniscus lens having a convex surface on the object side and the third lens is a negative meniscus lens having a concave surface on the object side, lateral chromatic aberration and axial chromatic aberration can be corrected well. Is possible, and the half angle of view is 32.
Good imaging performance is obtained up to about 5 °. The lateral chromatic aberration is the difference between the d-line and the g-line of the lateral aberration, and the axial chromatic aberration is the difference between the d-line and the g-line of the spherical aberration.

According to the invention described in the above (2), the conditional expression [1] is a condition for favorably correcting lateral chromatic aberration and longitudinal chromatic aberration, and also relating to workability of a lens optical material. If the upper limit of conditional expression [1] is exceeded, the workability of the lens will be poor and the cost will be high. Conversely, if the lower limit of conditional expression [1] is exceeded, lateral chromatic aberration and axial chromatic aberration cannot be corrected well. Conditional expression [1] has a range of 5
0 <ν ₂ <70, preferably 55 <ν ₂ <65. Conditional expression [2] is a condition relating to the cost of the lens optical material in order to correct axial chromatic aberration and lateral chromatic aberration. If the upper limit of conditional expression [2] is exceeded, axial chromatic aberration and magnification will not be satisfied. Chromatic aberration is undercorrected. Conversely, the cost of the lens material exceeding the lower limit of conditional expression [2] increases. The range of the conditional expression [2] is 20 <ν ₃ <35, preferably 25 <ν ₃ <33.

According to the invention described in the above (3), the conditional expression [3] is a condition under which chromatic aberration (axial chromatic aberration and chromatic aberration of magnification), distortion, exit pupil position and the like are appropriate. If the lower limit of conditional expression [3] is exceeded, it is difficult to sufficiently correct distortion, and it is difficult to keep the exit pupil position at an appropriate distance from the imaging surface. Conversely, when the value exceeds the upper limit of conditional expression [3], it becomes difficult to correct chromatic aberration. The range of conditional expression [3] is -0.5 <
f / f ₃ <−0.1, preferably −0.4 <f /
f ₃ <−0.2.

According to the invention described in the above (4), the conditional expression [4] is a condition for favorably correcting lateral chromatic aberration and for the size of the lens outer diameter. If the lower limit of conditional expression [4] is exceeded, it is difficult to satisfactorily correct lateral chromatic aberration. If the upper limit of conditional expression [4] is exceeded, it becomes difficult to reduce the outer diameter of the lens. Conditional expression [4] has a range of 0.
45 <t ₁ / f ₁ <0.8, preferably 0.55
<T ₁ / f ₁ <0.7.

According to the invention described in the above (5), the conditional expression [5] is a condition relating to good correction of chromatic aberration of magnification and field curvature and workability. When the value exceeds the upper limit of conditional expression [5],
It becomes difficult to reduce the chromatic aberration of magnification generated in the first lens, and it is difficult to reduce the Petzval sum due to the contribution of the image side surface of the first lens, and thus it is difficult to reduce the field curvature. Conversely, if the lower limit of conditional expression [5] is exceeded, the radius of curvature of both surfaces will be too small, resulting in poor workability. The range of conditional expression [5] is -0.
6 <(r ₂ −r ₁ ) / (r ₂ + r ₁ ) <− 0.3, preferably −0.5 <(r ₂ −r ₁ ) / (r ₂ + r ₁ ) <−
0.35.

According to the invention described in the above (6), when plastic is used, it is lightweight, and the shape other than the optical surface of the lens can be relatively freely formed, so that it is easy to assemble and suitable for mass production and inexpensive. Becomes

According to the invention described in the item (7), the conditional expression [6] is a condition for improving the balance between the total lens length and various lens aberrations such as spherical aberration. If the lower limit of conditional expression [6] is exceeded, the total lens length will be long. If the upper limit of conditional expression [6] is exceeded, it becomes difficult to satisfactorily correct spherical aberration, curvature of field, and the like. When all the lenses are plastic lenses, if the lower limit of conditional expression [6] is exceeded, the focus change due to the temperature change becomes large.
Range of the conditional expression [6] is _{-0.7 <f / f 1 <-0.4} , preferably _{-0.6 <f / f 1 <-0.45} .

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the taking lens according to the present invention will be described below.
The symbols used in the examples are as follows.

F: focal length of the entire system f ₁ : focal length of the first lens group f ₃ : focal length of the third lens group F: F number ω: half angle of view r: radius of curvature of the refraction surface d: refraction surface N _d : refractive index of lens material at d-line ν _d : Abbe number of lens material ν ₂ : Abbe number of second lens ν ₃ : Abbe number of third lens t ₁ : on axis of first lens Thickness r ₁ : radius of curvature of the first surface r ₂ : radius of curvature of the second surface The shape of the aspherical surface used in the present invention is such that the coordinates take the X axis in the optical axis direction and the height in the direction perpendicular to the optical axis. When Y is set, it is represented by the equation (1).

[0023]

(Equation 1)

In equation (1), Κ is the conic constant of the aspheric surface, A ₄ ,
A ₆ , A ₈ , and A ₁₀ indicate aspheric coefficients.

(Example 1) FIG. 1 shows a sectional view of a lens of Example 1. In FIG. 1, the glass block closest to the image formation means a low-pass filter, a cover glass of a CCD, and the like. FIG. 2 shows a lens aberration diagram of lateral aberration (a) and spherical aberration, astigmatism and distortion (b). The lateral chromatic aberration is the difference between the d-line and the g-line of the lateral aberration. On-axis chromatic aberration is the difference between spherical aberration d-line and g-line. Tables 1 and 2 show lens data.

[0026]

[Table 1]

[0027]

[Table 2]

The first lens is made of polycarbonate plastic and has both surfaces aspherical. The second lens is made of acrylic plastic and has both surfaces aspherical. Further, the third lens is made of polycarbonate plastic and both surfaces are aspherical. The aperture stop is arranged between the first lens and the second lens. The focal length f is 6.02 mm, the image height is 4 mm,
The half angle of view can be used up to 32.5 °.

Due to the appropriate distribution of the refractive power of the lenses, the temperature change should be about 30 ° and the change in the lens back focus should be about 0.08 mm even though all the lenses are made of plastic. Was completed.

(Embodiment 2) The lens configuration of Embodiment 2 is the same as that of Embodiment 1 shown in FIG. FIG. 3 shows lateral aberration (a) and lens aberration diagrams of spherical aberration, astigmatism, and distortion (b). Tables 3 and 4 show lens data.

[0031]

[Table 3]

[0032]

[Table 4]

The aperture stop is disposed between the first lens and the second lens, has a focal length f of 6.0 mm and an image height of 4 m.
m, and the half angle of view can be used up to 32.5 °.

(Embodiment 3) The lens configuration of Embodiment 3 is the same as that of Embodiment 1 shown in FIG. Lateral aberration (a) and spherical aberration,
FIG. 4 shows a lens aberration diagram of astigmatism and distortion (b). Tables 5 and 6 show lens data.

[0035]

[Table 5]

[0036]

[Table 6]

The first lens uses an acrylic plastic. The aperture stop is arranged between the first lens and the second lens. The focal length f is 6.04 mm, the image height is 4 mm, and the half angle of view can be used up to 32.5 °.

Next, Table 7 shows the values of the conditional expressions of the above-described first to third embodiments.

[0039]

[Table 7]

As shown in Table 7, each embodiment satisfies conditional expressions [1] to [6].

[0041]

According to the above configuration, the following effects can be obtained. It is possible to provide a photographing lens suitable for use with an imaging device, which has three lenses, is capable of satisfactorily correcting various aberrations including chromatic aberration of magnification, and can use a half angle of view up to about 32.5 °.

[Brief description of the drawings]

FIG. 1 is a sectional view of a lens according to a first embodiment.

FIG. 2 is a diagram showing lens aberrations of Example 1 including lateral aberration (a), spherical aberration, astigmatism, and distortion (b).

3A and 3B are lens aberration diagrams of lateral aberration (a) and spherical aberration, astigmatism, and distortion (b) of Example 2.

FIG. 4 is a diagram illustrating lens aberrations of lateral aberration (a) and spherical aberration, astigmatism, and distortion (b) in Example 3.

Claims (7)

[Claims] 1. A negative meniscus first lens having a convex surface facing the object side, a positive second lens, and a negative meniscus third lens having a concave surface facing the image side, in order from the object side. A photographing lens characterized by being constituted. 2. An Abbe number of the second lens is ν ₂ ,
When the Abbe number of the third lens and [nu _3, the imaging lens according to claim 1, characterized in that the following condition is satisfied. 50 <ν ₂ <70 20 <ν ₃ <35 Wherein the focal length of the phototaking lens system is f, the focal length of the third lens and the f _3, according to claim 1 or 2, characterized by satisfying the following condition
The taking lens according to. −0.5 <f / f ₃ <−0.1 Wherein the focal length of the first lens is f _1,
When the axial thickness of the first lens was t _1, the imaging lens according to claim 1, 2 or 3, characterized by satisfying the following conditional expression. 0.45 <t ₁ / f ₁ <0.8 5. When the radius of curvature of the object side surface of the first lens is r ₁ and the radius of curvature of the image side surface of the first lens is r ₂ , the following conditional expression is satisfied. Item 5. The photographic lens according to item 1, 2, 3, or 4. −0.6 <(r ₂ −r ₁ ) / (r ₂ + r ₁ ) <− 0.3 6. The first lens, the second lens, and the third lens.
The photographing lens according to claim 1, wherein all the lenses are formed of plastic lenses. 7. The focal length of the phototaking lens system is f, the focal length of the first lens and the f _1, any of claims 1 to 6, characterized by satisfying the following condition 2. The photographing lens according to claim 1. −0.7 <f / f ₁ <−0.4