JP2002228925A - Retrofocus wide-angle lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide excellent telecentricity and back focus sufficient but not too long, to completely obtain a wide angle and to realize high image forming performance and the reduction of cost. SOLUTION: This restrofocus wide-angle lens is composed of five lenses as a whole, that is, a 1st lens having negative refractive power, a 2nd lens being a biconvex lens, a 3rd lens being a biconcave lens, a 4th lens having positive refractive power, and a 5th lens being a biconvex lens in order from an object side, and the respective lenses satisfy following conditions. 0.9<d2/d4<1.5, 0.8<bf/f<1.3, 0.8<bf/ls<1.1 Provided that d2 means a distance between the 1st and the 2nd lenses, d4 means a distance between the 2nd and the 3rd lenses, (bf) means air-converted length to an image surface from the image surface side of the 5th lens, (f) means the focal distance of the lens entire system and (ls) means the diagonal length of the surface size of the image surface.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a retrofocus type wide-angle lens used for a video camera, a digital camera and the like.

[0002]

2. Description of the Related Art In recent years, there have been developed many digital cameras and video cameras which can easily take and view images as compared with silver halide cameras. In addition, digital cameras are rapidly spreading as image input devices such as personal computers.

As a lens configuration used for a digital camera or a video camera, a retrofocus wide-angle lens used for a silver halide single-lens reflex camera is often employed.

[0004] Further, the digital camera uses an image pickup device (CCD or the like) having a smaller image pickup screen size than a silver halide camera. Required. Furthermore, in order to improve the performance of the camera, the angle of a light beam incident on an image pickup device (such as a CCD) provided on the image plane should be as perpendicular to the image plane formed by the image pickup device as possible. Is desirable.

[0005] This is because if the incident angle greatly deviates from the angle perpendicular to the image plane, color unevenness will be caused. As described above, the terms exit pupil position and telecentricity are used as values representing the angle of incidence of light rays on the periphery of the image sensor. It is good.

In a retrofocus wide-angle lens used in a digital camera, a filter such as a low-pass filter or a color filter is often arranged between a photographing lens and an image sensor. It is necessary to set the back focus of the lens so that a space for disposing the filter can be secured.

However, if the back focus is set too long to secure the space for the filter,
The retrofocus wide-angle lens becomes too large, and the entire camera becomes large even though the imaging screen size can be reduced.

Therefore, in order to reduce the size of the retrofocus wide-angle lens, for example, as disclosed in Japanese Patent Publication No. 54-9888, a negative lens, a positive lens, a negative lens, a negative lens, having a small number of lenses. It is conceivable to make the entire lens system as small as possible by using a retrofocus wide-angle lens composed of five positive lenses, but the lens design conditions described in the above publication are designed for silver halide cameras. Therefore, it cannot be used for a digital camera having an image plane formed of an image sensor because of its poor telecentric characteristics.

[0009]

Recently, Japanese Patent Application Laid-Open No. 10-213742 and Japanese Patent Application Laid-Open No.
As disclosed in Japanese Patent Application Laid-Open No. 6-206, there has been proposed a lens configuration of a retrofocus wide-angle lens composed of five lenses for a digital camera with improved telecentric characteristics. However, under the lens design conditions in the above two publications, it cannot be said that the wide-angle lens is sufficiently widened, and sufficient imaging performance is obtained for an image sensor having a large number of pixels due to the compactness. There is a problem that it cannot be said that it can be said.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has a sufficient and not too large back focus while having good telecentricity. It is an object of the present invention to provide a retrofocus wide-angle lens that achieves high imaging performance and low cost.

[0011]

In order to achieve the above object, the retrofocus wide-angle lens according to the first aspect of the present invention has a negative refractive power of a meniscus shape having a convex surface facing the object side in order from the object side. Retrofocus consisting of a total of five lenses: a first lens having a power, a second lens having a biconvex lens, a third lens having a biconcave lens, a fourth lens having a positive refractive power, and a fifth lens having a biconvex lens. Type wide-angle lens, wherein each lens satisfies the following conditions.

0.9 <d2 / d4 <1.5 (1) 0.8 <bf / f <1.3 (2) 0.8 <bf / ls <1.1 (3) where d2: Air space between the first lens and the second lens, d4: air space between the second lens and the third lens, bf: length in air conversion from the image plane side to the image plane of the fifth lens, f: Focal length of the whole lens system, ls: diagonal length of surface size of image plane.

The retrofocus wide-angle lens according to the second aspect of the present invention is the retrofocus wide-angle lens according to the first aspect, wherein the second lens and the fifth lens satisfy the following conditions. It is characterized by.

0.7 <| r3 / r4 | <1.5 (4) 0.5 <| r9 / r10 | <1.5 (5) where r3 is the radius of curvature of the object side surface of the second lens, r4 : Radius of curvature of the image side surface of the second lens, r9: radius of curvature of the object side surface of the fifth lens, and r10: radius of curvature of the image side surface of the fifth lens.

Further, the invention of the retrofocus wide-angle lens according to the third aspect is the invention of the retrofocus wide-angle lens according to the first or second aspect, wherein the second lens and the third lens are disposed between the second lens and the third lens. And a stop and a light shielding plate, and the second lens and the fifth lens satisfy the following conditions.

0.9 <| r3 / r4 | <1.1 (6) 0.9 <| r9 / r10 | <1.1 (7) where r3 is the radius of curvature of the object side surface of the second lens, r4 : Radius of curvature of the image side surface of the second lens, r9: radius of curvature of the object side surface of the fifth lens, and r10: radius of curvature of the image side surface of the fifth lens.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of a retrofocus type wide-angle lens according to the present invention will be described with reference to the drawings. Although the retrofocus type wide-angle lens of the present invention is shown in this embodiment as applied to a digital camera, it is not limited to a digital camera, but can be applied to an imaging optical system of another imaging device such as a video camera. Also, it can be applied to a projection device such as a liquid crystal projector.

FIGS. 1, 3, and 5 show lens configuration diagrams corresponding to Embodiments 1 to 3, which are numerical embodiments described later, and have a meniscus shape with a convex surface facing the object side from the object side. The first lens L1 having a negative refractive power, the second lens L2 of a biconvex lens, the third lens L3 of a biconcave lens, the fourth lens L4 having a positive refractive power, and the fifth lens L5 of a biconvex lens have a total of 5 This is a retrofocus wide-angle lens composed of two lenses. Note that the second lens L
A filter unit in which a stop F and a light blocking plate B are disposed between the second lens L3 and the third lens L3, and a filter such as a low-pass filter or a color filter is stored between the fifth lens L5 and the image plane C. A is provided.

According to the present invention, in the retrofocus wide-angle lens having the above configuration, d2 is the air gap between the first lens L1 and the second lens L2, and d4 is the second lens L2.
The air gap between b and the third lens L3, bf, is set to the fifth lens L
5, the length in air conversion from the image plane side to the image plane C (in a state where no filter is provided), f is the focal length of the entire lens system, and ls is the diagonal length of the surface size of the image plane C. 0.9 <d2 / d4,1.5 (1) that satisfies conditional expression (1) 0.8 <bf / f <1.3 (2) that satisfies conditional expression (2) Conditional expression (3) 0.8 <bf / ls <1.1 (3) The lens arrangement is designed so as to satisfy all of the conditions.

Further, according to the present invention, when designing a lens within the range of the conditional expressions (1) to (3), r3 is the radius of curvature of the object side surface of the second lens L2, and r4 is the second lens. When the radius of curvature of the image side surface of L2, r9 is the radius of curvature of the object side surface of the fifth lens L5, and r10 is the radius of curvature of the image side surface of the fifth lens L5, conditional expression (4) is satisfied. 0.7 <| r3 /R4|<1.5 (4) The second lens L2 and the fifth lens L satisfy the following condition: 0.5 <| r9 / r10 | <1.5 (5)
5 is preferably designed.

Conditional expression (1) is a conditional expression showing the relationship between the air gap between the first lens L1 and the second lens L2 and the air gap between the second lens L2 and the third lens L3. If the lower limit of the range of the conditional expression (1) is exceeded, it becomes difficult to secure a sufficient back focus. If the back focus is forcibly lengthened, the refractive power of the first lens L1 must be increased, and aberration correction is performed. Becomes difficult. On the other hand, when the value exceeds the upper limit of the range of the conditional expression (1), the total length of the lens from the lens vertex on the object side surface of the first lens L1 to the image plane C formed by the image sensor becomes long, and a compact lens configuration is obtained. Can not do.

Conditional expression (2) is a conditional expression relating to the relationship between the back focus and the focal length of the entire lens system. If the lower limit of the range of conditional expression (2) is exceeded, the back focus becomes too short, and a low-pass filter or the like is used. It is not possible to secure enough space for filters. On the other hand, when the value exceeds the upper limit of the range of the conditional expression (2), the total length of the retrofocus wide-angle lens increases, and the negative refractive power of the first lens L1 has to be increased. Become.

Conditional expression (3) is a conditional expression relating to the relationship between the back focus and the diagonal length of the surface size of the image plane C. If the lower limit of the range of the conditional expression (3) is exceeded, it will be difficult to secure a sufficient back focus or the angle of view will be too large, and it will be difficult to secure the performance in the peripheral portion of the screen.
On the other hand, when the value exceeds the upper limit of the range of the conditional expression (3), the back focus becomes long, and a compact lens configuration cannot be obtained.

The conditional expression (4) is a conditional expression relating to the relationship between the radius of curvature of the front surface (object side) S3 and the rear surface (image surface side) S4 of the second lens L2. When the value goes below the lower limit of the range of the conditional expression (4), the refraction effect of the front surface S3 of the second lens L2 becomes too large, the spherical aberration is undercorrected, and the negative displacement of the distortion remains largely left. On the other hand, when the value exceeds the upper limit of the range of the conditional expression (4), the curvature of field is negatively displaced, so that it becomes difficult to correct the distortion and the distortion remains positively displaced.

Conditional expression (5) is a conditional expression relating to the relationship between the radius of curvature of the front surface (object side) S9 and the rear surface (image surface side) S10 of the fifth lens L5. If the lower limit of the range of the conditional expression (5) is exceeded, the positive refractive power moves to the object side, so that the negative displacement of the distortion increases and the total length of the retrofocus wide-angle lens increases. On the other hand, when the value exceeds the upper limit of the range of the conditional expression (5), the spherical aberration is negatively displaced, and the distortion is positively displaced.

Further, according to the present invention, the second lens L2 and the third lens
When the stop F and the light shielding plate B are disposed between the lens L3 and the lens unit L3, the range of the conditional expression (4) is further changed to the conditional expression (6). 0.9 <| r3 / r4 | <1.1 ( It is preferable to limit the conditional expression (5) to the following expression (6): 0.9 <| r9 / r10 | <1.1 (7) that satisfies the conditional expression (7).

In the case where the stop F and the light shielding plate B are provided between the second lens L2 and the third lens L3, a sufficient backing is achieved by limiting to conditional expressions (6) and (7). It is possible to reduce the cost of a retrofocus wide-angle lens while obtaining focus.

Conditional expression (6) defines the front surface S of the second lens L2.
3 is a conditional expression regarding the relationship between the radius of curvature of the rear surface S4 and the range of the conditional expression (4) is further limited. Examples 2 and 3 described below satisfy the condition of the conditional expression (6). However, Example 1 is out of this range.

The present invention, as shown in particular in Example 3,
It is preferable that | r3 / r4 | = 1.0 as a design value. By setting this value, the radii of curvature of both surfaces of the second lens L2 become the same diameter, the lens configuration can be simplified, the manufacture of the lens can be facilitated, and the second lens L2
The mounting direction of the second lens L2 can be improved, and the cost of the digital camera to which the retrofocus wide-angle lens is mounted can be further reduced. .

Conditional expression (7) defines the front surface S of the fifth lens L5.
9 is a conditional expression relating to the relationship between the radius of curvature of the rear surface S10 and the conditional expression (5), and the conditional expression (5) is further limited.
Example 1 is out of range.

The present invention, particularly as shown in Example 3,
It is preferable that | r9 / r10 | = 1.0 as a design value. By setting this value, the radii of curvature of both surfaces of the fifth lens L5 become the same diameter, the lens configuration can be simplified, and the manufacture of the lens can be facilitated.
Since the mounting direction of the fifth lens L5 can be improved without causing an error in the mounting direction of the lens surface, the cost of the digital camera to which the retrofocus wide-angle lens is mounted can be further reduced. .

Further, in the present embodiment, the lens surfaces of all the lenses are constituted by spherical surfaces. However, the performance can be further improved by making some lenses aspherical.

Hereinafter, in the lens configuration according to the present embodiment, numerical examples performed by changing numerical values required for lens design so as to fall within the range of the conditional expressions (1) to (5) will be described below. Example 1 to Example 3 are shown.

In each embodiment, f represents the focal length of the entire lens system, Fno represents the F number, 2w represents the angle of view,
S1 to S10 represent the surface number of the lens or the filter unit, r is the radius of curvature, d is the lens core thickness or air gap, Nd is the refractive index of the d-line, νd is the Abbe number, bf
Denotes a length in air conversion from the image plane side of the fifth lens to the image plane, and ls denotes a diagonal length of the image screen size. [Example 1] f = 10.0, Fno = 2.8, 2w = 6
With a lens configuration of 2.3 degrees,

[0035]

[Table 1] , Bs = 9.48 and ls = 11.74.

FIG. 1 shows a lens configuration diagram in the first embodiment.
An aberration curve diagram is shown in FIG. [Example 2] f = 10.0, Fno = 2.8, 2w = 6
2.2 degree lens configuration,

[0037]

[Table 2] Then, bf = 10.88 and ls = 11.74.

FIG. 3 is a diagram showing the lens arrangement in the second embodiment.
An aberration curve diagram is shown in FIG. Example 3 f = 10.0, Fno = 2.8, 2w = 6
With a lens configuration of 2.3 degrees,

[0039]

[Table 3] In this case, bf = 10.41 and ls = 11.74.

FIG. 5 is a diagram showing a lens configuration according to the third embodiment.
FIG. 6 shows an aberration curve diagram.

Next, Table 4 below shows values with respect to the respective conditional expressions in the first, second, and third embodiments.

[0042]

[Table 4] As is clear from Table 4, from the conditional expression (1) to the conditional expression (5)
Is within the range of the conditional expressions in all of the first, second, and third embodiments.

As for conditional expressions (6) and (7), only the second and third embodiments fall within the range of the conditional expressions, and the first embodiment falls outside the range.

In FIGS. 2, 4 and 6, the solid line (d) shows the aberration for the d-line and the dotted line (g) shows the aberration for the g-line. Also, FIG.
In the astigmatism diagrams in FIGS. 4 and 6, the solid line (D
S) indicates astigmatism with respect to the d-line on the sagittal surface, and dotted line (DT) indicates astigmatism with respect to the d-line on the meridional surface.

[0045]

As described above, according to the present invention, it is possible to obtain a sufficient back focus in which a filter can be arranged while maintaining good telecentricity, and it is possible to reduce the size without making the back focus too large. As a result, it is possible to provide a retrofocus wide-angle lens capable of realizing sufficient wide-angle, high imaging performance, and low cost.

[Brief description of the drawings]

FIG. 1 is a lens configuration diagram of a retrofocus wide-angle lens according to a first embodiment of the present invention.

FIG. 2 is an aberration diagram illustrating aberration performance of the retrofocus wide-angle lens according to the first embodiment.

FIG. 3 is a lens configuration diagram of a retrofocus wide-angle lens according to a second embodiment of the present invention.

FIG. 4 is an aberration diagram showing aberration performance of the retrofocus wide-angle lens according to the second embodiment.

FIG. 5 is a diagram illustrating a configuration of a retrofocus wide-angle lens according to a third exemplary embodiment of the present invention.

FIG. 6 is an aberration diagram showing aberration performance of the retrofocus wide-angle lens according to the third embodiment.

[Explanation of symbols]

 L1 First lens L2 Second lens L3 Third lens L4 Fourth lens L5 Fifth lens F Aperture B Light shield plate

Claims (3)

[Claims] 1. A first lens having a negative refractive power of a meniscus shape having a convex surface facing the object side, a second lens of a biconvex lens, a third lens of a biconcave lens, and a positive refractive power in order from the object side. This is a retrofocus wide-angle lens composed of five lenses as a whole of a fourth lens and a fifth lens of a biconvex lens, and each lens satisfies the following conditions. 0.9 <d2 / d4 <1.5 (1) 0.8 <bf / f <1.3 (2) 0.8 <bf / ls <1.1 (3) where d2 is the first lens Air spacing between the second lens, d4: air spacing between the second and third lenses, bf: length in air conversion from the image plane side to the image plane of the fifth lens, f: whole lens system Ls: diagonal length of the surface size of the image plane. 2. The retrofocus wide-angle lens according to claim 1, wherein the second lens and the fifth lens satisfy the following conditions. 0.7 <| r3 / r4 | <1.5 (4) 0.5 <| r9 / r10 | <1.5 (5) where r3: radius of curvature of the object side surface of the second lens, r4: second R9: radius of curvature of the object side surface of the fifth lens, r10: radius of curvature of the image side surface of the fifth lens. 3. The retrofocus wide-angle lens according to claim 1, wherein a stop and a light-shielding plate are provided between the second lens and the third lens, and the second lens and the fifth lens are connected to each other. The lens is characterized by satisfying the following conditions. 0.9 <| r3 / r4 | <1.1 (6) 0.9 <| r9 / r10 | <1.1 (7) where r3: radius of curvature of the object side surface of the second lens, r4: second R9: radius of curvature of the object side surface of the fifth lens, r10: radius of curvature of the image side surface of the fifth lens.