1. Field of the Invention
The present invention relates to an image pickup lens for forming an optical image of a subject on an image sensor, such as a CCD (charge coupled device), a CMOS (complementary metal oxide semiconductor), or the like, and an image pickup apparatus having the image pickup lens mounted thereon to perform imaging, such as a digital still camera or the like. The invention also relates to a portable terminal device, such as a camera-equipped cell phone, a personal digital assistance (PDA), or the like.
2. Description of the Related Art
Recently, along with the spread of personal computers to homes and the like, digital still cameras capable of inputting image information obtained by imaging a landscape, a person, or the like to a personal computer have been spreading rapidly. In addition, more and more cell phones have built-in camera modules for image input. Such devices with imaging capabilities employ image sensors such as CCDs, CMOSs, and the like. In recent years, these types of image sensors have been downsized greatly and, consequently, imaging devices as a whole and image pickup lenses to be mounted on the devices have also been required to have more compact sizes. At the same time, the pixel count of image sensors has been increasing, thereby causing a growing demand for improvement of image pickup lenses in resolution and performance.
Image pickup lenses, each formed of three or four lenses, are disclosed in U.S. Pat. Nos. 6,476,982, 7,466,497, 7,715,119, 7,453,654, and 7,633,690, and U.S. Patent Application Publication No. 2009015944, as well as in Japanese Unexamined Patent Publication Nos. 2007-017984, and 2009-020182. As described in these documents, for a four-element image pickup lens, in particular, a configuration of positive, negative, positive, and positive power arrangement from the object side or a configuration of positive, negative, positive, and negative power arrangement from the object side is known. In the case of such four-element image pickup lenses, the object side surface of the most image side lens often has a convex shape in a paraxial region (adjacent to the optical axis). In the mean time, Japanese Unexamined Patent Publication No. 2007-017984 discloses, in Examples 5 and 9, a configuration of positive, negative, positive, and negative power arrangement with the object side surface of the most image side lens having a concave shape adjacent to the optical axis of the lens.
As described above, downsizing and pixel count increase have been in progress for recent image sensors. For image pickup lenses of portable camera modules, in particular, cost reduction and compactness have been the major demands, but as the pixel count of image sensors even for portable camera modules tends to be increased, a demand for performance improvement is also growing. Consequently, development of wide variety of lenses comprehensively taking into account the cost, performance, and compactness is anticipated, and from the viewpoint of performance, development of inexpensive and high performance image pickup lenses with a perspective of possible application to digital cameras is anticipated. The lenses described in the aforementioned patent documents have a shortcoming, for example, in compatibility between image forming performance and compactness. Japanese Unexamined Patent Publication No. 2007-017984 discloses various types of four-element image pickup lenses, but it is hard to say that optimization conditions have been studied for each configuration example. Note that the present invention is a utilization invention of the invention described in Japanese Unexamined Patent Publication No. 2009-020182. As a result of further consideration of the balance between downsizing and performance for the image pickup lens described in Japanese Unexamined Patent Publication No. 2009-020182, the object of the present invention has been solved.
The present invention has been developed in view of the problems described above, and it is an object of the present invention to provide an image pickup lens reduced in overall length with enhanced image forming performance.
An image pickup lens of the present invention includes the following from an object side in the order listed below: a first lens having a positive refractive power; a second lens having a negative refractive power; a third lens having a positive refractive power; and a fourth lens having an object side surface which is concave or flat adjacent to an optical axis of the lens and a negative refractive power adjacent to the optical axis. The image pickup lens satisfies Conditional Expression (1) given below in which R3 is a paraxial radius of curvature of an object side surface of the second lens, and R4 is a paraxial radius of curvature of an image side surface of the second lens.
0.3<|(R4+R3)/(R4−R3)|<1.5 (1)
The image pickup lens of the present invention may provide advantageous effects for total length reduction and high image forming performance by optimizing the structure of each lens in a lens configuration of four lenses in total. In particular, the image pickup lens satisfies Conditional Expression (1) whereby the structure of the second lens is optimized. The image pickup lens of the present invention is advantageously configured for reducing a total length and obtaining high image forming performance even though the object side surface of the most image side lens (fourth lens) has a flat or concave shape adjacent to the optical axis. Then, by employing the following preferable configurations as appropriate, the total length reduction and performance enhancement may be facilitated.
0.3<|f4/f|<0.80 (2)
0.4<f1/f<1.1 (3)
0.2<f3/f<1.6 (4)
0.5<|f2/f|<2.0 (5)
20<ν1−ν2 (6)
where, f is an overall focal length, f1 is a focal length of the first lens, f2 is a focal length of the second lens, f3 is a focal length of the third lens, and f4 is a focal length of the fourth lens, ν1 is an Abbe number of the first lens with respect to d-line, and ν2 is an Abbe number of the second lens with respect to d-line.
Preferably, the image pickup lens of the present invention includes an aperture disposed on the object side of a surface apex position of an image side surface of the first lens on the optical axis.
Preferably, in the image pickup lens of the present invention, each of the first, second, third, and fourth lenses has an aspherical shape on each side.
In the image pickup lens of the present invention, it is particularly preferable that the image side surface of the fourth lens has a concave shape adjacent to the optical axis and a region in which the negative refractive power becomes weak toward the periphery in comparison with a region adjacent to the optical axis.
An image pickup apparatus of the present invention is an apparatus, including the image pickup lens of the present invention and an image sensor for outputting an imaging signal according to an optical image formed by the image pickup lens.
A portable terminal device of the present invention is a device, including the image pickup apparatus of the present invention and a display unit for displaying an image taken by the image pickup apparatus.
The image pickup apparatus or the portable terminal device of the present invention may obtain a high resolution imaging signal based on a high resolution optical image obtained by the image pickup lens of the present invention.
The image pickup lens of the present invention may realize total length reduction and high image forming performance by optimizing the shape and the like of each lens in a lens configuration of four lenses in total.
The image pickup apparatus or the portable terminal device of the present invention outputs an imaging signal according to an optical image formed by the image pickup lens of the present invention having high image forming performance, so that the apparatus or the device may obtain a high resolution image.
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
The image pickup lens according to the present embodiment includes from the object side in the order of aperture St, first lens G1, second lens G2, third lens G3, and fourth lens G4 along optical axis Z1.
Aperture St is an optical aperture stop which is preferable to be disposed on the object side of the surface apex of the image side surface of lens G1 on optical axis Z1, thereby being disposed on the most object side of the lens system. Here, the term “most object side” as used herein includes not only the case in which aperture St is disposed at the surface apex position of the object side surface of first lens G1 as, for example, in the configuration shown in
Image plane Simg includes an image sensor, such as a CCD or the like. Various types of optical members CG may be disposed between fourth lens G4 and the image sensor according to the camera side structure on which the lens is mounted. For example, flat plate optical members, such as a cover glass for protecting the image plane and an infrared cut filter, may be disposed. In this case, for example, a flat plate cover glass with a coating having a filter effect, such as infrared cut filter, ND filter, or the like, applied thereon may be used as optical member CG. In the image pickup lens, all of lenses G1 to G4 or at least one lens surface may have a coating having a filter effect, such as infrared cut filter, ND filter, or the like, or an anti-reflection coating.
First lens G1 has a positive refractive power. Preferably, first lens G1 has a biconvex shape adjacent to the optical axis.
Second lens G2 has a negative refractive power. Second lens G2 may be a lens having, adjacent to the optical axis, a biconcave shape (e.g., example configuration in
Third lens G3 has an image side surface which is convex adjacent to the optical axis and a positive refractive power. For example, an object side surface of third lens G3 is concave adjacent to the optical axis.
Fourth lens G4 has an object side surface which is concave (e.g., example configures shown in
Preferably, in each of first lens G1, second lens G2, third lens G3, and fourth lens G4, at least one surface is aspherical. The image side surface of fourth lens G4, in particular, has a concave shape adjacent to the optical axis and a region in which the negative refractive power becomes weak toward the periphery in comparison with a region adjacent to the optical axis. Further, it is preferable that the image side surface of fourth lens G4 has an aspherical shape having an inflexion point within an effective diameter. Still further, it is preferable that the image side surface of fourth lens G4 has an aspherical shape having a pole at a position other than the center of optical axis within the effective diameter. More specifically, it is preferable that, for example, the image side surface of fourth lens G4 is an aspherical surface having a concave shape toward the image side adjacent to the optical axis and a convex shape toward the image side in a peripheral region.
Here, if an aspherical shape is to be employed, second lens G2, third lens G3, and fourth lens G4 tend to have a complicated shape with a large size in comparison with first lens G1. Therefore, it is preferable that each of second lens G2, third lens G3, and fourth lens G4 is made of a resin material in view of workability and cost. Where manufacturing cost is important, it is preferable that first lens G1 is also made of a resin material, but first lens G1 may be made of a glass material in order to improve performance.
Preferably, the image pickup lens satisfies Conditional Expression (1) given below, in which R3 is a paraxial radius of curvature of the object side surface of second lens G2 and R4 is a paraxial radius of curvature of the image side surface of second lens G2.
0.3<|(R4+R3)/(R4−R3)|<1.5 (1)
Further, it is preferable that the image pickup lens selectively satisfies the following conditions as appropriate, in which f is an overall focal length, f1 is a focal length of first lens G1, f2 is a focal length of second lens G2, f3 is a focal length of third lens G3, and f4 is a focal length of fourth lens G4, ν1 is an Abbe number of first lens G1 with respect to d-line, and ν2 is an Abbe number of second lens G2 with respect to d-line.
0.3<|f4/f|<0.80 (2)
0.4<f1/f<1.1 (3)
0.2<f3/f<1.6 (4)
0.5<|f2/f|<2.0 (5)
20<ν1−ν2 (6)
Camera unit 1 includes, for example, a camera module shown in
In camera unit 1, an optical image formed by image pickup lens 20 is converted to an electrical imaging signal by the image sensor and the imaging signal is outputted to the signal processing circuit provided on the apparatus body. The use of the image pickup lens of the present embodiment as image pickup lens 20 of such camera-equipped cell phone allows a sufficiently aberration corrected high resolution imaging signal to be obtained. Cell phone body may generate a high resolution image based on the imaging signal.
The image pickup lens of the present embodiment may be applied to various types of image pickup apparatuses and portable terminal devices that employ image sensors, such as CCD, CMOS, and the like. The image pickup apparatus or portable terminal device of the present embodiment is not limited to a camera-equipped cell phone and it may be, for example, a digital still camera, a PDA, or the like.
An operation and advantageous effects of the image pickup lens configured in the aforementioned manner will now be described. The image pickup lens according to the present embodiment may provide advantageous effects for total length reduction and high image forming performance by arranging the powers of the lenses from the object side in the order of positive, negative, positive, and negative, appropriately setting a surface shape of each lens, and satisfying a predetermined conditional expression in a lens configuration of four lenses in total. In particular, the image pickup lens is advantageously configured for reducing the total length and obtaining high image forming performance even though the object side surface of the most image side lens (fourth lens G4) has a flat or concave shape adjacent to the optical axis. Further, the negative refractive power of fourth lens G4 provides an advantageous effect of ensuring a sufficient back focus. If positive refractive power of fourth lens G4 is too strong, it is difficult to ensure a sufficient back focus.
Further, in the image pickup lens, the use of an aspherical surface for at least one surface of each of first lens G1, second lens G2, third lens G3, and fourth lens G4 provides an advantageous effect for maintaining aberration performance. In fourth lens G4, in particular, the light flux is separated with respect to each angle of view in comparison with first lens G1, second lens G2, and third lens G3. By making the image side surface of fourth lens G4, which is the lens surface closest to the image sensor, concave toward the image side adjacent to the optical axis and convex toward image side in a peripheral portion, aberration with respect to each angle of view is corrected appropriately and the incident angle of the light flux on the image sensor is controlled below a predetermined angle. This may reduce the unevenness in light amount over the entire region of the image plane and provide an advantageous effect for correcting curvature of field, distortion, and the like.
Generally, it is preferable that image pickup lens systems have telecentricity, that is, it is preferable that the incident angle of the chief ray becomes substantially parallel to the optical axis (incident angle on the image plane becomes close to zero with respect to normal line). In order to ensure the telecentricity, it is preferable that aperture St is disposed at a position as close to the object side as possible. On the other hand, if aperture St is disposed at a position further away from the object side surface of first lens G1 in the object side direction, the distance between aperture St and the object side surface of first lens G1 is added to the optical path, which is disadvantageous for downsizing the overall configuration. Consequently, telecentricity may be ensured while reducing the total length by disposing aperture St at a position on optical axis Z1 corresponding to the surface apex position of the object side surface of first lens G1 or a position on optical axis Z1 between the surface apex position of the object side surface of first lens G1 and the surface apex position of the image side surface of first lens G1. Where the telecentricity is more important, aperture St may be disposed at a position on optical axis Z1 between the surface apex position of the object side surface of first lens G1 and edge position E (
Conditional Expression (1) given above is related to the shape and refractive power of second lens G2. If |(R4+R3)/(R4−R3)| exceeds the upper limit of Conditional Expression (1), the refractive power of second lens becomes too weak, causing a disadvantageous effect for the total length reduction. While if |(R4+R3)/(R4−R3)| exceeds the lower limit of Conditional Expression (1), the refractive power of second lens becomes too strong, causing difficulty in aberration correction. In order to reduce the total length and to obtain high image forming performance, it is preferable that the numerical range of Conditional Expression (1) is as follows.
0.35<|(R4+R3)/(R4−R3)|<1.45 (1-1)
In order to obtain still better performance, it is preferable that |(R4+R3)/(R4−R3)| is in the following range.
0.6<|(R4+R3)/(R4−R3)|<1.1 (1-2)
Conditional Expression (2) given above is related to focal length f4 of fourth lens G4, and if |f4/f| exceeds upper limit of Conditional Expression (2) and the refractive power of fourth lens G4 becomes weak, it is difficult to reduce the total length. On the other hand, if |f4/f| exceeds lower limit of Conditional Expression (2), the refractive power of fourth lens G4 becomes strong and it is necessary to cancel out the increased refractive power of fourth lens G4 by increasing the refractive power of third lens G3, thereby causing degradation in off-axis performance. In order to obtain better performance, it is preferable that the numerical range of Conditional Expression (2) is as follows.
0.35<|f4/f|<0.70 (2-1)
In order to obtain still better performance, it is preferable that |f4/f| is in the following range.
0.4<|f4/f|<0.70 (2-2)
Conditional Expression (3) given above is related to focal length f1 of first lens G1, and if f1/f falls below the numerical range, the refractive power of first lens G1 becomes too strong, causing increase in spherical aberration, and it is difficult to ensure sufficient back focus. On the other hand, if f1/f exceeds the numerical range, it is difficult to reduce the total length and to correct curvature of field, astigmatism, and the like. In order to obtain better performance, it is preferable that the numerical range of Conditional Expression (3) is as follows.
0.45<f1/f<1.0 (3-1)
In order to obtain still better performance, it is preferable that f1/f is in the following range.
0.5<f1/f<0.9 (3-2)
Conditional Expression (4) given above is related to focal length f3 of third lens G3, and if f3/f falls below the numerical range and the positive refractive power of third lens G3 becomes too strong, the performance is degraded in addition to difficulty to ensure back focus. On the other hand, if f3/f exceeds the numerical range, the positive refractive power of third lens G3 becomes too weak, causing difficulty in aberration correction. In order to obtain better performance, it is preferable that the numerical range of Conditional Expression (4) is as follows.
0.3<f3/f<1.5 (4-1)
In order to obtain still better performance, it is preferable that f3/f is in the following range.
0.35<f3/f<1.1 (4-2)
Conditional Expression (5) given above is related to focal length f2 of second lens G2, and if f2/f falls below the numerical range, the positive refractive power of second lens G2 becomes too strong, resulting in increased aberration. On the other hand, if f2/f exceeds the numerical range, the refractive power of second lens G2 becomes too weak, causing difficulty in correcting curvature of field, astigmatism, and the like. In order to obtain better performance, it is preferable that the numerical range of Conditional Expression (5) is as follows.
0.8<f2/f<1.9 (5-1)
In order to obtain still better performance, it is preferable that f2/f is in the following range.
0.9<f2/f<1.8 (5-2)
Conditional Expression (5) given above defines dispersions of first lens G1 and second lens G2 and if the numerical range is satisfied by the first lens G1 and second lens G2, on-axis chromatic aberration may be reduced. In order to obtain better performance, it is preferable that the numerical range of Conditional Expression (6) is as follows.
25<ν1−ν2<40 (6-1)
In order to obtain still better performance, it is preferable that ν1−ν2 is in the following range.
28<ν1−ν2<32 (6-2)
As described above, according to the image pickup lens of the present embodiment, the total length reduction and high image forming performance may be realized. Further, according to the image pickup apparatus or portable terminal device of the present embodiment, an imaging signal is outputted according to an optical image formed by the image pickup lens reduced in the total length and enhanced in image forming performance, so that downsizing of the apparatus or device as a whole may be realized. Further, a high resolution imaging signal may be obtained and a high resolution image may be obtained based on the imaging signal.
Specific Numerical Examples of the image pickup lens of the present invention will now be described. Hereinafter, a plurality of Numerical Examples is collectively described part by part.
[Table 1] and [Table 2] show specific lens data corresponding to the configuration of image pickup lens in
In the image pickup lens according to Example 1, each of first lens G1 to fourth lens G4 has an aspherical shape on each side. In the basic data of [Table 1], values of radii of curvature adjacent to the optical axis are shown as the radii of curvature of the aspherical surfaces.
[Table 2] shows aspherical surface data of the image pickup lens according to Example 1. In the values shown as aspherical surface data, the symbol “E” indicates that the numerical value that follows is power to base 10, and the value preceding the symbol is multiplied by the value represented by the exponential function to base 10. For example, 1.0E-02 refers to 1.0×10−2.
As for the aspherical surface data, values of each of coefficients Ai and K in Formula (A) given below which represents an aspherical surface shape. More specifically, Z represents a length of a perpendicular line (mm) drawn from a point on an aspherical surface at a height of h from the optical axis to the tangent plane (a plane perpendicular to the optical axis) to the apex of the aspherical surface.
Z=C·h
2/{1+(1−K·C2·h2)1/2}+ΣAi·hi (A)
where:
Z: depth of aspherical surface (mm)
H: distance (height) from optical axis to lens surface (mm)
K: eccentricity
C: paraxial curvature 1/R
(R: paraxial radius of curvature)
ΣAi·hi: sum of Ai·hi when i=3 to n (n: integer not less than 3)
Ai: ith order aspherical surface coefficient
In the Aspherical Surfaces of the Image Pickup Lens According to Example 1, aspherical surface coefficients An are indicated using A3 to A10 orders as effective based on Aspherical Surface Formula (A) given above.
As in Numerical Example 1 described above, specific lens data corresponding to the configuration of image pickup lens in FIG. 2 are shown in [Table 3] and [Table 4] as Numerical Example 2. Likewise, specific lens data corresponding to the configurations of image pickup lenses in
[Table 23] summarizes values related to each conditional expression for each Example. As shown in [Table 23], the value of each Example falls within the numerical range of each conditional expression.
The spherical aberration, astigmatism, and distortion of image pickup lens according to Example 1 are shown in
Likewise, the spherical aberration, astigmatism, and distortion of image pickup lens according to Example 2 are shown in
As is clear from the numerical data and aberration diagrams, the total length reduction and high image forming performance are realized in each Example.
It should be appreciated that the present invention is not limited to the embodiments and Examples described above, and various modifications and changes may be made. For example, values of the radius of curvature, surface separation, and refractive index of each lens element are not limited to those shown in each Numerical Example and may take other values.
Number | Date | Country | Kind |
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2010-101150 | Apr 2010 | JP | national |