Patent Grant
7924510
This application is based on and claims priority under 35 U.S.C §119 from Japanese Patent Application No. 2007-132334, filed on May 18, 2007, the entire disclosure of which is herein incorporated by reference.
1. Field of the Invention
The present invention relates to an imaging lens and an imaging apparatus equipped with the imaging lens, and relates more particularly to an imaging lens that is appropriate, for example, for mounting in an onboard camera, a portable terminal camera and a surveillance camera which employ an imaging device, such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor), and an imaging apparatus equipped with this imaging lens.
2. Description of Related Art
Recently, imaging devices such as CCD and CMOS have been very much downsized and improved in pixel density. Subsequently, the availability and use of more compact imaging devices have made possible great reductions in sizes of image apparatus main bodies, and this, in turn, has created a continuing demand for smaller, lighter imaging lens for smaller, lighter imaging apparatuses.
On the other hand, with respect to onboard camera and surveillance camera applications, there is a demand for compact, high-performance cameras that are highly resistant to the adverse effects of wide ranging, extreme weather variations, from bitterly cold winter weather in the open air to overheated summer automotive interiors in tropical areas. Furthermore, from the viewpoint of automotive use, the onboard cameras most in demand are those for which exposed lens portions are the smallest.
Imaging lenses in the related art are described, for example, in JP-A-55-45007 and JP-A-61-90115. The imaging lens disclosed in JP-A-55-45007 is a wide-angle retro-focus lens including six lenses, which is used for photography, and the imaging lens disclosed in JP-A-61-90115 is an objective lens for image formation, including five or six lenses, for which a wide range of chromatic aberration corrections has been performed.
However, the F values of the lens systems described in JP-A-55-45007 and JP-A-61-90115 are 3.5 and 3.76 to 4.5, respectively, which means dark optical systems. Since it is anticipated that onboard and surveillance cameras will be used at night, in that case, a bright optical system is required.
Furthermore, according to the imaging lenses described in JP-A-55-45007 and JP-A-61-90115, the back focal length ratios relative to the thicknesses of the lens systems in the optical axial direction (the length in the optical axial direction from the lens surface nearest to an object to the lens surface nearest to an image) are large, i.e., about 0.95 and 1 to 2, respectively. That is, the entire length for the optical systems in the related art, from the lens system to the imaging surface is long, and the recent demand for the downsizing of lens systems is not satisfied.
An object of an illustrative, non-limiting embodiment of the present invention is to provide a compact imaging lens, which performs a good optical performance and which has a small F value, and an imaging apparatus equipped with the imaging lens.
According to an aspect of the invention, there is provided an imaging lens including: in order from the object side:
a first lens group including a biconcave lens;
a second lens group including two lenses whose convex surfaces are arranged opposite to each other, the second lens group having a positive refractive power as a whole;
a third lens group including a biconcave lens; and
a fourth lens group including two lenses whose convex surfaces are arranged opposite to each other, the fourth lens group having a positive refractive power as a whole,
wherein a back focal length BF of the entire system (the imaging lens), a length L, in a optical axial direction from a surface of the entire system nearest to an object to a surface of the entire system nearest to an image, and a refractive index N4 of the biconcave lens of the third lens group at the d-line, satisfy conditional expressions (1) and (2) below.
0.20<BF/L<0.44 (1)
N4>1.8 (2)
In the imaging lens, each of the convex surfaces of the fourth lens group, which are arranged opposite to each other, may a radius of curvature whose absolute value is smaller than that of the other surface of each lens.
In the imaging lens, each of the convex surfaces of the second lens group, which are arranged opposite to each other, may have a radius of curvature whose absolute value is smaller than that of the other surface of each lens.
In the second lens group and the fourth lens group, there may be an air space between the convex surfaces arranged opposite to each other.
In the imaging lens, a focal length f1 of the first lens group and a focal length f of the entire system may satisfy conditional expression (3).
0.97<|f1|/f<1.92 (3)
Further, in the imaging lens, a focal length f2 of the second lens group and the focal length f of the entire system may satisfy conditional expression (4) below.
0.65<|f2|/f<1.30 (4)
Further, in the imaging lens, a focal length f3 of the third lens group and the focal length f of the entire system may satisfy conditional expression (5) below.
0.49<|f3|/f<1.03 (5)
In addition, in the imaging lens, an Abbe number ν4 of the biconcave lens of the third lens group at the d-line may satisfy conditional expression (6) below.
ν4<30 (6)
According to an aspect of the invention, there is provided an imaging apparatus including the imaging lens described above; and an imaging device for converting an optical image formed by the imaging lens into an electric signal.
The respective values represented by the conditional expressions (1) to (6) are based on the d-line (wavelength of 587.6 nm) as a reference wavelength. In this specification, the d-line is employed as a reference wavelength unless an explanatory note is especially provided.
The features of the invention will appear more fully upon consideration of the exemplary embodiment of the invention, which are schematically set forth in the drawings, in which:
In the imaging lens according to an exemplary embodiment of the invention, downsizing can be provided by satisfying the conditional expression (1), and when an appropriate material for the lens is selected so that the lens satisfies the conditional expression (2), and when the shapes of the individual lenses and the refractive power for the lens groups is property designated, a small F value and a good optical performance can be obtained. That is, according to an exemplary embodiment of the invention, a compact imaging lens which has a good optical performance and a small F value, and an imaging apparatus which is equipped with the imaging lens can be provided.
An imaging lens and an imaging apparatus equipped with this imaging lens will now be described detail according to exemplary embodiments of the present invention, referring to drawings. First, the imaging lens will be described, and thereafter, the imaging apparatus will be described.
In the imaging lens 1, a first lens group G1, a second lens group G2, a third lens group G3 and a fourth lens group G4 are arranged in order from the object side along the optical axis Z. The first lens group G1 includes a biconcave lens L1, and the second lens group G2 includes two lenses L2 and L3 whose convex surfaces are arranged opposite to each other with an air space in between, and has as a whole a positive refractive power. The third lens group G3 includes a biconcave lens L4, and the fourth lens group G4 includes two lenses L5 and L6 whose convex surfaces are arranged opposite to each other with an air space in between, and has as a whole a positive refractive power.
In the imaging lens 1 in
Further, referring to
Although not shown in
A detailed description will now be given of the structure of the imaging lens 1 and of the obtained operating effects. The imaging lens 1 is so designed that the refractive power of the entire system is positive and the refractive powers of the respective lens groups is negative, positive, negative and positive in order from the object side,. Since adjacent lens groups have refractive powers for which the signs are inverted, an aberration that may occur in a group can be offset by its adjacent group, so that all aberrations can be appropriately corrected.
In the first lens group G1, the lens L1 is made to have a biconcave shape, a satisfactorily strong refractive power can be obtained, despite the fact that a single lens is used. Therefore, this structure can contribute both to a reduction in the number of lenses used and to downsizing. As for the third lens group G3, since the lens L4 also has a biconcave shape, the same effects can be obtained as are provided by the lens L1.
Further, as is apparent from
On the other hand, the light ray heights of the positive lenses L2, L3, L5 and L6 are increased, and a great change in the light ray height occurs between the negative lens group and the positive lens group. In order to reduce an aberration that occurs between the groups, surfaces of the positive lenses opposite to the negative lenses may be a surface which has the greater absolute value of a radius of curvature and is directed toward the image side, and the imaging lens 1 is so configured. That is, the lens of the second lens group G2 (corresponding to the lens L2), which is the positive lens group nearest to the object, is arranged so that a surface having the greater absolute value for a radius of curvature is directed toward the object side and the lens of the second lens group G2 (corresponding to the lens L3) nearest to an image is so arranged that the surface having the greater absolute value of a radius of curvature is directed toward the image side. As a result, the effect of an aberration that occurs can be reduced.
In order to obtain a good positive refractive power for the second lens group G2, which is a positive lens group, the surface that is not opposite to the negative lens group may have a smaller absolute value for a radius of curvature. Therefore, in the second lens group G2 of the imaging lens 1, the convex surfaces of the respective lenses, for which the absolute values of a radius of curvature are smaller than those of the other surfaces in the respective lenses, are arranged opposite to each other.
The surfaces of the fourth lens group G4, which is the positive lens group, are arranged in the same manner as are those for the second lens group G2. The lens (corresponding to the lens L6) of the fourth lens group G4 nearest to the image is not directed toward the negative lens group, but is directed toward the imaging position P. Since the light ray height is changed from high to low to focus the light flux from the lens L6 to the imaging position P, the same technical concept as mentioned above can be applied. Also, for the fourth lens group G4, the convex surfaces of the lenses, which has the smaller absolute values of the radius of curvature than that of the other surfaces in the each lens, are arranged opposite to each other.
Furthermore, in the imaging lens 1, a back focal length BF for the entire system, a length L, in the optical axial direction from the surface of the entire system nearest to an object to the surface nearest to an image, and a refractive index N4 of the lens L4 of the third lens group G3 at the d-line satisfy the following conditional expressions (1) and (2) below.
0.20<BF/L<0.44 (1)
N4>1.8 (2)
The conditional expression (1) defines a ratio of the back focal length to the length of the lens system in the optical axial direction. When the ratio exceeds the upper limit of the conditional expression (1), the back focal length is extended, and the size of the entire optical system is increased, so that downsizing the apparatus is difficult. When the ratio is less than the lower limit of the conditional expression (1), an appropriate length can not be obtained for the back focal length, and it is difficult to arrange various filters and a cover glass between the lens system and the imaging surface.
The conditional expression (2) defines the refractive power of the biconcave lens L4. The lens L4 is a negative lens located substantially in the middle of the lens system, between the second lens group G2 and the fourth lens group G4, which has a positive refractive power, and is an important lens for obtaining the optical performance. When the value is less than the lower limit for the conditional expression (2), the radius of curvature of the lens is reduced, and machining of the lens is difficult. In addition, the value of an aberration that occurs is increased, and a satisfactory optical performance is difficult to obtain.
In order to obtain the simplicity for machining and the improvement of the optical performance, the conditional expression (2-1) below may be satisfied.
N4>1.88 (2-1)
For further simplicity for machining and additional improvement of the optical performance, the conditional expression (2-2) below may be satisfied.
N4>1.9 (2-2)
Although the imaging lens 1 is formed of a small number of lenses as described above, an aberration can be appropriately corrected while a small F value is maintained, and downsizing can also be provided.
Further, in the imaging lens 1, a focal length fi of the first lens group G1 and a focal length f of the entire system may satisfy conditional expression (3).
0.97<|f1|/f<1.9 (3)
The conditional expression (3) defines the refractive power of the first lens group G1 relative to the entire system. When the ratio exceeds the upper limit of the conditional expression (3), the refractive power of the first lens group G1 is too low, a back focal length having an appropriate length can not be obtained, and it is difficult to arrange various filters and a cover glass between the lens system and the imaging surface. When the ratio is less than the lower limit for the conditional expression (3), the refractive power of the first lens group G1 becomes too strong, and the effect of distortion is increased.
Moreover, in the imaging lens 1, a focal length f2 of the second lens group G2 and a focal length f of the entire system may satisfy conditional expression (4) below.
0.65<|f2|/f<1.30 (4)
The conditional expression (4) defines the refractive power of the second lens group G2, relative to the entire system. When the ratio exceeds the upper limit for the conditional expression (4), the refractive power of the second lens group G2 is too low, and a distortion or a field curvature is under-corrected. When the ratio is less than the lower limit of the conditional expression (4), the refractive power of the second lens G2 is too strong, and it is difficult to obtain a back focal length having a satisfactory length and the relative illumination.
Further, in the imaging lens 1, a focal length f3 of the third lens group G3 and a focal length f of the entire system may satisfy conditional expression (5) below.
0.49<|f3|/f<1.03 (5)
The conditional expression (5) defines the refractive power of the third lens group G3 relative to the entire system. When the ratio exceeds the upper limit of the conditional expression (5), spherical and other aberrations are under-corrected, and it is difficult to provide a bright optical system having a small F value. When the ratio is less than the lower limit of the conditional expression (5), high-order aberrations become too strong, and spherical and other aberrations are over-corrected. Furthermore, it is difficult to provide a bright optical system having a small F value.
In the imaging lens 1, an Abbe number ν4 of the lens L4 of the third lens group L3 at the d-line may satisfy conditional expression (6) below.
ν4<30 (6)
The conditional expression (6) defines the Abbe number of the biconcave lens L4. When the Abbe number exceeds the upper limit of the conditional expression (6), correction of a chromatic aberration is difficult.
Furthermore, in order to appropriately correct a chromatic aberration, the Abbe number ν4 of the lens L4 at the d-line may satisfy
ν4<25 (6-1).
In addition, in the imaging lens 1, the length L along the optical axis from the surface of the entire system nearest to the object to the surface nearest to the image, the back focal length BF of the entire system and the focal length f of the entire system may satisfy conditional expression (7) below.
3.08<(L+BF)/f<6.37 (7)
The conditional expression (7) defines a ratio of the entire length of the optical system, extended from the lens system to the imaging surface, relative to the focal length. When the ratio exceeds the upper limit of the conditional expression (7), the length of the entire optical system is extended, the size of the entire optical system is increased, and downsizing is difficult. When the ratio is less than the lower limit of the conditional expression (7), a back focal length having an appropriate length can not be obtained, and it is difficult to arrange various filters and a cover glass between the lens system and the imaging surface.
Since the lens L1 is the lens nearest to the object, when the imaging lens is used in a severely adverse environment by being mounted on, for example, an onboard camera, the lens L1 may be made of a material whose surface is not likely to be deteriorated by wind and rain, and may be strong enough to resist the effects of temperature changes due to direct sunlight and of chemicals like oils and detergents, i.e., a material that is waterproof, weatherproof, acid resistant and chemical resistant. Further, preferably, a hard material, not easily cracked, should be employed for the lens L1, and glass or transparent ceramics, for example, may be employed. A ceramic material has a greater hardness and a higher heat resistance than has normal glass.
Furthermore, when an imaging lens 1 is to be mounted in an onboard camera, for example, the imaging lens 1 should function well within a wide temperature range, extending from open air winter temperatures in sub-zero areas to greatly elevated summer temperatures inside automobiles in tropical areas. Therefore, only glass lenses may be used, and specifically, the imaging lens 1 may be suitable for employment in temperatures ranging from −40° C. to 125° C. Further, to enable the manufacture of low cost imaging lenses, only spherical lenses may be used.
Next, specific numerical examples for the imaging lens 1 of the present invention will now be described.
The specification values and the design specifications for an imaging lens according to an example 1 are shown in Table 1. In Table 1, surface numbers are provided in ascending order for i-th (i=1, 2, 3, . . . ) surfaces that are sequentially arranged in order from the surface of a component nearest to an object toward an image side. Ri indicates a radius of curvature for the i-th (i=1, 2, 3, . . . ) surface, and Di indicates on-axis surface spacing, along the optical axis Z, between the i-th (i=1, 2, 3, . . . ) surface and the (i+1)-th surface. Ndj indicates a refractive index at the d-line (wavelength 587.6 nm), of the j-th lens (j=1, 2, 3, . . . ), the surface number of which sequentially ascends from the lens nearest to the object as the first lens toward the image side. νdj indicates the Abbe number of the j-th lens at the d-line. A lens group correlated with the right-most column is denoted by a symbol. In Table 1, the unit “mm” is employed for a radius of curvature and the on-axis surface spacing, and when the radius of curvature is convex toward the object side, this is regarded as positive, while when the radius of curvature is convex toward the image side, this regarded as negative.
In Table 1, FNo. indicates an F value, ω indicates a half angle of view, IH indicates an image height, f indicates the focal length of the entire system, BF indicates a back focal length, and L indicates the length of the entire system along the optical axis Z, from the surface nearest to the object to the surface nearest to the image. These definitions for the symbols in Table 1 are applied for examples that will be described later.
The cross-sectional view of the lens structure of the example 1 is shown in
Specification values for an imaging lens of an example 2 are shown in Table 2, and a cross-sectional view of the lens structure thereof is shown in
Specification values for an imaging lens of an example 3 are shown in Table 3, and the cross-sectional view of the lens structure thereof is shown in
Specification values for an imaging lens of an example 4 are shown in Table 4, and a cross-sectional view of the lens structure thereof is shown in
Specification values for an imaging tens of an example 5 are shown in Table 5, and a cross-sectional view of the lens structure thereof is shown in
Specification values for an imaging lens of an example 6 are shown in Table 6, and a cross-sectional view of the lens structure thereof is shown in
Specification values for an imaging lens of an example 7 are shown in Table 7, and a cross-sectional view of the lens structure thereof is shown in
Specification values for an imaging lens of an example 8 are shown in Table 8, and a cross-sectional view of the lens structure thereof is shown in
Specification values for an imaging lens of an example 9 are shown in Table 9, and a cross-sectional view of the lens structure thereof is shown in
Specification values for an imaging lens of an example 10 are shown in Table 10, and a cross-sectional view of the lens structure thereof is shown in
Specification values for an imaging lens of an example 11 are shown in Table 11, and a cross-sectional view of the lens structure thereof is shown in
Specification values for an imaging lens of an example 12 are shown in Table 12, and a cross-sectional view of the lens structure thereof is shown in
Values corresponding to the conditional expressions (1) to (7) for the imaging lenses of the examples 1 to 12 are shown in Table 13. As is apparent from Table 13, all of the imaging lenses in the examples 1 to 12 satisfy the conditional expressions (1) to (7), and further, the imaging lenses in the examples 1 to 11 satisfy the conditional expressions (2-1), (2-2) and (6-1).
As is apparent from
The imaging lens 1 described above and the imaging lenses in the examples 1 to 12 can be employed for onboard cameras to obtain pictures to the front, side and rear of an automotive.
As described above, the imaging lens 1 according to this embodiment is a small, bright optical system having an appropriate optical performance, and the externally mounted cameras 2 and 3 and the internally mounted camera 4 can also be compactly constructed. With this system, a clear image can be formed on the imaging surface of the imaging device 6.
The present invention has been described by employing the embodiment and the numerical examples. However, the present invention is not limited to the embodiment and the numerical examples, and can be variously modified. For example, the radiuses of curvatures for the individual lens elements and the values for the on-axis surface spacing and the refractive indexes are not limited to those indicated in the numerical examples, other values can be employed.
Furthermore, an example wherein the present invention is employed for an onboard camera has been described as the embodiment for the imaging apparatus. However, the present invention is not limited to this application, and can also be applied, for example, for a portable terminal camera or a surveillance camera.
| Number | Date | Country | Kind |
|---|---|---|---|
| P2007-132334 | May 2007 | JP | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 3838911 | Ikeda | Oct 1974 | A |
| Number | Date | Country |
|---|---|---|
| 55-45007 | Mar 1980 | JP |
| 61-90115 | May 1986 | JP |
| 2002-162562 | Jun 2002 | JP |
| 2004-354572 | Dec 2004 | JP |
| 2005-24969 | Jan 2005 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20080285159 A1 | Nov 2008 | US |
