Patent Application
20150260961
The present application claims priority under 35 USC §119 to Japanese Patent Application No. 2014-047114 filed on Mar. 11, 2014 and Japanese Patent Application No. 2014-218918 filed on Oct. 28, 2014. Each of the above applications is hereby expressly incorporated by reference in its entirety, into the present application.
The present invention is related to a fixed focus imaging lens for forming optical images of subjects onto an imaging element such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor). The present invention is also related to an imaging apparatus provided with the imaging lens that performs photography such as a digital still camera, a cellular telephone with a built in camera, a PDA (Personal Digital Assistant), a smart phone, a tablet type terminal, and a portable gaming device.
Accompanying the recent spread of personal computers in households, digital still cameras capable of inputting image data such as photographed scenes and portraits into personal computers are rapidly becoming available. In addition, many cellular telephones, smart phones, and tablet type terminals are being equipped with camera modules for inputting images. Imaging elements such as CCD's and CMOS's are employed in these devices having photography functions. Recently, miniaturization of these imaging elements is advancing, and there is demand for miniaturization of the entirety of the photography devices as well as imaging lenses to be mounted thereon. At the same time, the number of pixels in imaging elements is increasing, and there is demand for high resolution and high performance of imaging lenses. Performance corresponding to 5 megapixels or greater, and more preferably 8 megapixels or greater, is desired.
In response to such demands, imaging lenses having a five lens configuration, which is a comparatively large number of lenses, have been proposed, and imaging lenses having a six lens configuration, which is an even greater number of lenses, have been proposed for further improvements in performance. For example, Taiwanese Patent Publication Nos. 201333575 and 201333518 propose imaging lenses having six lens configurations.
Meanwhile, there is demand for imaging lenses having comparatively short total lengths for use in apparatuses such as portable terminals, smart phones, and tablet type terminals in particular to realize wider angles of view in addition to shorter total lengths, in order to be compatible with imaging elements having large image sizes that satisfy demand for an increased number of pixels.
However, the angles of view of the imaging lenses disclosed in Taiwanese Patent Publication Nos. 201333575 and 201333518 are too narrow, and it is difficult for these lenses to satisfy the above demand for wider angles of view.
The present invention has been developed in view of the foregoing points. The object of the present invention is to provide an imaging lens that can achieve a shortening of the total length and a wide angle of view while being compatible with the higher numbers of pixels in imaging elements, and realizes high imaging performance from a central angle of view to peripheral angles of view. It is another object of the present invention to provide an imaging apparatus equipped with the imaging lens, which is capable of obtaining high resolution photographed images.
An imaging lens of the present invention consists essentially of six lenses, including:
a first lens having a positive refractive power and a convex surface toward the object side;
a second lens having a negative refractive power;
a third lens having a positive refractive power;
a fourth lens having a positive refractive power;
a fifth lens of a biconcave shape; and
a sixth lens having a negative refractive power, provided in this order from the object side;
the imaging lens satisfying the following conditional formulae:
0<f/f1<1.25 (1)
−0.68<f/f2<0 (2)
wherein f is the focal length of the entire system, f1 is the focal length of the first lens, and f2 is the focal length of the second lens.
Note that in the imaging lens of the present invention, the expression “consists essentially of six lenses” means that the imaging lens of the present invention may also include lenses that practically have no power, optical elements other than lenses such as a stop and a cover glass, and mechanical components such as lens flanges, a lens barrel, a camera shake correcting mechanism, etc., in addition to the six lenses. In addition, the shapes of the surfaces of the lenses and the signs of the refractive indices thereof are considered in the paraxial region in the case that the lenses include aspherical surfaces.
The optical performance of the imaging lens of the present invention can be further improved by adopting the following favorable configurations.
In the imaging lens of the present invention, it is preferable for the sixth lens to be of a meniscus shape having a convex surface toward the object side.
In the imaging lens of the present invention, it is preferable for the fourth lens to be of a meniscus shape having a concave surface toward the object side.
In the imaging lens of the present invention, it is preferable for the second lens to be of a meniscus shape having a convex surface toward the object side.
In the imaging lens of the present invention, it is preferable for the first lens to be of a meniscus shape having a convex surface toward the object side.
In the imaging lens of the present invention, it is preferable for the third lens to have a convex surface toward the object side.
The imaging lens of the present invention may satisfy one or arbitrary combinations of Conditional Formulae (3) through (10) and (1-1) through (5-1) below.
0.31<f/f1<1.2 (1-1)
−0.68<f/f2<−0.1 (2-1)
0.15<f/f3<3 (3)
0.15<f/f3<1.7 (3-1)
0.65<f/f4<3 (4)
0.68<f/f4<2.1 (4-1)
−3<f/f6<−0.5 (5)
−20.1<f/f6<−0.8 (5-1)
0.5<(L1r+L1f)/(L1r−L1f)<3 (6)
−0.55<(L5r+L50/(L5r−L50<1 (7)
−7.5<(L4r+L4f)/(L4r−L4f)<0 (8)
−1.4<f·P34<0 (9)
0.5<f·tan ω/L6r<20 (10)
wherein f is the focal distance of the entire system, f1 is the focal length of the first lens, f2 is the focal length of the second lens, f3 is the focal length of the third lens, f4 is the focal length of the fourth lens, f6 is the focal length of the sixth lens, L1f is the paraxial radius of curvature of the surface of the first lens toward the object side, L1r is the paraxial radius of curvature of the surface of the first lens toward the image side, L4f is the paraxial radius of curvature of the surface of the fourth lens toward the object side, L4r is the paraxial radius of curvature of the surface of the fourth lens toward the image side, L5f is the paraxial radius of curvature of the surface of the fifth lens toward the object side, L5r is the paraxial radius of curvature of the surface of the fifth lens toward the image side, L6r is the paraxial radius of curvature of the surface of the sixth lens toward the image side, ω is half the maximum angle of view when focused on an object at infinity, and P34 is the refractive power of an air lens formed between the surface of the third lens toward the image side and the surface of the fourth lens toward the object side, the refractive power of the air lens being obtained by Formula (P) below:
wherein Nd3 is the refractive index of the third lens with respect to the d line, Nd4 is the refractive index of the fourth lens with respect to the d line, L3r is the paraxial radius of curvature of the surface of the third lens toward the image side, L4f is the paraxial radius of curvature of the surface of the fourth lens toward the object side, and D7 is an air space distance between the third lens and the fourth lens along the optical axis.
An imaging apparatus of the present invention is equipped with the imaging lens of the present invention.
According to the imaging lens of the present invention, the configuration of each lens element is optimized within a lens configuration having six lenses as a whole. Therefore, a lens system that can achieve a short total length and a while angle of view while being compatible with an increased number of pixels and has high imaging performance from a central angle of view to peripheral angles of view can be realized.
The imaging apparatus of the present invention is equipped with any one of the imaging lenses of the present invention having high imaging performance. Therefore, the apparatus size of the imaging lens in the direction of the optical axis can be shortened, and the imaging apparatus of the present invention is capable of obtaining high resolution photographed images.
Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings.
The imaging lens L of the embodiment of the present invention is favorably employed in various imaging devices that employ imaging elements such as a CCD and a CMOS. The imaging lens L of the embodiment of the present invention is particularly favorable for use in comparatively miniature portable terminal devices, such as a digital still camera, a cellular telephone with a built in camera, a smart phone, a tablet type terminal, and a PDA. The imaging lens L is equipped with a first lens L1, a second lens L2, a third lens L3, a fourth lens L4, a fifth lens L5, and a sixth lens L6, provided along the optical axis Z1 in this order from the object side.
Various optical members CG may be provided between the sixth lens L6 and the imaging element 100, depending on the configuration of the camera to which the lens is applied. A planar optical member such as a cover glass for protecting the imaging surface and an infrared cutoff filter may be provided, for example. In this case, a planar cover glass having a coating having a filtering effect such as an infrared cutoff filter coating or an ND filter coating, or a material that exhibits similar effects, may be utilized as the optical member CG
Alternatively, the optical member CG may be omitted, and a coating may be administered on the sixth lens L6 to obtain the same effect as that of the optical member CG. In this case, the number of parts can be reduced, and the total length can be shortened.
It is preferable for the imaging lens L to be equipped with an aperture stop St positioned at the object side of the surface of the second lens L2 toward the object side. In the case that the aperture stop St is positioned at the object side of the surface of the second lens L2 toward the object side in this manner, increases in the incident angles of light rays that pass through the optical system and enter the image formation plane (imaging element) can be suppressed, particularly at peripheral portions of an imaging region. Note that the expression “positioned at the object side of the surface of the second lens L2 toward the object side” means that the position of the aperture stop in the direction of the optical axis is at the same position as the intersection of marginal axial rays of light and the surface of the second lens L2 toward the object side, or more toward the object side than this position. It is preferable for the apertures stop St to be positioned at the object side of the surface of the first lens L1 toward the object side, in order to cause this advantageous effect to become more prominent. Note that the expression “positioned at the object side of the surface of the first lens L1 toward the object side” means that the position of the aperture stop in the direction of the optical axis is at the same position as the intersection of marginal axial rays of light and the surface of the first lens L1 toward the object side, or more toward the object side than this position. In the embodiments, the lenses of the fifth through eighth Examples (
Alternatively, the apertures stop St may be positioned between the first lens L1 and the second lens L2. In this case, the total length can be shortened, while aberrations can be corrected in a well balanced manner by the lens positioned at the object side of the aperture stop St and the lenses positioned at the image side of the aperture stop St. In the embodiments, the lenses of the first through fourth Examples (
In the imaging lens L, the first lens L1 has a positive refractive power in the vicinity of the optical axis. This configuration is advantageous from the viewpoint of shortening the total length of the lens. In addition, the first lens L1 has a convex surface toward the object side in the vicinity of the optical axis. Thereby, moving the position of the rearward principal point of the first lens L1 toward the object side is facilitated, and a shortening of the total length of the lens can be more favorably realized. In addition, it is preferable for the first lens L1 to be of a meniscus shape having a convex surface toward the object side in the vicinity of the optical axis. In this case, moving the position of the rearward principal point of the first lens L1 toward the object side is facilitated further, and a shortening of the total length of the lens can be favorably realized.
The second lens L2 has a negative refractive power in the vicinity of the optical axis. Thereby, longitudinal chromatic aberration and spherical aberration can be favorably corrected. In addition, it is preferable for the second lens L2 to be of a meniscus shape having a convex surface toward the object side in the vicinity of the optical axis. In this case, astigmatism can be favorably corrected, while suppressing the generation of spherical aberration.
Both the third lens L3 and the fourth lens L4 have a positive refractive power in the vicinity of the optical axis. Both the fifth lens L5 and the sixth lens L6 have a negative refractive power in the vicinity of the optical axis. If the first lens L1 through the fourth lens L4 are considered to be a positive first lens group, and the fifth lens L5 and the sixth lens L6 are considered to be a negative second lens group, the imaging lens L will have a telephoto type configuration. Therefore, a shortening of the total length of the lens can be favorably realized. In addition, by arranging the imaging lens L to be of a telephoto type configuration and by configuring the third lens L3 and the fourth lens L4 within the positive first lens group constituted by the first lens L1 through the fourth lens L4 to have positive refractive powers, the refractive power of the positive first lens group constituted by the first lens L1 through the fourth lens L4 as a whole can be increased, while suppressing the refractive powers of the third lens L3 and the fourth lens L4. As a result, the total length of the lens can be shortened, while spherical aberration, astigmatism, and the like can be favorably corrected.
In addition, it is preferable for the third lens to have a convex surface toward the object side in the vicinity of the optical axis. In this case, spherical aberration can be favorably corrected. In addition, the third lens L3 may be of a biconvex shape. In this case, spherical aberration can be more favorably corrected. Alternatively, the third lens L3 may be of a meniscus shape having a convex surface toward the object side in the vicinity of the optical axis. In this case, moving the position of the rearward principal point of the third lens L3 is facilitated, and a shortening of the total length of the lens can be favorably realized.
It is preferable for the fourth lens L4 to be of a meniscus shape having a concave surface toward the object side in the vicinity of the optical axis. In this case, astigmatism can be favorably corrected.
The fifth lens L5 is of a biconcave shape in the vicinity of the optical axis. For this reason, correction of astigmatism is facilitated by the fifth lens L5 having a concave surface toward the object side. In addition, this configuration is advantageous from the viewpoint of widening the angle of view. Further, the total length of the lens can be favorably shortened by the fifth lens L5 having a concave surface toward the image side.
In addition, it is preferable for the sixth lens L6 to be of a meniscus shape having a convex surface toward the object side in the vicinity of the optical axis. In this case, moving the position of the rearward principal point of the imaging lens L is facilitated, and a shortening of the total length of the lens can be favorably realized. Further, field curvature can be favorably corrected.
In addition, it is preferable for the surface of the sixth lens L6 toward the image side to be of an aspherical shape having at least one inflection point at a position in an inwardly radial direction from the intersection of a principal light ray at a maximum angle of view and the surface of the sixth lens L6 toward the image side to the optical axis. By adopting this configuration, increases in the incident angles of light rays that pass through the optical system at and enter the image formation plane (imaging element) can be suppressed, particularly at the peripheral portions of the imaging region. In addition, distortion can be favorably corrected, by the surface of the sixth lens L6 toward the image side being of an aspherical shape having at least one inflection point at a position in an inwardly radial direction from the intersection of a principal light ray at a maximum angle of view and the surface of the sixth lens L6 toward the image side to the optical axis. Note that the “inflection point” on the surface of the sixth lens L6 toward the image side refers to a point at which the shape of the surface of the sixth lens L6 toward the image side changes from a convex shape to a concave shape (or from a concave shape to a convex shape) with respect to the image side. In addition, in the present specification, the expression “a position in an inwardly radial direction from the intersection of a principal light ray at a maximum angle of view and the surface toward the image side to the optical axis” refers to positions at the intersection of a principal light ray at a maximum angle of view and the surface toward the image side to the optical axis and positions radially inward toward the optical axis from these positions. In addition, the inflection point on the surface of the sixth lens L6 toward the image side may be provided at any position at the intersection of a principal light ray at a maximum angle of view and the surface of the sixth lens L6 toward the image side to the optical axis and at positions radially inward toward the optical axis from these positions.
In addition, in the case that each of the first lens L1 through the sixth lens L6 that constitute the imaging lens L is a single lens, the number of lens surfaces will be greater than that for a case in which any of the first lens L1 through the sixth lens L6 is a cemented lens. Therefore, the degree of freedom in the design of each lens will increase. As a result, shortening of the total length and increase in resolution will be facilitated.
According to the imaging lens L described above, the configurations of each of the first lens L1 through the sixth lens L6 are optimized as lens elements in a lens configuration having a total of six lenses. Therefore, a lens system that achieves a shortened total length and a widened angle of view, which is compatible with an increased number of pixels and has high imaging performance from a central angle of view to peripheral angles of view, can be realized.
It is preferable for at least one of the surfaces of each of the first lens L1 through the sixth lens L6 of the imaging lens L to be an aspherical surface, in order to improve performance.
Next, the operation and effects of conditional formulae related to the imaging lens L will be described in greater detail. Note that it is preferable for the imaging lens L to satisfy any one of the following conditional formulae, or arbitrary combinations of the following conditional formulae. It is preferable for the conditional formulae to be satisfied to be selected as appropriate according to the items required of the imaging lens L.
First, it is preferable for the focal length f1 of the first lens L1 and the focal length f of the entire system to satisfy Conditional Formula (1) below.
0<f/f1<1.25 (1)
Conditional Formula (1) defines a preferable range of numerical values for the ratio of the focal length f of the entire system with respect to the focal length f1 of the first lens L1. By securing the refractive power of the first lens L1 such that the value of f/f1 is not less than or equal to the lower limit defined in Conditional Formula (1), the positive refractive power of the first lens L1 will not become excessively weak with respect to the refractive power of the entire system. As a result, a shortening of the total length of the lens can be favorably realized. By suppressing the refractive power of the first lens L1 such that the value of f/f1 is not greater than or equal to the upper limit defined in Conditional Formula (1), the positive refractive power of the first lens L1 will not become excessively strong with respect to the refractive power of the entire system. As a result, spherical aberration can be favorably corrected. In addition, by configuring the imaging lens L such that the value of f/f1 is not greater than or equal to the upper limit defined in Conditional Formula (1), astigmatism can be favorably corrected, and is advantageous from the viewpoint of realizing a wider angle of view. It is more preferable for Conditional Formula (1-1) to be satisfied, even more preferable for Conditional Formula (1-2) to be satisfied, and still more preferable for Conditional Formula (1-3) to be satisfied, in order to cause these advantageous effects to become more prominent.
0.31<f/f1<1.2 (1-1)
0.31<f/f1<1.1 (1-2)
0.6<f/f1<1 (1-3)
In addition, it is preferable for the focal length f2 of the second lens L2 and the focal length f of the entire system to satisfy Conditional Formula (2) below.
−0.68<f/f2<0 (2)
Conditional Formula (2) defines a preferable range of numerical values for the ratio of the focal length f of the entire system with respect to the focal length f2 of the second lens L2. By suppressing the refractive power of the second lens L2 such that the value of f/f2 is not less than or equal to the lower limit defined in Conditional Formula (2), the negative refractive power of the second lens L2 will not become excessively strong with respect to the refractive power of the entire system. As a result, a shortening of the total length of the lens can be favorably realized. It is more preferable for the lower limit of Conditional Formula (2-2) to be satisfied, even more preferable for the lower limit of Conditional Formula (2-3) to be satisfied, and still more preferable for the lower limit of Conditional Formula (2-4) to be satisfied, in order to cause the advantageous effect obtained by satisfying the lower limit of Conditional Formula (2) to become more prominent. By securing the refractive power of the second lens L2 such that the value of f/f2 is not greater than or equal to the upper limit defined in Conditional Formula (2), the negative refractive power of the second lens L2 will not become excessively weak with respect to the refractive power of the entire system. As a result, spherical aberration and longitudinal chromatic aberration can be favorably corrected. It is more preferable for the upper limit of Conditional Formula (2-1) to be satisfied, even more preferable for the upper limit of Conditional Formula (2-3) to be satisfied, and still more preferable for the upper limit of Conditional Formula (2-4) to be satisfied, in order to cause the advantageous effect obtained by satisfying the upper limit of Conditional Formula (2) to become more prominent.
−0.68<f/f2<−0.1 (2-1)
−0.65<f/f2<0 (2-2)
−0.6<f/f2<−0.1 (2-3)
−0.55<f/f2<−0.15 (2-4)
In addition, it is preferable for the focal length f3 of the third lens L3 and the focal length f of the entire system to satisfy Conditional Formula (3) below.
0.15<f/f3<3 (3)
Conditional Formula (3) defines a preferable range of numerical values for the ratio of the focal length f of the entire system with respect to the focal length f3 of the third lens L3. It is preferable to secure the refractive power of the third lens L3 such that the value of f/f3 is not less than or equal to the lower limit defined in Conditional Formula (3). In this case, the refractive power of the third lens L3 will not become excessively weak with respect to the refractive power of the entire system, which is advantageous from the viewpoint of shortening the total length of the lens. It is more preferable for the lower limit of Conditional Formulae (3-2) through (3-4) to be satisfied, in order to cause the advantageous effect obtained by satisfying the lower limit of Conditional Formula (3) to become more prominent. By suppressing the refractive power of the third lens L3 such that the value of f/f3 is not greater than or equal to the upper limit defined in Conditional Formula (3), the negative refractive power of the third lens L3 will not become excessively strong with respect to the refractive power of the entire system. As a result, the generation of spherical aberration can be favorably suppressed. It is more preferable for the upper limit of Conditional Formula (3-1) to be satisfied, even more preferable for the upper limit of Conditional Formula (3-3) to be satisfied, and still more preferable for the upper limit of Conditional Formula (3-4) to be satisfied, in order to cause the advantageous effect obtained by satisfying the upper limit of Conditional Formula (3) to become more prominent.
0.15<f/f3<1.7 (3-1)
0.23<f/f3<3 (3-2)
0.23<f/f3<1.7 (3-3)
0.23<f/f3<0.6 (3-4)
In addition, it is preferable for the focal length f4 of the fourth lens L4 and the focal length f of the entire system to satisfy Conditional Formula (4) below.
0.65<f/f4<3 (4)
Conditional Formula (4) defines a preferable range of numerical values for the ratio of the focal length f4 of the fourth lens L4 with respect to the focal length f of the entire system. By securing the refractive power of the fourth lens L4 such that the value of f/f4 is not less than or equal to the lower limit defined in Conditional Formula (4), the positive refractive power of the fourth lens L4 will not become excessively weak with respect to the refractive power of the entire system, which is advantageous form the viewpoint of shortening of the total length of the lens. By suppressing the refractive power of the fourth lens L4 such that the value of f/f4 is not greater than or equal to the upper limit defined in Conditional Formula (4), the positive refractive power of the fourth lens L4 will not become excessively strong with respect to the refractive power of the entire system. As a result, the generation of spherical aberration can be favorably suppressed. It is more preferable for Conditional Formula (4-1) to be satisfied, and even more preferable for Conditional Formula (4-2) to be satisfied, in order to cause these advantageous effects to become more prominent.
0.68<f/f4<2.1 (4-1)
0.7<f/f4<1.25 (4-2)
In addition, it is preferable for the focal length f6 of the sixth lens L6 and the focal length f of the entire system to satisfy Conditional Formula (5) below.
−3<f/f6<−0.5 (5)
Conditional Formula (5) defines a preferable range of numerical values for the ratio of the focal length f of the entire system with respect to the focal length f6 of the sixth lens L6. By suppressing the refractive power of the sixth lens L6 such that the value of f/f6 is not less than or equal to the lower limit defined in Conditional Formula (5), the negative refractive power of the sixth lens L6 will not become excessively strong with respect to the refractive power of the entire system. As a result, increases in the incident angles of light rays that pass through the optical system at intermediate angles of view and enter the image formation plane (imaging element) can be suppressed. By securing the refractive power of the sixth lens L6 such that the value of f/f6 is not greater than or equal to the upper limit defined in Conditional Formula (5), the negative refractive power of the sixth lens L6 will not become excessively weak with respect to the refractive power of the entire system. As a result, the total length of the lens can be favorably shortened. It is more preferable for Conditional Formula (5-1) to be satisfied, and even more preferable for Conditional Formula (5-2) to be satisfied, in order to cause these advantageous effects to become more prominent.
−2.1<f/f6<−0.8 (5-1)
−1.4<f/f6<−0.9 (5-2)
In addition, it is preferable for the paraxial radius of curvature L1f of the surface of the first lens L1 toward the object side and the paraxial radius of curvature L1r of the surface of the first lens L1 toward the image side to satisfy Conditional Formula (6) below.
0.5<(L1r+L1f)/(L1r−L10<3 (6)
Conditional Formula (6) defines a preferable range of numerical values related to the paraxial radius of curvature L1f of the surface of the first lens L1 toward the object side and the paraxial radius of curvature L1r of the surface of the first lens L1 toward the image side. By configuring the imaging lens such that the value of (L1r+L1f)/(L1r−L1f) is not less than or equal to the lower limit defined in Conditional Formula (6), the total length of the lens can be favorably shortened. By configuring the imaging lens such that the value of (L1r+L1f)/(L1r−L1f) is not greater than or equal to the upper limit defined in Conditional Formula (6), the generation of spherical aberration can be suppressed, and spherical aberration can be favorably corrected. It is preferable for Conditional Formula (6-1) to be satisfied, more preferable for Conditional Formula (6-2) to be satisfied, and even more preferable for Conditional Formula (6-3) to be satisfied, in order to cause these advantageous effects to become more prominent.
1.5<(L1r+L1f)/(L1r−L10<3 (6-1)
1.65<(L1r+L1f)/(L1r−L10<2.7 (6-2)
1.7<(L1r+L1f)/(L1r−L10<2.5 (6-3)
In addition, it is preferable for the paraxial radius of curvature L5f of the surface of the fifth lens L5 toward the object side and the paraxial radius of curvature L5r of the surface of the fifth lens L5 toward the image side to satisfy Conditional Formula (7) below.
−0.55<(L5r+L5f)/(L5r−L50<1(7)
Conditional Formula (7) defines a preferable range of numerical values related to the paraxial radius of curvature L5f of the surface of the fifth lens L5 toward the object side and the paraxial radius of curvature L5r of the surface of the fifth lens L5 toward the image side. By configuring the imaging lens such that the value of (L5r+L5f)/(L5r−L5f) is not less than or equal to the lower limit defined in Conditional Formula (7), the absolute value of the paraxial radius of curvature of the surface of the fifth lens L5 toward the image side will be prevented from becoming excessively small. As a result, spherical aberration can be favorably corrected. By configuring the imaging lens such that the value of (L5r+L5f)/(L5r−L5f) is not greater than or equal to the upper limit defined in Conditional Formula (7), the absolute value of the paraxial radius of curvature of the surface of the fifth lens L5 toward the object side will be prevented from becoming excessively large. As a result, astigmatism can be favorably corrected. It is more preferable for Conditional Formula (7-1) to be satisfied, and even more preferable for Conditional Formula (7-2) to be satisfied, in order to cause these advantageous effects to become more prominent.
−0.4<(L5r+L5f)/(L5r−L5f)<1 (7-1)
−0.3<(L5r+L5f)/(L5r−L5f)<0.6 (7-2)
In addition, it is preferable for the paraxial radius of curvature L4f of the surface of the fourth lens L4 toward the object side and the paraxial radius of curvature L4r of the surface of the fourth lens L4 toward the image side to satisfy Conditional Formula (8) below.
−7.5<(L4r+L4f)/(L4r−L4f)<0 (8)
Conditional Formula (8) defines a preferable range of numerical values related to the paraxial radius of curvature L4f of the surface of the fourth lens L4 toward the object side and the paraxial radius of curvature L4r of the surface of the fourth lens L4 toward the image side. By configuring the imaging lens such that the value of (L4r+L4f)/(L4r−L4f) is not less than or equal to the lower limit defined in Conditional Formula (8), the absolute value of the paraxial radius of curvature of the surface of the fourth lens L4 toward the image side will be prevented from becoming excessively small. As a result, spherical aberration can be favorably corrected. By configuring the imaging lens such that the value of (L4r+L4f)/(L4r−L4f) is not greater than or equal to the upper limit defined in Conditional Formula (8), the absolute value of the paraxial radius of curvature of the surface of the fourth lens L4 toward the object side will be prevented from becoming excessively large. As a result, astigmatism can be favorably corrected. It is more preferable for Conditional Formula (8-1) to be satisfied, and even more preferable for Conditional Formula (8-2) to be satisfied, in order to cause these advantageous effects to become more prominent.
−5.2<(L4r+L4f)/(L4r−L40<−0.6 (8-1)
−3<(L4r+L40/(L4r−L40<−1 (8-2)
In addition, it is preferable for the focal length f of the entire system and the refractive power P34 of an air lens formed between the surface of the third lens L3 toward the image side and the surface of the fourth lens L4 toward the object side to satisfy Conditional Formula (9) below.
−1.4<f·P34<0 (9)
Here, P34 is obtained by Formula (P) below:
wherein Nd3 is the refractive index of the third lens L3 with respect to the d line, Nd4 is the refractive index of the fourth lens L4 with respect to the d line, L3r is the paraxial radius of curvature of the surface of the third lens L3 toward the image side, L4f is the paraxial radius of curvature of the surface of the fourth lens L4 toward the object side, and D7 is an air space distance between the third lens and the fourth lens along the optical axis.
Refractive power is an inverse of focal length. Therefore, if the focal length of the air lens formed between the surface of the third lens L3 toward the image side and the surface of the fourth lens L4 toward the object side is designated as f34a, Conditional Formula (9) defines a preferable range of numerical values for the ratio of the focal length f of the entire system with respect to the focal length f34a. By configuring the imaging lens such that the value of f·P34 is not less than or equal to the lower limit defined in Conditional Formula (9), the refractive power of the air lens formed between the surface of the third lens L3 toward the image side and the surface of the fourth lens L4 toward the object side will not be excessively strong. As a result, distortion can be favorably corrected. By configuring the imaging lens such that the value of f·P34 is not greater than or equal to the upper limit defined in Conditional Formula (9), the refractive power of the air lens formed between the surface of the third lens L3 toward the image side and the surface of the fourth lens L4 toward the object side will not be excessively weak. As a result, the astigmatism can be favorably corrected. It is more preferable for Conditional Formula (9-1) to be satisfied, and even more preferable for Conditional Formula (9-2) to be satisfied, in order to cause these advantageous effects to become more prominent.
−0.9<f·P34<−0.15 (9-1)
−0.6<f·P34<−0.2 (9-2)
In addition, it is preferable for the focal distance f of the entire system, the half angle of view to when in a state of focus on an object at infinity, and the paraxial radius of curvature L6r of the surface of the sixth lens L6 toward the image side to satisfy Conditional Formula (10) below.
0.5<f·tan ω/L6r<20 (10)
Conditional Formula (10) defines a preferable range of numerical values for the ratio of a paraxial image height (Ham) with respect to the paraxial radius of curvature L6r of the surface of the sixth lens L6 toward the image side. By configuring the imaging lens such that the value of f·tan ω/L6r is not less than or equal to the lower limit defined in Conditional Formula (10), the absolute value of the paraxial radius of curvature L6r of the surface of the sixth lens L6 toward the image side, which is the surface most toward the image side in the imaging lens L, will not be excessively large with respect to the paraxial image height (f·tan ω). Thereby, field curvature can be sufficiently corrected while realizing a shortening of the total length. Note that field curvature can be favorably corrected from a central angle of view to peripheral angles of view in the case that in the case that the sixth lens L6 is of an aspherical shape having a concave surface toward the image side and at least one inflection point as illustrated in the imaging lenses L of each of the Examples, and in the case that the lower limit of Conditional Formula (10) is satisfied. Therefore, this configuration facilitates realization of a wide angle of view. In addition, by configuring the imaging lens such that the value of f·tan ω/L6r is not greater than or equal to the upper limit defined in Conditional Formula (10), the absolute value of the paraxial radius of curvature L6r of the surface of the sixth lens L6 toward the image side, which is the surface most toward the image side in the imaging lens, will not be excessively small with respect to the paraxial image height (f·tan ω). Thereby, increases in the incident angle of light rays that pass through the optical system and enter the image formation plane (imaging element) can be suppressed, particularly at intermediate angles of view. It is preferable for Conditional Formula (10-1) to be satisfied, and more preferable for Conditional Formula (10-2) to be satisfied, in order to cause these advantageous effects to become more prominent.
0.7<f·tan ω/L6r<10 (10-1)
1<f·tan ω/L6r<5 (10-2)
As described above, in the imaging lens L according to the embodiments of the present invention, the configurations of each lens element is optimized in a lens configuration having a total of six lenses. Therefore, a lens system that achieves a shortened total length and a widened angle of view and has high imaging performance from a central angle of view to peripheral angles of view, can be realized.
In addition, in the case that the lens configurations of each of the first lens L1 through the sixth lens L6 are set such that the maximum angle of view in a state focused on an object at infinity is 77.6 degrees or greater as in the imaging lenses of the first through eighth embodiments, a widening of the angle of view and a shortening of the total length of the lens can be achieved, and the imaging lens L may be favorably applied for use in imaging apparatuses such as cellular telephones.
Further improved imaging performance can be realized by satisfying the above preferred conditions as appropriate. In addition, the imaging apparatuses according to the embodiments of the present invention output image signals corresponding to optical images formed by the high performance imaging lenses according to the embodiments of the present invention. Therefore, photographed images having high resolution from a central angle of view to peripheral angles of view can be obtained.
Next, specific examples of numerical values of the imaging lens of the present invention will be described. A plurality of examples of numerical values will be summarized and explained below.
Table 1 and Table 2 below show specific lens data corresponding to the configuration of the imaging lens illustrated in
Table 1 also shows the aperture stop St and the optical member CG In Table 1 “(St)” is indicated along with a surface number in the row of the surface number of the surface that corresponds to the aperture stop St, and “(IMG)” is indicated along with a surface number in the row of the surface number of the surface that corresponds to the imaging surface. The signs of the radii of curvature are positive for surface shapes having convex surfaces toward the object side, and negative for surface shapes having convex surfaces toward the image side. Note that the values of the focal length f (mm) of the entire system, the back focus Bf (mm), the F number Fno. and the maximum angle of view 2ω(°) in a state focused on an object at infinity are shown as data above the lens data. Note that the back focus Bf is represented as an air converted value.
In the imaging lens of Example 1, both of the surfaces of the first lens L1 through the sixth lens L6 are all aspherical in shape. In the basic lens data of Table 1, numerical values of radii of curvature in the vicinity of the optical axis (paraxial radii of curvature) are shown as the radii of curvature of the aspherical surfaces.
Table 2 shows aspherical surface data of the imaging lens of Example 1. In the numerical values shown as the aspherical surface data, the symbol “E” indicates that the numerical value following thereafter is a “power index” having 10 as a base, and that the numerical value represented by the index function having 10 as a base is to be multiplied by the numerical value in front of “E”. For example, “1.0E-02” indicates that the numerical value is “1.0·10−2”.
The values of coefficients An and KA represented by the aspherical surface shape formula (A) below are shown as the aspherical surface data. In greater detail, Z is the length (mm) of a normal line that extends from a point on the aspherical surface having a height h to a plane (a plane perpendicular to the optical axis) that contacts the apex of the aspherical surface.
wherein: Z is the depth of the aspherical surface (mm), h is the distance from the optical axis to the surface of the lens (height) (mm), C is the paraxial curvature=1/R (R is the paraxial radius of curvature), An is an nth ordinal aspherical surface coefficient (n is an integer 3 or greater), and KA is an aspherical surface coefficient.
Specific lens data corresponding to the configurations of the imaging lenses illustrated in
Similarly, the aberrations of the imaging lens of Example 2 through Example 8 are illustrated in
Table 17 shows values corresponding to Conditional Formulae (1) through (10) respectively summarized for each of Examples 1 through 8.
As can be understood from each set of numerical value data and from the diagrams that illustrate aberrations, each of the Examples realize a shortening of the total length of the lens, a widened angle of view, and high imaging performance.
Note that the imaging lens of the present invention is not limited to the embodiments and Examples described above, and various modifications are possible. For example, the values of the radii of curvature, the distances among surfaces, the refractive indices, the Abbe's numbers, the aspherical surface coefficients, etc., are not limited to the numerical values indicated in connection with the Examples of numerical values, and may be other values.
In addition, the Examples are described under the presumption that they are to be utilized with fixed focus. However, it is also possible for configurations capable of adjusting focus to be adopted. It is possible to adopt a configuration, in which the entirety of the lens system is fed out or a portion of the lenses is moved along the optical axis to enable automatic focus, for example.
2.0655481E−03
Note that the above paraxial radii of curvature, the distances among surfaces, the refractive indices, and the Abbe's numbers were obtained by measurements performed by specialists in the field of optical measurement, according to the methods described below.
The paraxial radii of curvature were obtained by measuring the lenses using an ultra high precision three dimensional measurement device UA3P (by Panasonic Factory Solutions K. K.) by the following procedures. A paraxial radius of curvature Rm (m is a natural number) and a conical coefficient Km are preliminarily set and input into UA3P, and an nth order aspherical surface coefficient An of an aspherical shape formula is calculated from the input paraxial radius of curvature Rm and conical coefficient Km and the measured data, using a fitting function of UA3P. C=1/Rm and KA=Km−1 are considered in the aforementioned aspherical surface shape formula (A). Depths Z of an aspherical surface in the direction of the optical axis corresponding to heights h from the optical axis are calculated from Rm, Km, An, and the aspherical surface shape formula. The difference between the calculated depths Z and actually measured depth values Z′ are obtained for each height h from the optical axis. Whether the difference is within a predetermined range is judged. In the case that the difference is within the predetermined range, Rm is designated as the paraxial radius of curvature. On the other hand, in the case that the difference is outside the predetermined range, the value of at least one of Rm and Km is changed, set as Rm+1 and Km+1, and input to UA3P. The processes described above are performed, and judgment regarding whether the difference between the calculated depths Z and actually measured depth values Z′ for each height h from the optical axis is within a predetermined range is judged. These procedures are repeated until the difference between the calculated depths Z and actually measured depth values Z′ for each height h from the optical axis is within a predetermined range. Note that here, the predetermined range is set to be 200 nm or less. In addition, a range from 0 to ⅕ the maximum lens outer diameter is set as the range of h.
The distances among surfaces are obtained by measurements using OptiSurf (by Trioptics), which is an apparatus for measuring the central thicknesses and distances between surfaces of paired lenses.
The refractive indices are obtained by performing measurements in a state in which the temperature of a measurement target is 25° C., using KPR-2000 (by K. K. Shimadzu), which is a precision refractometer. The refractive index measured with respect to the d line (wavelength: 587.6 nm) is designated as Nd. Similarly, the refractive index measured with respect to the e line (wavelength: 546.1 nm) is designated as Ne, the refractive index measured with respect to the F line (wavelength: 486.1 nm) is designated as NF, the refractive index measured with respect to the C line (wavelength: 656.3 nm) is designated as NC, and the refractive index measured with respect to the g line (wavelength: 435.8 nm) is designated as Ng. The Abbe's number vd with respect to the d line is obtained by calculations, substituting the values of Nd, NF, and NC obtained by the above measurements into the formula below.
νd=(Nd−1)/(NF−NC)
| Number | Date | Country | Kind |
|---|---|---|---|
| 047114/2014 | Mar 2014 | JP | national |
| 218918/2014 | Oct 2014 | JP | national |
