JP2013182132A - Imaging lens and imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make the entire optical length shorter, a sensor incident angle smaller, and a focus depth deeper, while maintaining especially, monochromatic performance and a magnification chromatic aberration, in an imaging lens having a four lens configuration.SOLUTION: The imaging lens substantially comprises four lenses configured by arranging, in order from an object side, a positive first lens L1 whose lens surface Sa1 on the object side has a convex shape in a paraxial region, a negative second lens L2, a positive third lens L3 whose lens surface Sa3 on the object side has a concave shape in the paraxial region, and a negative fourth lens L4 formed so that the peripheral part of a lens surface Sb4 on an image side has positive power. The imaging lens satisfies a conditional expression of 0≤ΔN≤0.1 (1), a conditional expression of 0≤Δν≤10 (2), and a conditional expression of -2.0<f2/f<-0.5 (3) simultaneously, where ΔN is the absolute value of the difference between the maximum and minimum values of the refractive index with respect to the d-line of the first to fourth lenses L1 to L4 and Δν is the absolute value of the difference between the maximum and minimum values of the Abbe number with respect to the d-line of the first to fourth lenses L1 to L4.

Description

  The present invention relates to an imaging lens composed of four lenses and an imaging apparatus using the imaging lens.

  Conventionally, an optical image formed through an imaging lens is received by an imaging element such as a CCD (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor) and captured to obtain a color image, for example, a digital still camera, A mobile terminal with a camera, an information terminal (PDA: Personal Digital Assistance), a smartphone, a portable game machine, and the like are known.

  For example, Patent Document 1 and Patent Document 2 are known as such a four-lens imaging lens.

  In Example 5 of the above-mentioned Patent Document 1, the same material is used for all four lenses in a four-lens configuration, thereby allowing axial chromatic aberration and correcting chromatic aberration of magnification to improve peripheral optical performance. An imaging lens configured to improve is disclosed. However, the imaging lens of Patent Document 1 cannot sufficiently cope with the recent increase in pixels and thickness.

  Conventionally, many imaging lenses having a four-lens structure in which an optical total length is short and various aberrations are well corrected are known. For example, a four-lens imaging lens described in Patent Document 2 is known. However, such a conventionally known four-lens imaging lens tends to have a shallow depth of focus.

Utility Model Registration No. 3146436 JP 2011-95301 A

  As described above, there has recently been a demand for an imaging lens that has good optical performance, particularly chromatic aberration of magnification and monochromatic aberration, and a short optical total length. On the other hand, in order to widen the in-focus range, a lens having a deep focal depth is also required.

  The present invention has been made in view of the above circumstances, and has good optical performance, in particular, while maintaining monochromatic performance and chromatic aberration of magnification, the optical total length is shortened, the sensor incident angle is decreased, and the depth of focus is increased. An object of the present invention is to provide a four-lens imaging lens that can be used and an imaging apparatus using the imaging lens.

  The four-lens imaging lens of the present invention includes, in order from the object side, a first lens having a positive power in which a lens surface on the object side forms a convex surface in a paraxial region, a second lens having a negative power, and an object A third lens having a positive power in which the lens surface on the side forms a concave surface in the paraxial region, and a fourth lens having a negative power and the peripheral portion of the lens surface on the image side having a positive power And conditional expression (1): 0 ≦ ΔN ≦ 0.1, (2): 0 ≦ Δν ≦ 10, (3): −2.0 <f2 / f <−0.5 is satisfied at the same time.

  Where ΔN is the absolute value of the difference between the maximum value and the minimum value of the refractive index with respect to the d-line of the lens constituting the imaging lens, and Δν is the maximum value of the Abbe number based on the d-line of the lens constituting the imaging lens. The absolute value of the difference from the minimum value, f is the focal length of the entire lens system, and f2 is the focal length of the second lens.

  According to the imaging lens of the present invention, the optical configuration is good with a lens configuration of 4 lenses as a whole, and in particular, while maintaining the monochromatic performance and the chromatic aberration of magnification, the optical total length is shorter, the sensor incident angle is smaller, and the focal depth is reduced. An imaging lens that can be deepened and an imaging device using the imaging lens, for example, a mobile terminal or a smartphone can be realized.

  The “peripheral portion on the lens surface” means at least a region off the paraxial region on the lens surface. This “peripheral portion on the lens surface” can be, for example, outside the aperture diameter or outside the half height of the diagonal of the image plane. Further, the “peripheral portion on the lens surface” is not necessarily limited to the case including the outermost peripheral portion within the effective diameter on the lens surface.

  An image pickup apparatus according to the present invention includes the four-lens image pickup lens and an image pickup element that picks up an optical image formed through the image pickup lens.

  In the imaging lens of the present invention, the optical performance can be further improved by satisfying the following preferable configuration.

  The lens surface on the image side of the fourth lens preferably has a positive power in a region including the outermost peripheral portion within the effective diameter.

  The imaging lens preferably has an Abbe number value of 50 or more and 60 or less for the d-line in any of the four lenses constituting the imaging lens.

  In the imaging lens, it is preferable that an aperture stop is disposed closer to the object side than the image side surface of the first lens.

  The imaging lens of the present invention includes the following conditional expressions (1 ′), (2 ′), (3 ′), (3 ″), (4), (4 ′), (5), (5 ′), ( It is preferable that any one of 6) to (12) is satisfied.In addition, as a preferable aspect, conditional expressions (1 ′), (2 ′), (3 ′), (3 ″), (4), ( 4 ′), (5), (5 ′), (6) to (12) may be satisfied, or any combination may be satisfied.

0 ≦ ΔN ≦ 0.05 (1 ′)
0 ≦ Δν ≦ 3 (2 ′)
−1.6 <f2 / f <−0.7 (3 ′)
-1.7 <f2 / f <-0.8 (3 ")
0.55 <f1 / f <1.0 (4)
0.6 <f1 / f <0.9 (4 ′)
0.0 ≦ f3 / f <1.5 (5)
0.4 <f3 / f <1.0 (5 ′)
-2.5 <f4 / f <0.0 (6)
0.01 <(ra4 + rb4) / (ra4-rb4) <1.30 (7)
0.01 <Sg12 / f <0.12 (8)
0.020 <U2 / f <0.240 (9)
0.020 <Sg34 / f <0.12 (10)
0.75 ≦ f12 / f ≦ 10 (11)
1.6 ≦ f / ra1 ≦ 3.0 (12)
However,
ΔN: absolute value of the difference between the maximum value and the minimum value of the refractive index for the d-line of the lens constituting the imaging lens Δν: the difference between the maximum value and the minimum value of the Abbe number for the d-line of the lens constituting the imaging lens Absolute value f: focal length of the entire lens system f1: focal length of the first lens f2: focal length of the second lens f3: focal length of the third lens f4: focal length of the fourth lens f12: first lens and second lens The combined focal length of the lens.

ra4: radius of curvature of the object side lens surface of the fourth lens rb4: radius of curvature of the image side lens surface of the fourth lens Sg12: air gap Sg34 between the first lens and the second lens Sg34: between the third lens and the fourth lens Air space U2 between: thickness of second lens (thickness on optical axis)
ra1: radius of curvature at the object-side lens surface of the first lens,
And

  An image pickup apparatus according to the present invention includes the four-lens image pickup lens.

  According to the imaging lens and the imaging apparatus of the present invention, the optical performance is good, in particular, the monochromatic performance and the chromatic aberration of magnification are maintained, the optical total length can be further shortened, the sensor incident angle can be reduced, and the depth of focus can be increased. A four-lens imaging lens can be realized. By using this imaging lens, an imaging device with a deep focal depth can be realized.

Sectional drawing which shows schematic structure of the imaging lens by embodiment of this invention Sectional drawing which shows schematic structure of the imaging lens of Example 1. FIG. Sectional drawing which shows schematic structure of the imaging lens of Example 2. Sectional drawing which shows schematic structure of the imaging lens of Example 3. Sectional drawing which shows schematic structure of the imaging lens of Example 4. Sectional drawing which shows schematic structure of the imaging lens of Example 5. Sectional drawing which shows schematic structure of the imaging lens of Example 6. FIG. 4 is an aberration diagram of the imaging lens of Example 1, where (a) shows spherical aberration, (b) shows astigmatism (field curvature), (c) shows distortion, and (d) shows lateral chromatic aberration. FIG. 4A is an aberration diagram of the imaging lens of Example 2, where (a) shows spherical aberration, (b) shows astigmatism (field curvature), (c) shows distortion, and (d) shows lateral chromatic aberration. FIG. 4A is an aberration diagram of the imaging lens of Example 3, where (a) shows spherical aberration, (b) shows astigmatism (field curvature), (c) shows distortion, and (d) shows lateral chromatic aberration. FIG. 6A is an aberration diagram of the imaging lens of Example 4. (a) shows spherical aberration, (b) shows astigmatism (field curvature), (c) shows distortion, and (d) shows lateral chromatic aberration. FIG. 6A is an aberration diagram of the imaging lens of Example 5. (a) shows spherical aberration, (b) shows astigmatism (field curvature), (c) shows distortion, and (d) shows lateral chromatic aberration. FIG. 9A is an aberration diagram of the imaging lens of Example 6. (a) shows spherical aberration, (b) shows astigmatism (field curvature), (c) shows distortion, and (d) shows lateral chromatic aberration. The figure which shows the mobile telephone terminal which is an example of the imaging device provided with the imaging lens which concerns on this invention. The figure which shows the smart phone which is an example of the imaging device provided with the imaging lens which concerns on this invention.

  The present invention relates to an image pickup lens for forming an optical image of a subject on an image pickup device such as a CCD or CMOS, a portable terminal device that mounts the image pickup lens for shooting, a digital still camera, a mobile phone with a camera, and information The present invention relates to an imaging device such as a terminal.

  Hereinafter, a four-lens imaging lens and an imaging apparatus including the imaging lens according to the present invention will be described with reference to the drawings.

  FIG. 1 is a cross-sectional view showing a schematic configuration of a portable terminal equipped with an imaging device including an imaging lens having a four-lens configuration according to the present invention. Note that arrows X, Y, and Z in FIG. 1 indicate three directions orthogonal to each other, as in FIGS. 2 to 8 described later, and the arrow Z direction indicates the same direction as the optical axis Z1. .

  A portable terminal 200 shown in FIG. 1 includes an imaging device 150 including an imaging lens 100 having a four-element structure and an imaging element 210 that is a solid-state imaging element such as a CCD or a CMOS. An image processing unit 220 that performs image processing on the image data Gs obtained by imaging by the imaging element 210 and a display unit 230 that displays an image represented by the processed image data Gse generated by the image processing unit 220 are provided. ing.

  The imaging element 210 converts an optical image Im representing the subject 1 formed on the imaging surface 210J of the imaging element 210 through the imaging lens 100 into an electrical signal, and outputs image data Gs indicating the optical image Im. It is.

  The imaging lens 100 has, in order from the object side (the arrow-Z direction side in the figure), the first lens L1 having a positive power in which the object-side lens surface Sa1 forms a convex surface in the paraxial region, and has a negative power. In the second lens L2, the third lens L3 having a positive power in which the object-side lens surface Sa3 forms a concave surface in the paraxial region, and the lens surface Sb4 having a negative power on the image side, lenses in the periphery thereof The surface is substantially composed of four lenses having a fourth lens L4 having a positive power. Conditional expression (1): 0 ≦ ΔN ≦ 0.1, conditional expression (2): 0 ≦ Δν ≦ 10 Conditional expression (3): −2.0 <f2 / f <−0.5 is satisfied at the same time.

  In FIG. 1, the lens surface on the image side of the first lens L1 is indicated by a symbol “Sb1”, the lens surface on the object side of the second lens L2 is indicated by a symbol “Sa2”, and the image side of the second lens L2 The lens surface is denoted by “Sb2”, the image side lens surface of the third lens L3 is denoted by “Sb3”, and the object side lens surface of the fourth lens L4 is denoted by “Sa4”.

  The configuration relating to the imaging lens 100 is an essential configuration for the imaging lens of the present invention.

  Hereinafter, selectable configurations for the imaging lens of the present invention will be described.

  A region other than the “peripheral portion on the lens surface of the fourth lens L4”, for example, the paraxial region on the lens surface Sb4 and the outermost peripheral portion within the effective diameter are regions having positive power or regions having no power. Or an area with negative power. More specifically, the paraxial region on the lens surface Sb4 and the outermost peripheral portion within the effective diameter can be a convex surface, a flat surface, or a concave surface.

  The imaging lens 100 may include an optical element LL having no power and an aperture stop St. The optical element LL is desirably a filter having a narrow wavelength band, but is not limited to such a case, and may be a low-pass filter or a filter that cuts a specific wavelength band. Further, the optical element LL is not limited to the case where the optical element LL is disposed between the imaging lens 100 and the imaging element 210, and can also be disposed between the object side of the imaging lens 100 and between the lenses constituting the imaging lens 100. Further, the aperture stop St can be disposed between the lenses constituting the imaging lens 100, disposed on the object side or the image side of the imaging lens 100, or disposed inside the lens constituting the imaging lens 100. .

  Here, prior to description of specific embodiments, matters common to the embodiments will be described.

  The imaging lens 100 of each embodiment includes, in order from the object side, a first lens having a positive power in which a lens surface on the object side forms a convex surface in a paraxial region, a second lens having a negative power, and an object side lens surface. A third lens having a positive power in which the lens surface forms a concave surface in the paraxial region, and a fourth lens having a negative power and a peripheral portion of the lens surface on the image side having a positive power. It is composed of substantially four lenses, and conditional expression (1): 0 ≦ ΔN ≦ 0.1, conditional expression (2): 0 ≦ Δν ≦ 10, conditional expression (3): −2.0 < It is configured to satisfy f2 / f <−0.5 at the same time. Since it is configured as described above, it is possible to obtain good optical performance, and in particular, while maintaining the monochromatic performance and chromatic aberration of magnification, it is possible to shorten the optical total length, reduce the sensor incident angle, and increase the depth of focus. it can.

  Regarding the fourth lens, “the peripheral portion of the lens surface on the image side has positive power” specifically means that, for example, even if the lens surface on the image side is a concave surface near the axis, The peripheral portion has a convex shape and the like, and this makes it possible to reduce the sensor incident angle of the peripheral light even if the optical total length is short. Specifically, the peripheral portion can be outside the aperture diameter or outside the half height of the image plane diagonal dimension. Further, the outermost peripheral portion of the lens surface on the image side may be concave for the purpose of securing the thickness of the lens.

  According to the imaging lens 100, by allowing the occurrence of axial chromatic aberration, the optical total length can be shortened, the sensor incident angle can be reduced, and the depth of focus can be increased while maintaining the optical performance of each single color. it can. In addition, since the imaging lens 100 allows the generation of axial chromatic aberration, it is not necessary to include an achromatic lens structure more than necessary, and can be more easily downsized.

  In addition, by configuring each lens with substantially the same optical characteristics, that is, with materials having substantially the same refractive index and Abbe number, even if axial chromatic aberration is large, chromatic aberration of magnification (difference in magnification due to color) can be reduced. It becomes possible to make it smaller.

  Furthermore, since the lens power (power in the paraxial region) becomes positive, negative, positive and negative in order from the first lens, it is possible to further reduce the chromatic aberration of magnification. Note that even if the chromatic aberration of magnification is simply corrected, good optical performance cannot be obtained if the occurrence of spherical aberration, coma, astigmatism, field curvature, and distortion in a single color is large. As described above, by adopting positive / negative / positive / negative configurations, it becomes possible to satisfactorily correct aberrations in those single colors.

  If the upper limit of conditional expression (1): 0 ≦ ΔN ≦ 0.1 is exceeded, it becomes difficult to correct curvature of field while increasing the depth of focus.

  If the upper limit of conditional expression (2): 0 ≦ Δν ≦ 10 is exceeded, it will be difficult to correct lateral chromatic aberration while increasing the depth of focus.

  On the other hand, if the upper limit of conditional expression (3): −1.6 <f2 / f <−0.7 is exceeded, the power of the second lens becomes too large to maintain a short optical total length. . Or, it is disadvantageous for correction of curvature of field and securing of peripheral light quantity. Also, a predetermined sensor incident angle (image side telecentricity) cannot be obtained.

  On the other hand, if the lower limit of conditional expression (3) is not reached, the astigmatic difference from the intermediate field angle to the peripheral field angle increases. Further, there arises a problem that the optical total length becomes large.

  Note that if the imaging lens 100 is configured to exceed the upper limit of conditional expression (4): 0.55 <f1 / f <1.0, the power of the first lens becomes too small, and the configuration is like a retrofocus type. This is disadvantageous in reducing the optical total length. In addition, it becomes difficult to balance the lens design, for example, barrel distortion tends to occur when trying to maintain the entire optical length.

  On the other hand, if the imaging lens 100 is configured so as to fall below the lower limit of the conditional expression (4), the power of the first lens becomes too large, and the power relationship with other lenses collapses from a predetermined relationship. The curvature of field becomes under (undercorrected), and the balance between distortion and spherical aberration tends to be lost. In addition, the peripheral light amount becomes small.

  If the upper limit of conditional expression (5): 0.0 ≦ f3 / f <1.5 is exceeded, the optical total length tends to increase, and the curvature of field is corrected if an increase in the optical total length is to be suppressed. It becomes difficult to do.

  On the other hand, if the lower limit of the conditional expression (5) is not reached, the power of the third lens becomes negative, the axial aberration and the astigmatic difference are deteriorated, and barrel distortion is likely to occur. In addition, it is difficult to keep the emission angle (sensor incident angle) small and within a predetermined range.

  If the upper limit of conditional expression (6): −2.41 <f4 / f <0.0 is exceeded, the power of the fourth lens L4 becomes too strong, and the field curvature becomes under (undercorrected). In addition, barrel distortion increases. On the other hand, if the lower limit of conditional expression (6) is not reached, the power of the fourth lens becomes too weak, and the image side telecentricity deteriorates.

  Further, by satisfying conditional expression (7): 0.01 <(ra4 + rb4) / (ra4-rb4) <1.30 and conditional expression (8): 0.01 <Sg12 / f <0.12, Specifically, by setting the bending of the fourth lens within the range defined by the conditional expression (7) and the air gap between the first lens and the second lens within the range defined by the conditional expression (8), chromatic aberration is achieved. The occurrence of various aberrations other than the above can be suppressed, the image side telecentricity can be ensured (the incident angle to the imaging surface can be reduced), and the total optical length can be increased while ensuring the back focus. Can be shortened.

  If the upper limit of conditional expression (7) is exceeded, the image side telecentricity will deteriorate. On the other hand, if the lower limit of conditional expression (7) is not reached, the amount of field curvature and distortion occurring in the intermediate image height region will increase.

  If the upper limit of conditional expression (8) is exceeded, the spherical aberration will be overcorrected and the curvature of field will be under-corrected (undercorrected) for rays passing through the peripheral edge within the effective diameter. On the other hand, if the lower limit of conditional expression (8) is not reached, for each light beam, the difference between the light beam height at the image side lens surface of the first lens and the light beam height at the object side lens surface of the second lens is small. Become. Therefore, if the lens surface on the image side of the first lens and / or the lens surface on the object side of the second lens are aspheric surfaces, the effect of using the aspheric surfaces cannot be fully utilized. In addition, the spherical aberration is insufficiently corrected for the light ray passing through the peripheral edge within the effective diameter, and the field curvature is over (overcorrected).

  If the imaging lens 100 is configured to satisfy the conditional expression (9): 0.020 <U2 / f <0.240, specifically, the thickness of the second lens and the focal length of the entire lens system Is within the range defined by the conditional expression (9), the occurrence of spherical aberration and field curvature can be suppressed.

  If the imaging lens 100 is configured so as to exceed the upper limit of the conditional expression (9), the spherical aberration is excessively corrected, more specifically, the correction of light rays passing through the peripheral edge of the pupil is excessive, and the curvature of field is increased. Becomes under (undercorrection). On the other hand, when the imaging lens 100 is configured so as to fall below the lower limit of the conditional expression (9), the spherical aberration is insufficiently corrected, more specifically, the correction of light rays passing through the peripheral edge of the pupil is insufficient, and the curvature of field is reduced. Becomes over (overcorrection).

  Further, by satisfying conditional expression (10): 0.020 <Sg34 / f <0.12, specifically, the ratio between the thickness of the second lens and the focal length of the entire lens system is expressed by this conditional expression ( Within the range defined in 10), the occurrence of coma and astigmatism can be suppressed, and the image side telecentricity can be enhanced.

  Further, if the imaging lens 100 is configured so as to exceed the upper limit of the conditional expression (10), the light ray height on the image side lens surface of the third lens in the peripheral rays and the object side lens surface of the fourth lens are set. As a result, the amount of coma aberration increases and the effect on astigmatism increases, and it is difficult to keep the sensor incident angle small (maintain image side telecentricity). On the other hand, if the imaging lens 100 is configured so as to fall below the lower limit of the conditional expression (10), the field curvature will be under (undercorrected).

  If the lens surface on the object side of the second lens is concave in the paraxial region, it is disadvantageous for reducing the sensor incident angle, but it is advantageous for shortening the optical total length.

  If the second lens has negative power, it is advantageous in reducing the Petzval sum.

  It is desirable that each lens of the first lens to the fourth lens is formed with at least one aspherical surface. That is, each of the first lens L1 to the fourth lens L4 is preferably a double-sided aspherical lens or a single-sided aspherical lens.

  If the second lens has negative power in the paraxial region, the Petzval sum can be brought close to zero.

  The lens surface on the image side of the fourth lens has a extreme point (maximum point, minimum point) at a position other than the apex (position intersecting the optical axis on the lens surface) and deviating from the paraxial region, If the region including the outermost peripheral portion (peripheral portion) within the effective diameter has a positive power, the peripheral image height region of the optical image formed through the imaging lens 100 (the region where the image height is larger than the aperture diameter). ) Can improve telecentricity.

  If the lens surface on the image side of the fourth lens is concave in the paraxial region, it is disadvantageous for reducing the sensor incident angle, but it is advantageous for shortening the optical total length.

  Further, if the imaging lens 100 is configured so that the stop is disposed on the object side of the first lens, the image-side telecentricity is improved (the emission angle is nearly parallel to the optical axis), so that the total optical length is reduced. It can be shortened.

  In addition, the value of the numerical formula in each said conditional expression is calculated | required in consideration of the positive / negative of the value of a curvature radius or a focal distance. The radius of curvature of the lens surface is positive when convex on the object side and negative when convex on the image side.

  In addition, when the lens surface is an aspheric surface, unless otherwise specified, the unevenness of the lens surface, the positive / negative of the radius of curvature, and the positive / negative of the power (refractive power) are the unevenness and the radius of curvature in the paraxial region, respectively. , And power (refractive power). In addition, the unevenness | corrugation about the specific area | region in a lens surface, the positive / negative of a curvature radius, and the positive / negative of power (refractive power) shall determine an unevenness | corrugation and positive / negative for every area | region.

  The “imaging lens 100 substantially consisting of four lenses” means a lens having substantially no power other than the four lenses, an optical element other than a lens such as an aperture or a cover glass, a lens flange, It also includes a lens barrel, an image sensor, a mechanism portion such as a camera shake correction mechanism, and the like. As described above, the imaging lens 100 including four lenses may be configured by only four lenses, or other than the four lenses other than a lens or a lens having no power. It may have an optical element or the like.

  The term “consisting of n lenses” means that the lens is composed of n single lenses. In the case of including a cemented lens, the number of lenses that are composed of n lenses is n. The number of lenses is counted as being composed of the following lenses.

  The optical total length is obtained by passing, through the imaging lens 100, a focal position on the image side of the imaging lens 100 from the lens surface closest to the object side in the imaging lens 100 (a light beam (d-line) emitted from an infinite subject located on the object side). The distance on the optical axis to the convergence position). The distance from the lens surface disposed closest to the object side to the lens surface disposed closest to the image side is an actual length that is not converted into air, and the above focal point is determined from the lens surface disposed closest to the image side. The distance to the position (that is, the back focus) is determined using the air-converted back focus.

  Further, it is desirable that the first lens and the second lens constituting the imaging lens 100 satisfy the conditional expression (11): 0.75 ≦ f12 / f ≦ 10, and the conditional expression (11 ′): 0.75 ≦. It is more preferable that f12 / f ≦ 2.5 is satisfied, and it is further more preferable that conditional expression (11 ″): 1.25 ≦ f12 / f ≦ 2.5 is satisfied. The combined focal length of two lenses.

  Conditional expression (11) defines the range of the ratio between the combined focal length f12 of the first lens and the second lens and the focal length f of the entire lens system of the imaging lens 100.

  If the upper limit of conditional expression (11) is exceeded, it approaches the retrofocus type and the principal point moves to the image side, so that it becomes difficult in principle to shorten the optical total length. On the other hand, if the lower limit of conditional expression (11) is not reached, the curvature of field will be too low, or the amount of peripheral light will be too small.

  It is desirable that the object-side lens surface of the first lens constituting the imaging lens 100 satisfies the conditional expression (12): 1.6 ≦ f / ra1 ≦ 3.0. Here, ra1 is the radius of curvature of the lens surface on the object side of the first lens, and f is the focal length of the entire lens system of the imaging lens 100.

  This conditional expression (12) greatly affects the optical total length, the angle of view, and the angle of the light beam emitted to the image side (image side telecentricity). The range which keeps good property is prescribed | regulated.

  If the imaging lens 100 is configured so as to exceed the upper limit of the conditional expression (12), the spherical aberration becomes under (undercorrected), and the distortion becomes too negative (barrel type). On the other hand, if the imaging lens 100 is configured so as to fall below the lower limit of the conditional expression (12), it is disadvantageous for suppressing an increase in the total optical length.

  The image-side lens surface of the third lens constituting the imaging lens 100 desirably satisfies the conditional expression (13): 1.5 <| f / rb3 | <4.5. Here, rb3 is a curvature radius on the image side lens surface of the third lens.

  Conditional expression (13) defines the range of the ratio between the focal length f of the entire lens system of the imaging lens 100 and the radius of curvature rb3 on the image-side lens surface of the third lens.

  If the upper limit of conditional expression (13) is exceeded, the incident angle (sensor incident angle) on the imaging surface in the intermediate image height region becomes large (the image side telecentricity deteriorates), or the field curvature is over (corrected). Excessive), and pincushion-like distortion is likely to occur. On the other hand, if the lower limit of conditional expression (13) is not reached, the incident angle on the imaging surface in the peripheral image height region becomes large (telecentricity in the peripheral image height region deteriorates).

  The intermediate image height region is a region in the optical image that is located near the intermediate position between the position where the optical image formed through the imaging lens 100 intersects the optical axis and the maximum image height position of the optical image.

  The imaging lens 100 desirably satisfies conditional expression (14): 0.9 ≦ TL / f ≦ 1.25, and satisfies conditional expression (14 ′): 1.00 ≦ TL / f ≦ 1.23. It is more desirable to do.

  Conditional expression (14) defines the range of the ratio between the optical total length TL and the focal length f of the entire lens system of the imaging lens 100.

  If the lower limit of conditional expression (14) is not reached, the power of each lens must be increased, so that various aberrations are deteriorated and the optical performance is deteriorated. In particular, since the on-axis light beam and the off-axis light beam are too close to each other in the third lens and the fourth lens, there arises a problem that it is difficult to balance aberration for both the on-axis light beam and the off-axis light beam at the same time. On the other hand, if the upper limit of conditional expression (14) is exceeded, it is difficult to reduce the size of the imaging lens 100.

  The imaging lens 100 desirably satisfies the conditional expression (15): 100 ≦ f * f / (f−fF) ≦ 400, and the conditional expression (15 ′): 120 ≦ f * f / (f−fF). It is further desirable to satisfy ≦ 280. However, fF is the focal length of the entire lens system of the imaging lens 100 for the F line, and f is the focal length of the entire lens system of the imaging lens 100 for the d line. The numerical expression in the conditional expression (15) uses [mm] as a unit.

  Here, since the color misregistration is easily recognized by humans as the red wavelength (on the d-line basis, the longer wave side than the d-line), it is not desired to increase the red-axis chromatic aberration. Therefore, it is possible to add a greater weight to the blue wavelength and correct it to white in the post-process of the sensor output image, on the axis of the blue wavelength and the reference wavelength. Correction of chromatic aberration is most important in producing a white resolution depth.

  If the lower limit of the conditional expression (15) is not reached, the adverse effect due to the difference in the color of light emitted from the subject to be imaged becomes excessive (the difference in focal length increases more than necessary), and the white resolution is reduced. It becomes difficult to form an optical image capable of generating an image having a deep depth.

  If the upper limit of conditional expression (15) is exceeded, it becomes difficult to use the difference in focal length due to the difference in the color of light emitted from the subject to be imaged. It is difficult to achieve the purpose of forming an optical image that can be generated.

  In the imaging lens 100, if all four lenses are plastic lenses, an aspheric plastic lens can be easily formed by injection molding, the weight can be reduced, and the autofocus mechanism can be easily carried. The burden of current consumption for driving can be reduced. Such an imaging lens 100 is suitable for application to a portable terminal.

  In addition, since the imaging lens 100 can make the optical performance in a single color uniform for each region in the imaging surface, a filter having a narrow wavelength band is arranged in the optical path, or a monochrome image or a specific single color. It is also suitable for application to an imaging device for acquiring an image and an imaging device that performs image processing on the acquired image data to increase the depth of focus.

  The lens surface Sb4 on the image side of the fourth lens L4 in the imaging lens 100 may have a positive power in a region including the outermost peripheral portion within the effective diameter.

  The imaging lens 100 may further have the following configuration.

  The object-side lens surface Sa2 of the second lens L2 may be concave in the paraxial region.

  Each of the first lens L1 to the fourth lens L4 is preferably a double-sided aspherical lens or a single-sided aspherical lens.

  The image-side lens surface Sb4 of the fourth lens L4 has a maximum point (maximum point, minimum point) at a position deviating from the paraxial region, and an outermost peripheral portion (peripheral portion) within the effective diameter of the lens surface Sb4. It can have positive power in the containing region.

  The image-side lens surface Sb4 of the fourth lens L4 can be a concave surface in the paraxial region.

  In addition, a diaphragm St is disposed on the object side of the first lens L1, a diaphragm St is disposed between the first lens L1 and the second lens L2, or a diaphragm St is disposed inside the first lens L1. You can do it. For example, when the first lens L1 is a plastic lens, the first lens L1 including the diaphragm St can be formed by insert molding so that the diaphragm St is disposed inside the first lens L1. .

  In the imaging lens 100, all four lenses (the first lens L1 to the fourth lens L4) can be plastic lenses.

  Next, the operation of the mobile terminal 200 will be described.

  In the portable terminal 200, an optical image Im formed through the imaging lens 100 and intentionally generating axial chromatic aberration is captured by the image sensor 210 to acquire image data Gs.

  Next, the image processing unit 220 performs image processing on the image data Gs input from the image sensor 210 to generate processed image data Gse.

  The display unit 230 displays an image represented by the processed image data Gse input from the image processing unit 220.

  Next, with respect to Examples 1 to 7 showing specific numerical data related to the four-lens imaging lens of the present invention, FIGS. 2 to 7, FIGS. 8 to 13, Tables 1A, 1B, 2A, 2B,. Description will be made with reference to 6A, 6B and Table 7. In addition, the code | symbol in FIGS. 2-7 which corresponds with the code | symbol in FIG. 1 which shows the above-mentioned imaging lens 100 has shown the component corresponding to each other.

<Example 1>
Example 1 will be described below.

  FIG. 2 is a cross-sectional view illustrating a schematic configuration of the imaging lens of the first embodiment.

  Table 1A below shows lens data of the imaging lens of Example 1. In the lens data shown in Table 1A, the surface number i is the surface of the i-th (i = 1, 2, 3,...) Surface Si that sequentially increases toward the image side with the surface disposed closest to the object side being first. Indicates the number. The lens data in Table 1A are assigned surface numbers including the aperture St, the optical element LL having no power, and the imaging surface 210J on which the optical image Im is formed.

  The symbol Ri in Table 1A indicates the radius of curvature of the i-th (i = 1, 2, 3,...) Surface, and the symbol Di indicates the i (i = 1, 2, 3,...) -Th surface and the i + 1-th surface. The space | interval on the optical axis Z1 with a surface is shown. Symbols Ri and Di correspond to numbers corresponding to symbols Si (i = 1, 2, 3,...) Indicating lens surfaces, diaphragms, and the like.

  Further, the symbol Ndj in Table 1A is the d-line (wavelength 587...) Of the j-th (j = 1, 2, 3,...) Optical element that sequentially increases toward the image side with the most optical element on the object side as the first. 6 nm), and νdj represents the Abbe number based on the d-line of the j-th optical element. In Table 1A, the unit of curvature radius and surface interval is mm, and the curvature radius is positive when convex on the object side and negative when convex on the image side.

  In the lens data in Table 1A, the surface number is marked with * for the aspheric surface, and the value of the radius of curvature Ri of the aspheric surface indicates the value of the paraxial curvature radius.

In addition, since the optical system as described above can maintain a predetermined performance even when the dimensions of optical elements such as lenses are proportionally enlarged or proportionally reduced, the entire lens data is proportionally enlarged or proportionally reduced. The imaging lens can also be an embodiment according to the present invention.

  Further, Table 1B shows aspheric coefficients related to the imaging lens of Example 1. In other words, Table 1B shows the aspheric coefficients related to the aspheric surfaces with surface marks marked with * in Table 1A.

  The aspheric coefficients listed in Table 1B are created so that the aspheric shape is determined by applying the following aspheric formula.

Zf (ρ) = C · ρ ² / {1+ (1-K · C ² · ρ ² ) ^1/2 } + ΣAn · | ρ | ⁿ
However,
Zf (ρ): Depth of aspheric surface (mm)
ρ: distance (height) from the optical axis to the lens surface (mm), ρ = X ² + Y ²
K: aspheric coefficient representing a quadratic surface C: paraxial curvature = 1 / R (R: paraxial radius of curvature)
An: n-th order (n is an integer of 3 or more) aspheric coefficient

  8A to 8E show aberration diagrams of the imaging lens of Example 1. FIG. 8A shows spherical aberration (axial chromatic aberration), FIG. 8B shows astigmatism, FIG. 8C shows distortion, and FIG. 8D shows lateral chromatic aberration. FIG.

  In the astigmatism diagram (b), the solid line indicates the sagittal aberration, and the broken line indicates the tangential aberration. Further, “FNo. 3.23” described at the top of the spherical aberration diagram (a) means that the F number is 3.23. “Ω = 32.6 °” described at the top of the other aberration diagrams (b) to (d) means that the half angle of view is 32.6 °.

  Hereinafter, Table 7 will be described.

  Table 7 shows values and optical total lengths corresponding to mathematical expressions and variable symbols in the conditional expressions for the imaging lenses of Example 1 and Examples 2 to 6 to be described later. As can be seen from Table 7, the imaging lens of Example 1 satisfies all of the conditional expressions (1) to (15). Table 7 is shown at the end of the description of the examples.

  FIG. 2 showing the configuration of the imaging lens of Example 1 above, FIGS. 8A to 8D showing aberrations, Tables 1A and 1B showing lens data, and how to read Table 7 will be described later. The same applies to the diagrams and tables corresponding to 2 to 6, and therefore, the description of the examples described later is omitted.

<Example 2>
Example 2 will be described below.

  FIG. 3 is a cross-sectional view illustrating a schematic configuration of the imaging lens of the second embodiment.

  As can be seen from Table 7 above, the imaging lens of Example 2 satisfies all of the conditional expressions (1) to (15).

  9A to 9D are graphs showing various aberrations of the imaging lens of Example 2. FIG.

Table 2A below shows lens data of the imaging lens of Example 2, and Table 2B shows aspheric coefficients of the imaging lens of Example 2.

<Example 3>
Example 3 will be described below.

  FIG. 4 is a cross-sectional view illustrating a schematic configuration of the imaging lens of the third embodiment.

  As can be seen from Table 7, the imaging lens of Example 3 satisfies all conditional expressions (1) to (15).

  10A to 10D are diagrams illustrating various aberrations of the imaging lens of Example 3. FIG.

Table 3A below shows lens data of the imaging lens of Example 3, and Table 3B shows aspheric coefficients of the imaging lens of Example 3.

<Example 4>
Hereinafter, Example 4 will be described.

  FIG. 5 is a cross-sectional view illustrating a schematic configuration of the imaging lens of the fourth embodiment.

  As can be seen from Table 7, the imaging lens of Example 4 satisfies all of the conditional expressions (1) to (15).

  FIGS. 11A to 11D are diagrams illustrating various aberrations of the imaging lens of Example 4. FIGS.

Table 4A below shows lens data of the imaging lens of Example 4, and Table 4B shows aspheric coefficients of the imaging lens of Example 4.

<Example 5>
Example 5 will be described below.

  FIG. 6 is a cross-sectional view illustrating a schematic configuration of the imaging lens of the fifth embodiment.

  As understood from Table 7, the imaging lens of Example 5 satisfies all the conditional expressions other than the conditional expression (7).

  12A to 12D are diagrams illustrating various aberrations of the imaging lens of Example 5. FIG.

Table 5A below shows lens data of the imaging lens of Example 5, and Table 5B shows aspheric coefficients of the imaging lens of Example 5.

<Example 6>
Example 6 will be described below.

  FIG. 7 is a cross-sectional view illustrating a schematic configuration of the imaging lens of the sixth embodiment.

  As can be seen from Table 7, the imaging lens of Example 6 satisfies all conditional expressions (1) to (15).

  FIGS. 13A to 13D are graphs showing various aberrations of the imaging lens of Example 6. FIGS.

Table 6A below shows lens data of the imaging lens of Example 6, and Table 6B shows aspheric coefficients of the imaging lens of Example 6.

Table 7 below shows values and optical total lengths corresponding to the respective mathematical expressions and variable symbols in the conditional expressions for the imaging lenses of Examples 1 to 6, as described above.

  In the above embodiment, the example in which the present invention is applied to a portable terminal has been illustrated and described. However, the present invention is not limited to such applications. For example, a digital still camera, a portable terminal with camera, Or it is applicable also to an information terminal (PDA), a smart phone, a portable game machine, etc.

  FIG. 14 is an external view of a camera-equipped mobile terminal M1 that is an example of the imaging apparatus of the present invention. The camera-equipped portable terminal M1 includes an imaging lens ML1 according to an embodiment of the present invention, a CCD that captures an optical image formed by the imaging lens ML1, and outputs an imaging signal corresponding to the optical image. The image pickup device MS1 is provided. The image sensor MS1 is disposed on the imaging plane (imaging plane) of the imaging lens ML1.

  FIG. 15 shows an external view of a smartphone M2 that is an example of the imaging apparatus of the present invention. The smartphone M2 captures an imaging lens ML2 according to the embodiment of the present invention and an imaging element MS2 such as a CCD that captures an optical image formed by the imaging lens ML2 and outputs an imaging signal corresponding to the optical image. Is provided. The imaging element MS2 is disposed on the imaging surface (imaging surface) of the imaging lens ML2.

  In addition, this invention is not limited to the said embodiment and Example, A various deformation | transformation is possible. For example, the values of the radius of curvature, the surface spacing, the refractive index, the Abbe number, etc. of each lens element are not limited to the values shown in the above numerical examples, but can take other values.

L1 First lens L2 Second lens L3 Third lens L4 Fourth lens Sa1 Lens side on the object side of the first lens Sa3 Lens surface on the object side of the third lens Sb4 Lens surface on the image side of the fourth lens

Claims (20)

In order from the object side, the first lens having a positive power in which the lens surface on the object side forms a convex surface in the paraxial region, the second lens having a negative power, and the lens surface on the object side having a concave surface in the paraxial region. Four lenses substantially including a third lens having a positive power and a fourth lens having a negative power and a peripheral portion on the lens surface on the image side having a positive power. Consists of
An imaging lens having a four-lens structure, which satisfies the following conditional expressions (1), (2), and (3) simultaneously:
0 ≦ ΔN ≦ 0.1 (1)
0 ≦ Δν ≦ 10 (2)
−2.0 <f2 / f <−0.5 (3)
However,
ΔN: absolute value of the difference between the maximum value and the minimum value of the refractive index for the d-line of the lens constituting the imaging lens Δν: the difference between the maximum value and the minimum value of the Abbe number for the d-line of the lens constituting the imaging lens Absolute value f: focal length of the entire lens system f2: focal length of the second lens The imaging lens according to claim 1, further satisfying the following conditional expression (4):
0.55 <f1 / f <1.0 (4)
However,
f1: Focal length of the first lens The imaging lens according to claim 1 or 2, wherein the following conditional expression (5) is satisfied.
0.0 ≦ f3 / f <1.5 (5)
However,
f3: focal length of the third lens The imaging lens according to any one of claims 1 to 3, wherein the imaging lens further satisfies the following conditional expression (6).
-2.5 <f4 / f <0.0 (6)
However,
f4: focal length of the fourth lens The imaging lens according to any one of claims 1 to 4, wherein the following conditional expression (7) is satisfied.
0.01 <(ra4 + rb4) / (ra4-rb4) <1.30 (7)
However,
ra4: radius of curvature of the object side lens surface of the fourth lens rb4: radius of curvature of the image side lens surface of the fourth lens The imaging lens according to claim 1, wherein the following conditional expression (8) is satisfied.
0.01 <Sg12 / f <0.12 (8)
However,
Sg12: Air distance between the first lens and the second lens The imaging lens according to claim 1, wherein the following conditional expression (9) is satisfied.
0.020 <U2 / f <0.240 (9)
However,
U2: thickness of the second lens (thickness on the optical axis) The imaging lens according to claim 1, wherein the following conditional expression (10) is satisfied.
0.020 <Sg34 / f <0.12 (10)
However,
Sg34: Air distance between the third lens and the fourth lens   9. The imaging lens according to claim 1, wherein an image side lens surface of the fourth lens has a positive power in a region including an outermost peripheral portion within an effective diameter. .   The imaging lens according to any one of claims 1 to 9, wherein any of the four lenses constituting the imaging lens has an Abbe number value of 50 or more and 60 or less with respect to the d-line.   The imaging lens according to claim 1, wherein an aperture stop is disposed closer to the object side than an image side surface of the first lens. The imaging lens according to claim 1, wherein the following conditional expression (1 ′) is satisfied.
0 ≦ ΔN ≦ 0.05 (1 ′) The imaging lens according to claim 1, wherein the following conditional expression (2 ′) is satisfied.
0 ≦ Δν ≦ 3 (2 ′) The imaging lens according to claim 1, wherein the following conditional expression (3 ′) is satisfied.
−1.6 <f2 / f <−0.7 (3 ′) The imaging lens according to any one of claims 1 to 14, wherein the following conditional expression (4 ') is satisfied.
0.6 <f1 / f <0.9 (4 ′)
However,
f1: Focal length of the first lens The imaging lens according to claim 1, wherein the following conditional expression (5 ′) is satisfied.
0.4 <f3 / f <1.0 (5 ′)
However,
f3: focal length of the third lens The imaging lens according to claim 1, wherein the following conditional expression (3 ″) is satisfied.
-1.7 <f2 / f <-0.8 (3 ") The imaging lens according to claim 1, wherein the following conditional expression (11) is satisfied.
0.75 ≦ f12 / f ≦ 10 (11)
However,
f12: The combined focal length of the first lens and the second lens. The imaging lens according to any one of claims 1 to 18, wherein the following conditional expression (12) is satisfied.
1.6 ≦ f / ra1 ≦ 3.0 (12)
However,
ra1: radius of curvature at the object-side lens surface of the first lens,   An imaging apparatus comprising the imaging lens according to claim 1.