JP2007212954A - Single-focus imaging lens and imaging apparatus equipped therewith - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a compact imaging lens that has proper optical performance. <P>SOLUTION: The single focus imaging lens 10 is constituted of a first lens (L1) having positive optical power, a diaphragm, a second lens (L2) and a third lens (L3) arranged, in this order starting from the object side, and satisfies a conditional Expression (1): ¾f/f<SB>N</SB>¾10<SP>-2</SP>, where f: focal distance of the entire system on a d-line, and f<SB>N</SB>: focal distance of either the second lens (L2) or the third lens (L3) on the d-line. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a single focus imaging lens and an imaging device including the same. In particular, a single focus imaging lens suitable for a small imaging device (for example, a digital still camera, a small camera for a portable information terminal, etc.) provided with a (solid) imaging device such as a CCD (Charge Coupled Device), and the like The present invention relates to an imaging apparatus.

  In recent years, there has been a rapid increase in needs for an imaging apparatus (for example, a digital still camera) that is compact and has a high pixel count of, for example, 5 million pixels or more. Also, portable information terminals such as mobile phones and PDA terminals have come to be equipped with small cameras having a relatively high number of pixels, for example, 1 million pixels or more. Under such circumstances, there is a strong demand for an imaging lens having a small size and high imaging performance that can be used for an imaging device having a high number of pixels.

  2. Description of the Related Art Conventionally, as a compact imaging lens, an imaging lens having one or two lenses is known. However, with an imaging lens having one or two lenses, it is difficult to sufficiently correct the field curvature, as is apparent from the aberration theory, and it is difficult to obtain sufficiently high optical performance. For this reason, in recent years, an imaging lens having a three-lens configuration has become common.

As such a three-lens imaging lens, for example, Patent Document 1 discloses, in order from the object side, a meniscus first lens having a negative optical power with a convex surface facing the object side, and an object side. An imaging lens is disclosed that includes a second lens having a positive optical power with a concave surface facing and a third lens having a negative optical power. Patent Document 1 describes that the imaging lens described in Patent Document 1 is inexpensive while being compatible with high pixels.
JP 2005-115309 A

  However, the three-lens imaging lens described in Patent Document 1 has a relatively long back focus. For this reason, there exists a problem that the full length of an imaging lens cannot be shortened enough.

  Moreover, there exists a problem that the imaging lens comprised by four or more lenses is large sized.

  The present invention has been made in view of such problems, and an object thereof is to provide an imaging lens having a simple configuration including three lenses, a short back focus, a small size, and high optical performance. It is to provide.

In order to achieve the above object, a single-focus imaging lens according to the present invention includes a first lens having a positive optical power, an aperture, a second lens, and a third lens arranged in this order from the object side. And satisfying the following conditional expression (1).
| F / f _N | <10 ⁻² (1)
However,
f: the focal length of the entire system at the d-line,
f _N : the focal length at the d line of either one of the second lens and the third lens,
It is.

An imaging apparatus according to the present invention includes a single-focus imaging lens that forms an optical image, and an imaging device that converts the optical image formed by the single-focus imaging lens into an electrical signal. In the imaging apparatus according to the present invention, the single focus imaging lens includes a first lens having a positive optical power, an aperture, a second lens, and a third lens arranged in this order from the object side. The following conditional expression (1) is satisfied.
| F / f _N | <10 ⁻² (1)
However,
f: the focal length of the entire system at the d-line,
f _N : the focal length at the d line of either one of the second lens and the third lens,
It is.

  According to the present invention, it is possible to provide a small imaging lens having good optical performance.

  Hereinafter, an example of an embodiment of a single focus imaging lens according to the present invention will be described with reference to the drawings.

  1 to 3 are schematic configuration diagrams of a single focus imaging lens 10 according to first to third embodiments, respectively.

  Any of the single-focus imaging lenses 10 of the first to third embodiments may be an imaging device (a solid imaging device, for example, a CCD (charge coupled device), a COMS (complementary metal-oxide semiconductor), etc.). Is for forming an optical image. As will be described in detail later, the single focus imaging lens 10 can be mounted on a digital camera, a camera equipped with a portable information terminal, or the like.

  In each of the first to third embodiments, the single focus imaging lens 10 is configured by three lenses of a first lens (L1), a second lens (L2), and a third lens (L3). Yes.

  For example, when the single focus imaging lens is composed of two or less lenses, it is difficult to simultaneously correct the field curvature and the axial chromatic aberration, as is apparent from the aberration theory. For this reason, it is difficult to achieve high optical performance even if many aspheric surfaces are used.

  High optical performance (imaging performance) can be realized only by configuring the single focus imaging lens with three or more lenses. That is, a single focus imaging lens in which various aberrations such as field curvature (distortion aberration) and axial chromatic aberration are suppressed can be realized.

  Note that, from the viewpoint of realizing higher imaging performance, it is preferable to configure the single focus imaging lens with a larger number of lenses (for example, four or more lenses). However, as the number of lenses constituting a single focus imaging lens increases, the total thickness of the lenses constituting the single focus imaging lens, the gap between the lenses, and the space of the lens barrel that houses the single focus imaging lens tend to increase. is there. For this reason, when the number of lenses increases, the single focus imaging lens tends to increase in size. Therefore, from the viewpoint of realizing a small single-focus imaging lens, it is preferable to reduce the number of lenses constituting the single-focus imaging lens.

  From the above, in order to simultaneously achieve miniaturization and high optical performance, it is most preferable to configure the single focus imaging lens 10 with three lenses. Specifically, it is preferable that the single-focus imaging lens 10 is configured by three lenses (L1 to L3) having relatively large optical power that affects the imaging performance. Additional optical elements (eg, lenses, filters, etc.) having a relatively small optical power that has little influence on the image performance may be included.

  In each of the first to third embodiments, a diaphragm (STP) (for example, an aperture diaphragm) is disposed between the first lens (L1) and the second lens (L2).

  From the viewpoint of realizing high telecentricity, it is preferable to arrange the aperture (STP) as close to the object as possible. Further, from the viewpoint of reducing the light incident angle on the imaging surface, it is preferable to dispose the stop (STP) as close to the object as possible. From these viewpoints, it is most preferable to dispose the diaphragm (STP) closer to the object than the first lens (L1). However, if the stop (STP) is disposed closer to the object than the first lens (L1), the amount of change in the light beam height at the periphery of the screen of the image sensor increases. For this reason, the performance deterioration degree of the single focus imaging lens 10 due to the attachment error of the aperture stop (STP) (the relative position shift between the aperture stop (STP) and each lens and the image sensor) increases. That is, it is necessary to attach a diaphragm (STP) to each lens and image sensor with high accuracy. Therefore, manufacture becomes difficult.

  As described above, from the viewpoint of realizing the single focus imaging lens 10 that is easy to manufacture while improving the telecentricity and relatively reducing the light incident angle to the imaging surface, the aperture (STP) is made the first lens (L1). And the second lens (L2). The diaphragm (STP) may be provided separately from the first lens (L1) and the second lens (L2). The aperture on the image side lens surface of the first lens (L1) or the aperture on the object side lens surface of the second lens (L2) may be used as a stop (STP).

  The first lens (L1) is a lens having positive optical power (hereinafter, may be referred to as “positive lens”). By using the first lens (L1) disposed closest to the object as a positive lens, the back focus can be shortened, and the small single focus imaging lens 10 can be realized. From the viewpoint of shortening the back focus and the overall length, it is preferable that the first lens (L1) has the main positive optical power in the first to third lenses.

  Specifically, the first lens (L1) is preferably a meniscus lens (meniscus lens) having positive optical power with a convex surface facing the object side. According to this configuration, the principal point position of the first lens (L1) is closer to the object. Accordingly, a small single focus imaging lens 10 with a shorter back focus and a shorter overall length can be realized.

  In particular, when the first lens (L1) has a strong positive optical power, it is preferable that the first lens (L1) is substantially made of glass. Since resin has a high temperature dependence of the refractive index, for example, when the first lens (L1) is made of resin, if the temperature of the environment surrounding the single-focus imaging lens 10 changes, the first lens (L1) has a strong positive optical power. The refractive power of one lens (L1) changes greatly. For this reason, the image point position of the entire system of the single focus imaging lens 10 is greatly changed, and the optical performance is deteriorated. In contrast, glass has a relatively low temperature dependency of the refractive index. Therefore, by making the first lens (L1) substantially of glass, it is possible to suppress changes in the image point position of the entire system of the single focus imaging lens 10 due to temperature changes.

  Furthermore, glass is harder than resin or the like. Therefore, the scratch of the exposed first lens (L1) is suppressed by making the first lens (L1) substantially of glass.

  The second lens (L2) and / or the third lens (L3) may be substantially made of glass, or may be substantially made of resin. By employing the second lens (L2) and / or the third lens (L3) that are substantially made of resin, the manufacturing cost of the single focus imaging lens 10 can be reduced.

  In each of the first to third embodiments, it is preferable that one of the second lens (L2) and the third lens (L3) is a lens having extremely weak optical power.

  For example, when the second lens (L2) has a relatively strong positive optical power, the chromatic aberration of magnification of the single focus imaging lens 10 is reduced, but the axial chromatic aberration tends to increase. Conversely, when the second lens (L2) has a relatively strong negative optical power, although the axial chromatic aberration of the single focus imaging lens 10 is reduced, in order to reduce the lateral chromatic aberration along with the axial chromatic aberration, It is necessary to make the optical power of the peripheral portion of the second lens (L2) positive. Therefore, it tends to be difficult to process the second lens (L2). As described above, by using the second lens (L2) as a lens having extremely weak optical power, it is possible to realize the single focus imaging lens 10 in which both the longitudinal chromatic aberration and the lateral chromatic aberration are suitably corrected.

  For example, when the third lens (L3) has a relatively strong positive optical power, the positive combined optical power of the second lens (L2) and the third lens (L3) is relatively increased. The total length of the single focus imaging lens 10 tends to increase (that is, the single focus imaging lens 10 tends to increase in size). Conversely, when the third lens (L3) has a relatively strong negative optical power, the total length of the single focus imaging lens 10 can be shortened, but the incident angle to the imaging surface in the peripheral portion becomes large, Telecentricity tends to decrease. As described above, by using the third lens (L3) as a lens having extremely weak optical power, it is possible to realize the single focus imaging lens 10 that is small in size and has high telecentricity.

  One of the second lens (L2) and the third lens (L3) is a lens having extremely weak optical power, and the other is a negative lens (lens having negative optical power). Is preferred. By doing so, chromatic aberration can be further reduced. Further, the single focus imaging lens 10 can have a telephoto action, and the total length of the single focus imaging lens 10 can be further shortened.

  As described above, the first lens (L1) is a meniscus lens having a positive optical power with a convex surface facing the object side, and the second lens (L2) is a meniscus lens with a concave surface facing the object side. It is preferable. In this way, the lens surfaces are arranged in order from the object side, the lens surface (r1) having a positive optical power, the lens surface (r2) having a negative optical power (hereinafter, the first lens ( L1)), a lens surface (r4) having negative optical power, and a lens surface (r5) having positive optical power (hereinafter, the second lens (L2)). For this reason, negative refractive power will be divided | segmented into a 1st lens (L1) and a 2nd lens (L2). Therefore, the optical performance deterioration of the single focus imaging lens 10 at the time of relative eccentricity of the first lens (L1) and the second lens (L2) is suppressed.

  Note that at least one of the lens surfaces included in the first lens (L1), the second lens (L2), and the third lens (L3) is preferably an aspherical surface. By doing so, various aberrations such as spherical aberration can be reduced. In particular, the lens surface (image side surface) (r7) on the image surface side of the third lens (L3) is preferably an aspherical surface. On the image side surface (r7) of the third lens (L3), the luminous flux becomes thinner and the light ray height at the peripheral portion becomes the highest. Therefore, by making the image side surface (r7) of the third lens (L3) an aspherical surface, it is possible to satisfactorily correct the distortion, which is the principal ray aberration, while keeping the spherical aberration and the coma aberration small. That is, it is possible to realize the single focus imaging lens 10 in which all of the spherical aberration, the coma aberration, and the distortion that is the principal ray aberration are reduced.

  Furthermore, the image side surface (r7) of the third lens (L3) is preferably an aspherical lens surface having at least one inflection point. Specifically, in the region near the optical axis, a concave shape is formed on the image surface side, while in the peripheral region of the image surface side lens surface (r7), a convex shape is formed on the image surface side. In addition, it is preferable that the region near the optical axis formed in a concave shape and the peripheral region formed in a convex shape are continuously connected via an inflection point. In other words, the image surface side lens surface (r7) has a lens surface having a shape such that the image surface side lens surface (r7) once rises from the vicinity of the optical axis toward the outside of the image surface side lens surface (r7) and can be pulled toward the object side further toward the outside. It is preferable that It is preferable that the curvature radius of the peripheral region formed in a convex shape is set larger than the curvature radius of a region in the vicinity of the optical axis formed in a concave shape. According to this configuration, the incident angle of the off-axis light beam can be made relatively small, and the shading effect can be reduced. Accordingly, higher optical performance (imaging performance) can be realized.

The single focus imaging lens 10 in each of the first to third embodiments satisfies the following conditional expression (1).
| F / f _N | <10 ⁻² (1)
However,
f: the focal length of the entire system at the d-line,
f _N : the focal length at the d-line of one of the second lens (L2) and the third lens (L3),
It is.

  The conditional expression (1) defines the optical power of the first lens (L2) or the third lens (L3).

First, the case where f _N is the focal length of the second lens (L2) at the d-line will be described. In this case, by satisfying conditional expression (1), it is possible to realize the single focus imaging lens 10 in which both the longitudinal chromatic aberration and the lateral chromatic aberration are suitably corrected. For example, if the positive optical power of the second lens (L2) is too strong and out of the range of the conditional expression (1), the lateral chromatic aberration of the single focus imaging lens 10 is reduced, but the axial chromatic aberration tends to increase. It is in. On the other hand, if the negative optical power of the second lens (L2) is too strong and out of the range of the conditional expression (1), the axial chromatic aberration of the single focus imaging lens 10 is reduced, but the magnification together with the axial chromatic aberration In order to reduce chromatic aberration, it is necessary to make the optical power of the peripheral portion of the second lens (L2) positive. Therefore, it tends to be difficult to process the second lens (L2).

Next, the case where f _N is the focal length of the third lens (L3) at the d-line will be described. In this case, by satisfying the conditional expression (1), it is possible to realize the single focus imaging lens 10 that is small and has high telecentricity. For example, if the positive optical power of the third lens (L3) is too strong to deviate from the range of the conditional expression (1), the second lens (L2) and the third lens (L3) are relatively positive. The combined optical power increases and the total length of the single focus imaging lens 10 tends to increase (that is, the single focus imaging lens 10 tends to increase in size). Conversely, if the negative optical power of the third lens (L3) is too strong and falls outside the range of the conditional expression (1), the total length of the single focus imaging lens 10 can be shortened, but the imaging surface in the peripheral portion The angle of incidence on the surface increases and the telecentricity tends to decrease.

From the above viewpoint, it is preferable that the following conditional expression (1-a) is satisfied.
| F / f _N | <9 × 10 ⁻³ (1-a)
A parallel plate-shaped optical member such as a low-pass filter may be disposed between the image side surface (r7) of the third lens (L3) and the imaging surface. In that case, conditional expression (1) should just be satisfy | filled with the value which converted the arrange | positioned optical member into the air conversion distance. The same applies to other conditional expressions.

  Hereinafter, conditional expressions that are preferably satisfied by the single focus imaging lens 10 according to each embodiment will be described. However, it is not necessary to satisfy all the conditional expressions described below simultaneously. If each conditional expression is satisfied independently, the action and effect corresponding to the conditional expression can be obtained. Of course, it is needless to say that satisfying a plurality of conditional expressions is more preferable from the viewpoint of optical performance, miniaturization, manufacturing / assembly, and the like.

The following conditional expression (2) defines the optical power of the first lens (L1).
0.4 <f ₁ /f<1.0 (2)
However,
f ₁ : focal length of the first lens (L1) at the d-line,
f: the focal length of the entire system at the d-line,
It is.

  When the lower limit of conditional expression (2) is exceeded, the optical power of the first lens (L1) becomes strong, and correction of spherical aberration and coma tends to be difficult. Moreover, since the curvature radius of each lens surface (r1, r2) which comprises a 1st lens (L1) becomes small, the process of a 1st lens (L1) becomes difficult. On the other hand, when the upper limit of conditional expression (2) is exceeded, the optical power of the first lens (L1) becomes weak and the back focus of the single focus imaging lens 10 tends to become long. Accordingly, it is difficult to shorten the overall length of the single focus imaging lens 10.

From the above viewpoint, it is more preferable to satisfy the following conditional expression (2-a).
0.5 <f ₁ /f<0.9 (2-a)
It is preferable that the focal length (f _N ) of any one of the second lens (L2) and the third lens (L3) satisfies the conditional expression (1), and the other lens is a negative lens. . According to this configuration, chromatic aberration can be further reduced. Further, the single focus imaging lens 10 can have a telephoto action, and the total length of the single focus imaging lens 10 can be further shortened.

The following conditional expression (3) defines the optical power of the other negative lens.
_{-1.0 <f / f m <-0.3} ····· (3)
However,
f _m : the focal length of the other lens (the lens that does not satisfy the conditional expression (1) among the second lens (L2) and the third lens (L3)) at the d-line,
f: the focal length of the entire system at the d-line,
It is.

  When the upper limit of conditional expression (3) is exceeded, the telephoto action of the single focus imaging lens 10 tends to be small. Therefore, the total length of the single focus imaging lens 10 tends to be long, and the single focus imaging lens 10 tends to increase in size. On the other hand, if the lower limit of conditional expression (3) is exceeded, the optical power of the negative lens becomes strong, and the incident angle of the off-axis light beam on the imaging surface tends to increase. Accordingly, the telecentricity tends to decrease.

From the above viewpoint, it is more preferable to satisfy the following conditional expression (3-a).
_{-0.9 <f / f m <-0.4} ····· (3-a)
The following conditional expression (4) defines the shape factor on the object side surface (r1) of the first lens (L1) and the object side surface (r4) of the second lens (L2).
−0.3 <(r ₁ + r ₄ ) / (r ₁ −r ₄ ) <0.3 (4)
However,
r ₁ : radius of curvature on the optical axis of the object side surface of the first lens,
r ₄ : radius of curvature on the optical axis of the object side surface of the second lens,
It is.

  When the lower limit of conditional expression (4) is exceeded, the optical power of the object side surface (r1) of the first lens (L1) becomes strong, and correction of spherical aberration and coma tends to be difficult. In addition, the negative optical power of the object side surface (r4) of the second lens (L2) becomes small, the off-axis ray splashing is insufficient, and the telecentricity tends to be lowered. On the other hand, when the upper limit of conditional expression (4) is exceeded, the optical power of the object side surface (r1) of the first lens (L1) becomes weak, the total length of the single focus imaging lens 10 becomes long, and the single focus imaging lens 10 It tends to increase in size.

From the above viewpoint, it is more preferable to satisfy the following conditional expression (4-a).
−0.2 <(r ₁ + r ₄ ) / (r ₁ −r ₄ ) <0.2 (4-a)
Conditional formula (5) below defines the Abbe number of the material of the first lens.
50 <νld <80 (5)
However,
νld: Abbe number of the first lens,
It is.

  When the lower limit of conditional expression (5) is exceeded, axial chromatic aberration and lateral chromatic aberration tend to increase. On the other hand, if the upper limit of conditional expression (5) is exceeded, chromatic aberration can be further reduced, but the cost of the first lens (L1) tends to increase, and the cost of the single focus imaging lens 10 can be reduced. It becomes difficult. In addition, since the material exceeding the conditional expression (5) is actually only a material having a small refractive index, the curvature of the lens surface of the first lens (L1) must be strengthened. Therefore, it becomes difficult to process the first lens (L1).

  The single focus imaging lens 10 according to the first to third embodiments has been described above. However, the single focus imaging lens 10 may have a configuration capable of focusing for focus adjustment. Specifically, the entire single focus imaging lens 10 or a part of the first to third lenses may be configured to be displaceable in the optical axis direction with respect to the imaging surface.

  Further, the optical path may be bent by appropriately arranging a surface having no optical power, such as a reflective surface, a refractive surface, and a diffractive surface, in the optical path of the single focus imaging lens 10. By appropriately bending the optical path, the single focus imaging lens 10 can be made thin and compact. In addition, by using such a thin and compact single focus imaging lens 10, it is possible to realize a thin and compact imaging device.

  It is preferable that at least one of the first lens (L1), the second lens (L2), and the third lens (L3) is a lens that blocks infrared light. Specifically, for example, it is preferable to coat at least one lens surface of any one lens with an infrared light absorbing material, or to include an infrared absorbing material in any one lens. According to this configuration, the intensity of infrared light incident on the imaging surface of the imaging element can be reduced. Accordingly, the sensitivity of the image sensor can be made closer to the human eye's specific luminous intensity, and more natural color reproduction is possible.

  Note that “infrared light” refers to light having a wavelength of 780 nm to 2500 nm. In addition, a lens that shields infrared light refers to a lens that reflects and / or absorbs infrared light.

  Further, in order to cut off harmful light that causes ghost, flare, and the like, an optical path regulating member may be arranged at any location on the optical path of the single focus imaging lens 10.

  Further, a parallel plate-shaped optical member such as a low-pass filter is provided at any position on the optical path of the single focus imaging lens 10, for example, between the image side surface (r7) and the imaging surface (r8) of the third lens (L3). You may arrange. Since a (solid-state) imaging device such as a CCD captures the subject image formed by the single focus imaging lens 10 as a two-dimensional sampling image with a low aperture ratio, a high frequency more than half the sampling frequency becomes a false signal. By arranging an optical member such as a low-pass filter, a high-frequency component of such an image can be removed in advance.

  Further, when the single focus imaging lens 10 is used for a CCD having a microlens, shading in the peripheral portion of the screen on the imaging surface may be reduced by shifting the microlens of the CCD. By designing the CCD microlens in consideration of the incident angle of the light beam at each image height, shading at the periphery of the imaging surface can be reduced.

  In each of the first to third embodiments, the lens is composed only of a refractive lens that deflects incident light by refraction (that is, a lens that deflects at the interface between media having different refractive indexes). Explained the example. However, in the present invention, each lens constituting the single focus imaging lens 10 may be a lens of a type other than a refractive lens. For example, each lens is a diffractive lens that deflects incident light by diffractive action, a refractive / diffractive hybrid lens that deflects incident light by combining diffractive action and refracting action, and deflects incident light by refractive index distribution in the medium. A gradient index lens may be used. Among these, a low-cost refractive lens, diffractive lens, refractive / diffractive hybrid lens, and the like are preferable.

  In addition to the diaphragm (STP), a light flux restricting plate for cutting unnecessary light may be arranged as necessary.

  Next, an embodiment of an imaging apparatus including the single focus imaging lens 10 according to the above embodiment will be described. Here, a digital still camera equipped with the single focus imaging lens 10 and a portable information terminal will be described as examples, but the imaging apparatus according to the present invention is not limited to these.

  4 and 5 are perspective views of the digital still camera 1.

  A digital still camera (hereinafter referred to as “DSC”) 1 includes a camera body 14, a single focus imaging lens 10, and an image sensor (not shown) that converts an optical image formed by the single focus imaging lens 10 into an electrical signal. ), A strobe 11, a release button 12, and a display monitor 13.

  FIG. 6 is a front view of the portable information terminal 2. FIG. 7 is a rear view of the portable information terminal 2.

  The mobile information terminal 2 includes a mobile phone body 27, a speaker unit 21, a microphone unit 22, an input button 23, a display monitor 24, an antenna 25, a single focus imaging lens 10, and a single focus imaging lens 10. An image sensor (not shown) that converts the optical image thus converted into an electrical signal and a display monitor 26 are provided. The microphone unit 22 is for inputting an operator's voice as information. The speaker unit 21 is for outputting the voice of the other party. The input button 23 is used for an operator to input information. The display monitor 24 is for displaying information such as a photographed image and a telephone number of the operator himself or the other party. The antenna 25 is for transmitting and receiving communication radio waves.

  An object image received by an image sensor (not shown) is input to a processing means (not shown) built in the portable information terminal 2 and is displayed as an electronic image on the display monitor 24 or a monitor of a communication partner portable information terminal or the like. Or both. In addition, when transmitting an image to a communication partner portable information terminal or the like, the signal processing function included in the processing means converts the information of the object image received by the image sensor into a signal that can be transmitted. It has become.

  Thus, by mounting the single focus imaging lens 10 according to the above-described embodiment, a small and high-performance imaging device can be realized.

  Hereinafter, the single focus imaging lens embodying the present invention will be described more specifically with reference to construction data, various aberration diagrams, and the like. Numerical examples 1 to 3 described here are numerical examples corresponding to the first to third embodiments, respectively. Lens configuration diagrams (FIGS. 1 to 3) representing the first to third embodiments respectively show the lens configurations of the corresponding first to third embodiments.

  Note that ri (i = 0, 1, 2, 3,...) Is a radius of curvature (mm) of the i-th surface counted from the object side. di (i = 1, 2, 3,...) is the i-th axis upper surface interval (mm) counted from the object side. Ni (i = 1, 2, 3,...) And νi (i = 1, 2, 3,...) Are refractive indices (Nd) with respect to the d-line of the i-th optical element counted from the object side. Abbe number (νd). F represents an F number, and ω represents a half angle of view (°).

  The surface on which the aspheric coefficient is written indicates that it is an aspherical refractive optical surface or a surface having a refractive action equivalent to that of an aspherical surface, and is defined by the following formula (6) representing the aspherical surface shape. Shall. The aspheric data of each example is shown together with other data.

  However, in the above equation (6), the optical axis direction is a cylindrical coordinate system in which the axis toward the image plane side is the Z axis and the H axis extends in a direction perpendicular to the optical axis. CR: paraxial radius of curvature, K: conic coefficient, An: n-order aspheric coefficient.

  In Examples 1 to 3 below, the half angle of view (ω) is about 30 °, but it is possible to further widen the angle by appropriately designing the optical power and bending shape of each lens.

  8 to 10 are aberration diagrams corresponding to Examples 1 to 3, respectively. In each figure, the graph of (a) shows spherical aberration (SA) with respect to the d line, the graph of (b) shows astigmatism (AST), and the graph of (c) shows the distortion aberration (DIST) with respect to the d line. In the astigmatism diagram (b), the solid line represents data on the sagittal image plane, and the broken line represents data on the meridional image plane.

  Example 1

  The data shown in Table 2 is the aspheric coefficient of each lens surface in Example 1.

  -Example 2-

  The data shown in Table 4 is the aspheric coefficient of each lens surface in Example 2.

  -Example 3-

  The data shown in Table 6 is the aspheric coefficient of each lens surface in Example 3.

Table 7 below shows various values of the numerical examples and values corresponding to the parameters defined by the conditional expressions (1) to (5). The units of various values shown in Table 7 are f (mm), f _N (mm), f _m (mm), and f ₁ (mm).

  As is clear from Table 7, each numerical example satisfies the conditional expressions (1) to (5).

  Since the single focus imaging lens according to the present invention is small and has high optical performance, it is useful for cameras equipped with portable information terminals (cell phones, PDAs, etc.), surveillance cameras, PC cameras, digital still cameras, digital video cameras, and the like. is there.

It is a schematic block diagram of the single focus imaging lens 10 of 1st Embodiment (Numerical Example 1). It is a schematic block diagram of the single focus imaging lens 10 of 2nd Embodiment (Numerical Example 2). It is a schematic block diagram of the single focus imaging lens 10 of 3rd Embodiment (Numerical Example 3). It is a schematic perspective view at the time of seeing the digital still camera 1 from diagonally forward. It is a schematic perspective view at the time of seeing the digital still camera 1 from the diagonal construction method. 2 is a front view of a portable information terminal 2. FIG. 4 is a rear view of the portable information terminal 2. FIG. FIG. 6 is an aberration diagram of Numerical Example 1. FIG. 6 is an aberration diagram of Numerical Example 2. FIG. 10 is an aberration diagram of Numerical Example 3.

Explanation of symbols

STP ... Aperture 1 ... Digital still camera (DSC)
2 ... Portable information terminal 10 ... Single focus imaging lens 11 ... Strobe 12 ... Release buttons 13, 24, 26 ... Display monitor 14 ... Camera body 21 ... Speaker unit 22 .... Microphone part 23 ... Input button 25 ... Antenna 27 ... Mobile phone body

Claims (9)

A single focus imaging lens for forming an optical image on an imaging surface of an imaging element,
A single-focus imaging lens that is arranged in this order from the object side and includes a first lens having a positive optical power, an aperture, a second lens, and a third lens, and satisfies the following conditional expression (1);
| F / f _N | <10 ⁻² (1)
However,
f: the focal length of the entire system at the d-line,
f _N : the focal length at the d line of either one of the second lens and the third lens,
It is. The single focus imaging lens according to claim 1,
A single focus imaging lens satisfying the following conditional expression (2);
0.4 <f ₁ /f<1.0 (2)
However,
f ₁ : focal length at the d-line of the first lens,
f: the focal length of the entire system at the d-line,
It is. The single focus imaging lens according to claim 1 or 2,
The other lens is a negative lens, and a single focus imaging lens satisfying the following conditional expression (3);
_{-1.0 <f / f m <-0.3} ····· (3)
However,
f _m : focal length of d-line of the negative lens,
f: the focal length of the entire system at the d-line,
It is. In the single focus imaging lens according to any one of claims 1 to 3,
The first lens has a meniscus shape with a convex surface facing the object side, and the second lens has a meniscus shape with a concave surface facing the object side. In the single focus imaging lens according to any one of claims 1 to 4,
A single focus imaging lens satisfying the following conditional expression (4);
−0.3 <(r ₁ + r ₄ ) / (r ₁ −r ₄ ) <0.3 (4)
However,
r ₁ : radius of curvature on the optical axis of the object side surface of the first lens,
r ₄ : radius of curvature on the optical axis of the object side surface of the second lens,
It is. In the single focus imaging lens according to any one of claims 1 to 5,
A single focus imaging lens in which at least one of the second lens and the third lens is substantially made of resin. The single focus imaging lens according to any one of claims 1 to 6,
The first lens is a single focus imaging lens substantially made of glass. The single focus imaging lens according to any one of claims 1 to 7,
A single focus imaging lens satisfying the following conditional expression (5);
50 <νld <80 (5)
However,
νld: Abbe number of the first lens,
It is. An imaging apparatus comprising: a single focus imaging lens that forms an optical image; and an imaging device that converts an optical image formed by the single focus imaging lens into an electrical signal,
The single focus imaging lens includes a first lens having a positive optical power, an aperture, a second lens, and a third lens that are arranged in this order from the object side. The following conditional expression (1) is satisfied: Imaging device to satisfy;
| F / f _N | <10 ⁻² (1)
However,
f: the focal length of the entire system at the d-line,
f _N : the focal length at the d line of either one of the second lens and the third lens,
It is.

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