JP2004184987A - Imaging lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an imaging lens utilized as a compact and high-resolution imaging lens used for a camera mounted in a portable telephone, a PC camera, a monitor camera and a digital still camera or the like using an imaging device such as a CCD or a CMOS. <P>SOLUTION: The imaging lens is provided with at least an aperture diaphragm S, a lens group LG constituted of a first lens L1 having negative refractive power and a second lens L2 arranged to be bonded to or separated from the first lens L1 and having positive refractive power, a third lens L3 being a meniscus whose concave surface faces to an object side and having positive refractive power, and a fourth lens L4 having at least one refractive surface made aspherical and having weak negative refractive power in order from the object side to an image surface side. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention relates to an imaging lens used in a camera mounted on a mobile phone, a PC camera, a surveillance camera, a digital still camera, and the like using an imaging device such as a CCD or a CMOS.

  In recent years, there has been a strong demand for miniaturization and weight reduction of mobile phones, digital cameras and the like.

  In order to respond to this demand, there has been proposed an imaging lens that is reduced in size by reducing the number of imaging lenses used in a mobile phone, a digital still camera, and the like (see Patent Documents 1 and 2).

  Here, conditions suitable for an imaging lens when configuring a small camera module using an imaging element will be described.

  Condition 1 is that the device has a short overall length and can be mounted on a thin device, for example, a portable tool such as a mobile phone or an electronic organizer.

  Condition 2 is that a sufficient back focus is secured for inserting an optical low-pass filter, an infrared absorption filter, a cover glass for protection, and the like between the imaging lens and the imaging element.

  Condition 3 is to maintain appropriate telecentricity. This is because, when a light beam is obliquely incident on the image sensor, a reduction in aperture efficiency called shading occurs.

By configuring an imaging lens satisfying this condition and mounting the imaging lens on a small camera module, it is possible to provide a digital still camera or the like that has been required to be reduced in size and weight.

JP-A-10-90597 Japanese Patent Application No. 2002-215745

  However, the imaging lenses described in the above publications satisfy the above-mentioned conditions, but do not sufficiently satisfy them.

  In other words, although it is possible to sufficiently meet the demand for miniaturization, correction of various aberrations including chromatic aberration is not satisfactory for recent miniaturization of pixels of an image sensor, and further improvement in resolution is required. Is required.

  In recent years, demands for miniaturization and high optical performance have been increasing, and at the same time, there has been a growing demand for cost reduction. Therefore, it is necessary to reduce the number of lenses and glass and to use as much plastic material as possible at low cost. Moreover, even when a plastic material is used, a lens having a specification that is resistant to environmental changes such as temperature and humidity is required.

  Furthermore, the small camera module uses an image sensor, and it is necessary to maintain appropriate telecentricity compared to conventional compact cameras using silver halide film, and high aperture efficiency and long back focus are required. The ideas are fundamentally very different.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an imaging lens that is small and short in size as a whole, is low in cost, and is resistant to environmental changes.

  In order to achieve the above object, an imaging lens according to the present invention includes, in order from an object side to an image plane side, an aperture stop, a first lens having a negative refractive power, and a junction with or separated from the first lens. A second lens having a positive refractive power, a third lens having a positive meniscus refractive power with a concave surface facing the object side, and a weak negative refractive power having at least one refractive surface having an aspherical shape. And at least a fourth lens.

  With the above-described configuration of the imaging lens, the dimension in the optical axis direction can be reduced, and the fourth lens can have a sufficient back focus as a weak negative refractive power.

  In order to secure an appropriate telecentricity, an aperture stop is arranged closest to the object side. With this arrangement, the position of the exit pupil is lengthened to make it easier to secure telecentricity, and by making the first lens a negative meniscus lens, the negative refracting power makes it easier to secure telecentricity.

  Furthermore, the use of an aspherical surface for the fourth lens also facilitates ensuring telecentricity.

  Another imaging lens according to the present invention is characterized in that the following conditional expression (1) is satisfied with respect to the dimension of the imaging lens in the optical axis direction.

(1) TL / Y <3.3
Where TL is the distance along the optical axis from the surface closest to the object to the image plane
Y: Maximum image height The above conditional expression (1) is a conditional expression relating to the dimension in the optical axis direction of the imaging lens defined by the ratio of the distance along the optical axis from the surface closest to the object side to the image plane and the maximum image height. If the upper limit is exceeded, the dimension of the imaging lens in the optical axis direction becomes too long, which is contrary to the object of the present invention of miniaturization.

  Another imaging lens according to the present invention is characterized in that the first lens and the second lens satisfy the following conditional expression (2) with respect to the Abbe number of the d-line.

(2) 10 <ν _L2 -ν _L1
Here, ν _{L1 is} the Abbe number of the first lens with respect to d-line.
ν _L2 : Abbe number of the second lens with respect to the d-line Above conditional expression (2) is a conditional expression relating to the achromaticity of the lens group defined by the difference between the Abbe number of the first lens and the second lens with respect to the d-line, and the lower limit When the value exceeds, the chromatic aberration on the axis and the magnification increases, and it becomes difficult to correct the chromatic aberration.

Another imaging lens according to the present invention is a lens group including at least the first lens and the second lens,
An imaging lens, wherein the following condition (3) is satisfied with respect to the focal length of the lens group and the third lens.

(3) 0.4 <f _L3 / f _LG <6.0
Where f _{LG is} the focal length of the lens group
f _L3 : focal length of the third lens The above conditional expression (3) is a conditional expression relating to the distribution of the refractive power of the third lens and the lens group defined by the ratio of the focal length of the third lens to the lens group. If the upper limit is exceeded, the refractive power burden of the lens group becomes excessive, and spherical aberration and coma aberration increase. In addition, the radius of curvature of the spherical surface of the lens constituting the lens group becomes small, and processing becomes difficult. On the other hand, if the lower limit is exceeded, the refracting power of the third lens becomes too strong, so that the balance of the overall aberrations tends to be lost, and it is particularly difficult to correct off-axis aberrations.

  Another imaging lens of the present invention is characterized in that the following conditions (4) and (5) are satisfied with respect to the focal length and the image-side surface shape of the fourth lens.

(4) -75 <f _L4 / f <-1.0
(5) 0.2 <r _L4-2 / f <1.0
Where f is the focal length of the entire imaging lens system
f _L4 : focal length of the fourth lens
_rL4-2 : radius of curvature of the fourth lens on the image side The conditional expression (4) defines the focal length of the fourth lens as a ratio of the focal length of the entire imaging lens system, and relates to the refractive power of the fourth lens. When the refractive power of the fourth lens is increased beyond the upper limit, it is advantageous for miniaturization, but it becomes difficult to correct the telecentricity and distortion of the peripheral portion. On the other hand, if the refractive power of the fourth lens becomes smaller than the lower limit, the back focus of the entire lens system becomes longer, and compactness cannot be maintained.

  The conditional expression (5) is a conditional expression relating to the image-side surface shape of the fourth lens in which the image-side radius of curvature of the fourth lens is defined by the ratio of the focal length of the entire imaging lens system. If the lower limit is exceeded, the telecentricity and the distortion will deteriorate.

  Further, another imaging lens according to the present invention is characterized in that the third lens and the fourth lens are formed of a plastic material, so that the cost and weight can be reduced and the lens surface has an aspheric surface. Becomes easy, and an imaging lens with good various aberrations can be obtained.

  Here, in the conditional expressions (1) to (4), more preferred conditional expressions are shown below.

(1a) 2.0 <TL / Y <2.8
(2a) 20.0 <ν _L2 −ν _L1 <40.0
(3a) 0.85 <f _L3 / f _LG <2.1
(4a) -11.0 <f _L4 / f <-1.4
(5a) 0.25 <r _L4-2 / f <0.5

The conditional expression (1a) is a conditional expression relating to the dimension of the imaging lens in the optical axis direction.

  In general, an optical system aimed at high resolution tends to have a longer overall length, but balancing the practical overall length and good imaging performance is extremely important in terms of the lens configuration.

  That is, the F number, the back focus, the amount of off-axis aberration, the pupil position, and the like greatly depend on the overall length.

  Therefore, when the value exceeds the upper limit of the conditional expression (1a), the overall length becomes too large, which is contrary to the object of the present invention.

  In addition, high-order field curvature and astigmatic coma are likely to occur, which tends to cause a decrease in the resolution of the peripheral portion, which is not preferable.

  On the other hand, if the lower limit of the conditional expression (1a) is exceeded, the thickness of each lens cannot be sufficiently secured, and the production becomes difficult.

  In addition, since the refractive power of the first lens must be increased, spherical aberration becomes under, and correction becomes difficult.

  The conditional expression (2a) is a conditional expression relating to the Abbe number of the first lens and the second lens with respect to the d-line, and particularly relates to correction of lateral chromatic aberration.

  If the upper limit of conditional expression (2a) is exceeded, chromatic aberration of magnification will be excessively corrected, making it difficult to balance with other aberrations. If the lower limit is exceeded, chromatic aberration of magnification cannot be corrected.

  The conditional expression (3a) is a conditional expression relating to the distribution of the refractive power of the third lens and the lens group. If the refractive power of the first lens is increased beyond the upper limit of conditional expression (3a), spherical aberration However, it is not preferable because it is difficult to correct the coma aberration and it is difficult to secure a sufficient back focus, and if the refractive power of the third lens exceeds the lower limit and the refractive power of the third lens becomes strong, the influence of the temperature change becomes large.

  Further, the chromatic aberration caused by the third lens is increased, and especially the off-axis performance is deteriorated.

  The conditional expression (4a) is a conditional expression relating to the refractive power of the fourth lens. If the refractive power of the fourth lens exceeds the upper limit of the conditional expression (4a) and the refractive power of the fourth lens becomes strong, the telecentricity will be deteriorated. However, it is not preferable because the influence of the temperature change becomes large, and if it exceeds the lower limit, it is disadvantageous for further downsizing.

  The above-mentioned conditional expression (5a) is a conditional expression relating to the image-side surface shape of the fourth lens. If the upper limit of conditional expression (5a) is exceeded, the refractive power of the image-side surface becomes small, which is disadvantageous for miniaturization. If the lower limit is exceeded, the periphery of the lens becomes extremely aspherical in order to correct telecentricity, which is not desirable.

  Further, when actually configuring the optical system, it is more desirable to satisfy the following conditions in addition to the above conditions.

(6) 0.3 <d _L1-2 / f <0.6
Where, d _{L1 to d2} : the distance from the object-side surface of the first lens to the image-side surface of the second lens
Distance along the optical axis (7-1) When the first lens and the second lens are joined to each other, it is preferable that the following conditional expression is satisfied.

-15.0 <r _L1-2 / {f (n _L2 -n _L1 )} <-1.0
Here, r _L1-2 : radius of curvature of the first lens on the image plane side
n _L1 : refractive index of the first lens
n _L2 : refractive index of the second lens (7-2) When the lens group is composed of the separated first and second lenses, it is preferable that the following conditional expression is satisfied.

-10.0 <r _L1-2 / {f (1-n _L1 )} <-0.3
(8) -6.0 <(r _L1-2 + r _L1-1 ) / (r _L1-2 -r _L1-1 ) <-1.5
Here, r _{L1-1 is the} radius of curvature of the first lens on the object side.
_rL1-2 : radius of curvature of the first lens on the image side (9) 0.5 < _rL3-2 / _rL3-1 <1.0
Here, r _L3-1 : the radius of curvature of the third lens on the object side r _L3-2 : the radius of curvature of the third lens on the image side To explain the above conditional expression, conditional expression (6) _satisfies the condition of the first lens. A conditional expression relating to a distance along an optical axis of a lens group, which defines a distance along an optical axis from an object-side surface to an image-side surface of the second lens by a ratio of a focal length of the entire imaging lens system, Exceeding the upper limit of conditional expression (6) is not preferable because the center thickness of the first lens and the second lens constituting the lens group becomes too large, which is disadvantageous for miniaturization and weight reduction.

  Further, the Petzval sum is corrected to the negative side, and as a result, the field curvature is largely inclined to the positive side, which is not preferable.

  On the other hand, if the lower limit of conditional expression (6) is exceeded, it becomes difficult to correct coma. Further, the Petzval sum is corrected to the positive side, and as a result, the field curvature is largely inclined to the negative side, which is not preferable. In order to more fully exert the effects of the present invention, it is preferable to set the upper limit of conditional expression (6) to 0.5 and the lower limit to 0.4.

  Next, the conditional expression (7-1) is a conditional expression relating to the refractive power of the cemented surface in the lens group when the first lens and the second lens are joined to each other. Exceeding the upper limit of -1) is advantageous for securing telecentricity, but it increases the refractive power of the joint surface, making it difficult to correct off-axis aberrations.

  On the other hand, if the lower limit of conditional expression (7-1) is exceeded, the divergence of the joint surface will be weakened, and the telecentricity will be degraded.

  In order to achieve the effect of the present invention more sufficiently, it is preferable to set the upper limit of conditional expression (7-1) to -2.0 and the lower limit to -10.0.

  Next, the conditional expression (7-2) is a conditional expression relating to the refracting power of the image-side surface of the first lens when the first lens and the second lens are separated from each other. If the upper limit is exceeded, it is advantageous to secure telecentricity. However, since the refractive power of the image-side surface of the first lens becomes strong, it becomes difficult to correct off-axis aberrations. On the other hand, if the lower limit is exceeded, the divergence of the image-side surface of the first lens will be weakened, and the telecentricity will be degraded.

  In order to achieve the effect of the present invention more sufficiently, it is preferable to set the upper limit of conditional expression (7-2) to -0.5 and the lower limit to -5.0.

  Next, the above conditional expression (8) is a conditional expression relating to the shape factor of the first lens L1, which enables a negative meniscus lens to achieve good imaging performance and has a good shape in terms of production technology. Is defined. If the upper limit of conditional expression (8) is exceeded, the occurrence of spherical aberration will increase, making correction difficult. Further, the shape becomes difficult to be processed, and the shape becomes difficult in production technology, which is not preferable. On the other hand, if the lower limit of conditional expression (8) is exceeded, spherical aberration will be insufficiently corrected.

  In order to achieve the effect of the present invention more sufficiently, it is preferable to set the upper limit of conditional expression (8) to -2.5 and the lower limit to -5.0.

  Conditional expression (9) is a conditional expression relating to the object-side and image-side surface shapes of the third lens. If the upper and lower limits are exceeded, the concentric shape and the effect are lost, and in particular, astigmatism, distortion, and telecentricity The property rapidly deteriorates.

  In order to achieve the effect of the present invention more sufficiently, it is preferable to set the upper limit of conditional expression (9) to 0.98 and the lower limit to 0.70.

  Thus, according to the imaging lens of the present invention, it has been possible to provide an imaging lens that can reduce the size and length of the entire module, is low-cost, and is resistant to environmental changes.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(FIG. 1) and (FIG. 2) are configuration diagrams of an imaging lens according to the present invention. (Table 1) to (Table 12) show configuration data of the first to twelfth embodiments of the present invention.

In the following embodiments, as shown in FIGS. 1 and 2, the aperture stop S is disposed closest to the object side, and the first lens L1, the second lens L2, the third lens L3, The fourth lens L4 and the cover glass CG are arranged, and LG is a lens group having a first lens L1 and a second lens L2.

In each of the embodiments, an aperture stop S is provided immediately before the object side of the first lens L1. In each embodiment, the radius of curvature of the i-th surface (including the aperture stop surface) counted from the object side is r _i , the distance between the i-th surface and the (i + 1) -th surface on the optical axis is d _i , The refractive index and Abbe number of the lens on the i-th surface counted from the side are denoted by n _i and v _i , respectively. The refractive index and Abbe number indicate values at the d-line (587.56 nm). The surface of * represents an aspherical surface, and each of the aspherical surfaces is represented by the following aspherical expression (Equation 1). F represents the focal length of the entire imaging lens system, F / NO represents brightness, and ω represents the angle of view.

  In this embodiment, a high-performance CCD or CMOS camera, monitor, and TV are configured with a small number of lenses, such as four lenses, while maintaining the brightness at an F value of about 2.8. And so on.

  The first to seventh and ninth to twelfth embodiments are cases where the first lens L1 and the second lens L2 are joined with the lens group LG, and the eighth embodiment is such that the lens group LG is separated. In this case, the first lens L1 and the second lens L2 are used.

  In the first to third and eighth to twelfth embodiments, by satisfying the above conditional expressions (1) to (8), various aberrations are reduced as shown in the data shown in FIGS. 3 to 5 and FIGS. Thus, all of the spherical aberration, astigmatism, and distortion are almost satisfactory values, and it can be seen that it is possible to obtain an imaging lens that achieves downsizing and has other favorable optical characteristics.

  In the fourth to seventh embodiments, the above-mentioned conditional expressions (1) to (8) are satisfied, and the preferable one among the above-mentioned conditional expressions (1a) to (5a) and the above-mentioned conditional expressions (6) to (9) is preferable. By satisfying the range, the various aberrations are as shown in the data shown in FIGS. 6 to 9, and all of the spherical aberration, astigmatism, and distortion are almost satisfactory values. It can be seen that a focus can be sufficiently secured, and an imaging lens with good other optical characteristics can be obtained.

  In all the embodiments, Zeonex (trade name), which is a heat-resistant resin, is used for the third lens L3 and the fourth lens L4. By using a plastic material, cost reduction and weight reduction can be achieved.

Note that the present invention is not limited to the above-described embodiment, and can be variously changed as needed.

As described above, according to the first embodiment of the present invention, the lens unit LG having the positive refractive power from the object side, the third lens L3 having the positive refractive power, and the negative refractive power By adopting a telephoto type configuration in which the fourth lens L4 is provided, miniaturization is possible. However, since the back focus becomes short, a sufficient back focus is secured by using the fourth lens L4 as a weak negative refractive power. In order to secure an appropriate telecentricity, the aperture stop S is arranged closest to the object side. With this arrangement, the exit pupil position can be lengthened. Further, by making the first lens L1 a negative meniscus lens, it is easy to ensure telecentricity by its negative refractive power. And the fourth lens
By using an aspherical shape for L4, it is easier to ensure telecentricity.

  In the third embodiment of the present invention, since the configuration is such that the conditional expression (2) is satisfied, it is possible to achieve downsizing which is the object of the present invention.

  The invention according to claim 2 of the present embodiment is configured so as to satisfy a predetermined conditional expression (2) regarding the Abbe number of the first lens L1 and the second lens L2, thereby reducing chromatic aberration. Correction can be appropriately performed, and an imaging lens having better optical characteristics can be obtained.

  Further, the invention according to claim 4 of the present embodiment is mainly configured to satisfy the conditional expression (3) regarding the refractive power of the lens group LG and the third lens L3, and thus mainly achieves spherical aberration. And coma can be corrected, and an imaging lens having better optical characteristics can be obtained.

  The invention according to claim 5 of the present embodiment is configured so as to satisfy predetermined conditional expressions (4) and (5) regarding the refractive power of the fourth lens L4 and the curvature on the image side. It is possible to secure a sufficient back focus, maintain appropriate telecentricity, and obtain an imaging lens with better optical characteristics.

  In the invention according to claim 6, the cost and weight can be reduced by using a plastic material for the third lens L3 and the fourth lens L4. In addition, it is easy to make the lens surface aspherical, and it is possible to obtain an imaging lens with good various aberrations.

FIG. 3 is an external view showing a configuration of an example in which the imaging lens of the present invention includes a first lens L1 and a second lens L2 to which a lens group LG is joined. FIG. 2 is an external view showing a configuration of an embodiment in a case where a lens group LG includes a separated first lens L1 and a second lens L2 in the imaging lens of the present invention. FIG. 6 is a diagram illustrating spherical aberration, astigmatism, and distortion at d-line (587.56 nm) in the first example. FIG. 14 is a diagram illustrating spherical aberration, astigmatism, and distortion at d-line (587.56 nm) in the second example. FIG. 14 is a diagram illustrating spherical aberration, astigmatism, and distortion at d-line (587.56 nm) in the third example. FIG. 14 is a diagram illustrating spherical aberration, astigmatism, and distortion at d-line (587.56 nm) in the fourth example. FIG. 14 is a diagram illustrating spherical aberration, astigmatism, and distortion at d-line (587.56 nm) in the fifth example. FIG. 14 is a diagram illustrating spherical aberration, astigmatism, and distortion at d-line (587.56 nm) in the sixth example. It is a figure showing spherical aberration, astigmatism, and distortion at d-line (587.56 nm) of the 7th example. It is a figure showing spherical aberration, astigmatism, and distortion at d-line (587.56 nm) of the 8th example. It is a figure which shows the spherical aberration in the d-line (587.56nm) of 9th Example, astigmatism, and distortion. It is a figure showing spherical aberration, astigmatism, and distortion at d-line (587.56 nm) of the 10th example. It is a figure showing spherical aberration, astigmatism, and distortion at d-line (587.56 nm) of the 11th example. FIG. 33 is a diagram illustrating spherical aberration, astigmatism, and distortion at d-line (587.56 nm) in a twelfth example.

Explanation of reference numerals

S aperture stop
L1 First lens
L2 Second lens
L3 Third lens
L4 4th lens
CG cover glass

Claims (6)

An aperture stop in order from the object side to the image plane side, a first lens having a negative refractive power, a second lens having a positive refractive power joined or separated from the first lens, An image pickup apparatus characterized by having at least a meniscus third lens having a positive refractive power with a concave surface facing the lens and a fourth lens having a weak negative refractive power with at least one refractive surface having an aspherical shape. lens. The imaging lens according to claim 1,
An imaging lens characterized by satisfying the following conditional expression (1) with respect to the dimension of the imaging lens in the optical axis direction.
(1) TL / Y <3.3
Where TL is the distance along the optical axis from the surface closest to the object to the image plane
Y: Maximum image height The imaging lens according to claim 1 or 2,
An imaging lens, characterized by satisfying the following conditional expression (2) with respect to the Abbe number of the first lens and the second lens with respect to d-line.
(2) 10 <ν _L2 -ν _L1
Here, ν _{L1 is} the Abbe number of the first lens with respect to d-line.
ν _L2 : Abbe number of the second lens with respect to d-line The imaging lens according to claim 1,
A lens group including at least the first lens and the second lens,
An imaging lens, wherein the following condition (3) is satisfied with respect to the focal length of the lens group and the third lens.
(3) 0.4 <f _L3 / f _LG <6.0
Where f _{LG is} the focal length of the lens group
f _L3 : focal length of the third lens The imaging lens according to any one of claims 1 to 4,
An imaging lens, wherein the following conditions (4) and (5) are satisfied with respect to the focal length and the image-side surface shape of the fourth lens.
(4) -75 <f _L4 / f <-1.0
(5) 0.2 <r _L4-2 / f <1.0
Where f is the focal length of the entire imaging lens system
f _L4 : focal length of the fourth lens
r _L4-2 : radius of curvature of the fourth lens on the image side The imaging lens according to any one of claims 1 to 5,
An imaging lens, wherein the third lens and the fourth lens are formed of a plastic material.

Images