Patent Application
20120293877
1. Field of the Invention
The present invention relates to an internal focus imaging lens having an anti-vibration function and suitable for digital cameras, video cameras, etc.
2. Description of the Related Art
Conventionally, numerous internal focus imaging lenses have been proposed that have an anti-vibration function of preventing image blur that occurs consequent to vibration. These imaging lenses further move a relatively light midsection lens group to perform focusing (see, for example, Japanese Patent Nos. 3745104, 3541283, and 3486541; Japanese Patent Application Laid-Open Publication Nos. 2008-145584 and 2005-321574).
Single lens reflex cameras are equipped with a mechanism that uses a mirror disposed just in front of the film, to reflect light that has passed through the imaging lens and thereby guide the light to the optical viewfinder to make the captured image and viewfinder image coincide. However, recently mirror-less single lens cameras that are significantly smaller in size and omit the reflective mirror that guides the image to the optical viewfinder have appeared. Originally, sub-reflective mirrors that guide the images to a distance measuring apparatus during auto-focusing were also disposed with these reflective mirrors. However, since mirror-less single lens cameras omit both the reflective mirror and the sub-reflective mirror, mainstream auto-focusing by phase difference detection cannot be performed in conventional single lens reflex cameras. Thus, in mirror-less single lens cameras, auto-focusing by contrast detection, which is mainstream among compact digital cameras and video cameras, is employed. However, in this case, with mainstream drive apparatuses such as DC motors and ultrasonic motors in conventional interchangeable lenses for single lens reflex cameras, necessary operations against wobble occurring with contrast detection auto-focusing are difficult. Consequently, drive apparatuses such as step motors used in compact digital cameras and video cameras are necessary.
Nonetheless, step motors have a low drive torque and with respect to a focusing lens group that is in a conventional optical system and that can be driven by a conventional DC motor or ultrasonic motor, numerous disadvantages arise related to the focusing lens group being too heavy to drive. With telephoto lenses, this tendency is strong. The same problems occur concerning the anti-vibration lens group. In particular, with a large diameter telephoto lens, since the lenses forming the anti-vibration lens group have a large diameter, anti-vibration lens group tends to be heavy. If the anti-vibration lens group becomes heavy, driving control at the time of vibration correction becomes difficult. In addition, if the lens diameter becomes large, the aberration that occurs also increases and consequently, to control the aberration, the number of lenses configuring the anti-vibration lens group has to be increased, making the anti-vibration lens group even heavier and creating a vicious circle.
The optical systems disclosed in the patent documents above are no exception and the driven focusing lens group and anti-vibration lens group are configured by multiple lenses and are thus, heavy, making driving control by a step motor, which has a low drive torque, difficult.
It is an object of the present invention to at least solve the above problems in the conventional technologies.
An imaging lens according to one aspect of the present invention includes, sequentially from an object side, a first lens group having a positive refractive power; a second lens group having a negative refractive power; a third lens group having a positive refractive power; and a fourth lens group having a negative refractive power. The third lens group is configured by a single lens element, and is moved along an optical axis, whereby focusing is performed.
The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings.
Referring to the accompanying drawings, exemplary embodiments of an imaging lens according to the present invention are explained in detail below.
The imaging lens according to the present invention includes sequentially from a side nearest an object (object side), a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a fourth lens group having a positive refractive power.
An object of the present invention is to provide a compact imaging lens having favorable imaging performance as well as a compact and light-weight focusing lens group and anti-vibration lens group. Thus, to achieve this object, the following conditions are set.
The imaging lens according to the present invention performs focusing by moving the third lens group along the optical axis. The third lens group is preferably configured by a single lens element to facilitate reductions in size and weight. In particular, configuration by a single lens is more preferable. A single lens element configuration of the third lens group, which is the focusing lens group, and reductions in size and weight enable the load on driving mechanism of the focusing lens group, power consumption and the outer diameter of the barrel to be reduced, and further enable a lens driving mechanism having a low drive torque (e.g., a step motor) to be employed. A single lens element may be a single ground lens, an aspheric lens, and a cemented aspheric lens; and does not include lenses that are not cemented to one another and have a gap therebetween such as 2 positive lenses.
Further, the imaging lens according to the present invention preferably satisfies the following conditional expressions, where f is the focal length of the entire optical system, f1 is the focal length of the first lens group, f2 is the focal length of the second lens group, f3 is the focal length of the third lens group, and f4 is the focal length of the fourth lens group.
0.4<f1/f<0.5 (1)
0.2<|f2|/f<0.4 (2)
0.4<f3/f<0.6 (3)
2.0<|f4|/f<16.0 (4)
Conditional expression (1) indicates a condition for suitably setting the positive refractive power of the first lens group. Below the lower limit of conditional expression (1), the positive refractive power of the first lens group becomes too strong and higher order spherical aberration easily occurs. When higher order spherical aberration occurs at the first lens group, correction of this aberration is difficult at another lens group. On the other hand, above the upper limit of conditional expression (1), the refractive power of the first lens group becomes too weak whereby the back focus increases, increasing the overall length of the optical system.
Conditional expression (2) indicates a condition for suitably setting the negative refractive power of second lens group. Below the lower limit of conditional expression (2), spherical aberration is overcorrected at the second lens group and as a result, favorable imaging performance for the optical system overall becomes difficult to maintain. On the other hand, above the upper limit of conditional expression (2), various types of aberration (particularly, spherical aberration) occurring at the first lens group, which has a positive refractive power, cannot be completely corrected at the second lens group, which has a negative refractive power, and thus, is undesirable.
Conditional expression (3) indicates a condition for suitably setting the positive refractive power of the third lens group, which is the focusing lens group. Below the lower limit of conditional expression (3), the number of lenses configuring the third lens group has to be increased in order to correct spherical aberration and coma flare. As a result, reductions in the size and weight of the focusing lens group become impossible. On the other hand, above the upper limit of conditional expression (3), the refractive power of the third lens group becomes too weak, the distance that the third lens group is moved to perform focusing increases, and imaging performance for close range imaging significantly deteriorates.
Conditional expression (4) indicates a condition for suitably setting the negative refractive power of the fourth lens group. Below the lower limit of conditional expression (4), the number of lenses configuring the fourth lens group has to be increased in order to correct spherical aberration and coma flare. As a result, reductions in the size of the optical system become difficult, thereby inviting decreases in optical transmission in the overall optical system, which is undesirable. On the other hand, above the upper limit of conditional expression (4), the back focus of the optical system becomes too long and the overall length of the optical system increases.
The imaging lens according to the present invention preferably satisfies the following conditional expression, where β3 is the lateral magnification of the third lens group and β4 is the lateral magnification of the fourth lens group.
1.5<β42−(β3×β4)2<2.5 (5)
Conditional expression (5) indicates a condition for suitably setting a combination of the lateral magnification of the third lens group, which is the focusing lens group, and the lateral magnification of the fourth lens group, which is disposed closer to the image than the third lens group. Satisfaction of conditional expression (5) enables the overall length of the optical system to be shortened and imaging performance to be improved. Below the lower limit of conditional expression (5), the distance that the third lens group is moved during focusing increases, thereby increasing the overall length of the optical system and inhibiting reductions in the size of the optical system. On the other hand, above the upper limit of conditional expression (5), aberration variation during focusing becomes prominent, and thus, is undesirable.
The imaging lens according to the present invention preferably satisfies the following conditional expression, where f1-2 is combined focal length of the first lens group and the second lens group and f is the focal length of the entire optical system.
1.0<f1-2/f<2.0 (6)
Conditional expression (6) indicates a condition for suitably setting a combination of the first lens group having a positive refractive power and the second lens group having a negative refractive power, the first and the second lens groups configuring a telephoto optical system. Below the lower limit of conditional expression (6), the optical convergence by the first lens group and the second lens group becomes strong, making the correction of various types of aberration difficult. On the other hand, above the upper limit of conditional expression (6), the configuration of the optical system approaches an a focal optical system (an optical system having a focal length at infinity) and the overall length of the optical system increases.
In the imaging lens according to the present invention, the first lens group preferably includes 1 or more negative lenses that satisfy the following conditional expression, where υdA is the Abbe number for the d-line of the negative lens.
υdA>60 (7)
The negative lens satisfying conditional expression (7) can be formed of an affordable glass material and therefore, by disposing even 1 negative lens formed of an affordable glass material in the first lens group, the manufacturing cost of the optical system can be reduced by the amount saved consequent to the use of the glass material.
In the imaging lens according to the present invention, a positive lens that satisfies the following conditional expression, where υdB is the Abbe number for d-line of the positive lens, is preferably disposed closest to the image, among the lenses of the first lens group (i.e., farthest on the image side of the first group).
υdB<35 (8)
By disposing farthest on the image side of the first lens group, a positive lens that satisfies conditional expression (8), the second lens group, which is disposed closer to the image than the first lens group, can be configured by 1 lens, enabling reductions in the size and weight of the optical system to be facilitated.
The imaging lens according to the present invention further has an anti-vibration correction function of correcting blur that occurs when the optical system is subject to vibration. For example, when the optical system is subject to vibration consequent to handshake, the anti-vibration lens group is moved in direction substantially perpendicular to the optical axis, whereby blur is corrected. In the imaging lens according to the present invention, the fourth lens group includes sequentially from the object side, a front group having a negative refractive power, an intermediate group having a negative refractive power, and a rear group having a positive refractive power. The intermediate group having a negative refractive power has a function of an anti-vibration lens group. In other words, the intermediate group is moved in a direction substantially perpendicular to the optical axis (shifting of lens center), whereby blur that occurs when the optical system is subject to vibration is corrected. The intermediate group is preferably configured by a single lens element to facilitate reductions in size and weight, and is more preferably configured by 1 lens. A single lens element configuration of the movable intermediate group and reductions in size and weight enable the load on the driving mechanism of the intermediate group to be decreased and power consumption to be reduced. Furthermore, a lens driving mechanism having a low drive torque (e.g., a step motor) can be employed. A single lens element is defined as described above.
The imaging lens according to the present invention preferably satisfies the following conditional expression, where fASM is the focal length of the intermediate group of the fourth lens group, fREAR is the focal length of the rear group of the fourth lens group, and f is the focal length of the entire optical system.
0.1<|fASM|/f<0.2 (9)
0.2<fREAR/f<0.3 (10)
Conditional expression (9) indicates a condition for suitably setting the negative refractive power of the intermediate group of the fourth lens group. Below the lower limit of conditional expression (9), the refractive power of the intermediate group of the fourth lens group becomes too strong, whereby the occurrence of spherical aberration and chromatic difference of magnification becomes prominent. Here, aberration that has become prominent is difficult to correct at another lens group. On the other hand, above the upper limit of conditional expression (9), the refractive power of the front group becomes too weak and the distance that the intermediate group, which is the anti-vibration lens group, is moved (shifting of lens center) increases, and consequently, the correction angle necessary for the anti-vibration function cannot be established.
Conditional expression (10) indicates a condition for suitably setting the positive refractive power of the rear group of the fourth lens group. Below the lower limit of conditional expression (10), overcorrected aberration occurs at the rear group of the fourth lens group and as a result, favorable imaging performance in the optical system overall becomes difficult to maintain. On the other hand, above the upper limit of conditional expression (10), various types of aberration occurring at the intermediate group in the fourth lens group and having a negative refractive power cannot be completely corrected.
The imaging lens according to the present invention preferably satisfies the following conditional expression, where βREAR is the lateral magnification of the rear group of the fourth lens group and βASM is the lateral magnification of the intermediate group of the fourth lens group.
−2.0<βREAR−(βASM×βREAR)<−1.6 (11)
Conditional expression (11) indicates a condition for suitably setting a combination of the lateral magnification of the intermediate group, which is the anti-vibration lens group and in the fourth lens group, and the lateral magnification of the rear group disposed on the image side of the intermediate group. Below the lower limit of conditional expression (11), aberration variation increases at the time of anti-vibration correction and thus, is undesirable. On the other hand, above the upper limit of conditional expression (11), the intermediate group of the fourth lens group is moved (shifting of lens center) over a greater distance at the time of anti-vibration correction, which increases the diameter of the optical system, and thus, is undesirable.
In the imaging lens according to the present invention, the fourth lens group may be configured as follows. In other words, the fourth lens group includes sequentially from the object side, the front group having a negative refractive power and a rear group having a positive refractive power. The front group having a negative refractive power has a function as an anti-vibration lens group and is moved substantially perpendicular to the optical axis (shifting of lens center), whereby blur occurring when the optical system is subject to vibration, is corrected. The front group is preferably configured by a single lens element to facilitate reductions in size and weight, and is more preferably configured by 1 lens. A single lens element configuration of the movable front group and reductions in size and weight enable the load on the driving mechanism of the front group to be decreased and power consumption to be reduced. Furthermore, a lens driving mechanism having a low drive torque (e.g., a step motor) can be employed. A single lens element is defined as described above.
When the fourth lens group is configured by 2 lens groups, the imaging lens according to the present invention preferably satisfies the following conditional expression, where fASF is the focal length of the front group, fREAR is the focal length of the rear group, and f is the focal length of the entire optical system.
0.1<|fASF|/f<0.2 (12)
0.2<fREAR/f<0.3 (10)
Conditional expression (12) corresponds to conditional expression (9) described above and when values deviate from the prescribed upper and lower limits, the disadvantages that arise are identical to those in the case of conditional expression (9).
When the fourth lens group is configured by 2 lens groups, the imaging lens according to the present invention preferably satisfies the following conditional expression, where βREAR is the lateral magnification of the rear group and βASF is the lateral magnification of the front group.
−2.0<{βREAR−(βASF×βREAR}<−1.6 (13)
Conditional expression (13) corresponds to conditional expression (11) described above and when values deviate from the prescribed upper and lower limits, the disadvantages that arise are identical to those in the case of conditional expression (11).
As described, according to the present invention, a compact imaging lens having a compact, light-weight focusing lens group and anti-vibration as well as favorable imaging performance can be realized. In particular, configuration of the focusing lens group and the anti-vibration lens group by a single lens element, respectively, enables further reductions in the size and weight of these movable lens groups. Consequently, the imaging lens is suitable for an imaging apparatus equipped with a lens drive apparatus such as a step motor having a low drive torque and can be used in an imaging apparatus equipped with a contrast detection autofocus mechanism. Further, satisfaction of the conditional expressions above suppresses the distance that the focusing lens group and anti-vibration, which are movable lens groups, are moved, thereby facilitating further reductions in the size of the optical system and enabling further improvement of the imaging performance of the optical system.
Hereinafter, with reference to the accompanying drawings, embodiments of the imaging lens according to the present invention will be described in detail. However, the present invention is not limited to the embodiments below.
The first lens group G11 includes, sequentially from the object side, a negative lens L111, a positive lens L112, a positive lens L113, a negative lens L114, a positive lens L115, a negative lens L116, and a positive lens L117. The positive lens L115 and the negative lens L116 are cemented.
The second lens group G12 is configured by a negative lens L121.
The third lens group G13 is configured by a positive lens L131. The surface of the object side of the positive lens L131 is aspheric. The third lens group G13 is moved along the optical axis, from the image plane IMG side toward the object side, whereby focusing from a state of infinity focus to a focus state for the minimum object distance is performed.
The fourth lens group G14 includes, sequentially from the object side, a front group G14F having a negative refractive power, an intermediate group G14M having a negative refractive power, an aperture stop AS, and a rear group G14R having a positive refractive power. The aperture stop AS suppresses the occurrence of aberration, which causes deterioration of the imaging performance of the optical system. The front group G14F includes, sequentially from the object side, a negative lens L141 and a positive lens L142. The negative lens L141 and the positive lens L142 are cemented. The intermediate group G14M is configured by a negative lens L143. Both surfaces of the negative lens L143 are aspheric. The intermediate group G14M has a function as the anti-vibration lens group. In other words, the intermediate group G14M is moved in a direction that is substantially perpendicular to the optical axis (shifting of lens center), whereby blur that occurs when the optical system is subject to vibration consequent to handshake, is corrected. The rear group G14R includes, sequentially from the object side, a negative lens L144, a positive lens L145, a positive lens L146, and a negative lens L147. The negative lens L144 and the positive lens L145 are cemented.
Various values related to the imaging lens according to the first embodiment are indicated below.
Among the values for each of the embodiments, r2, . . . indicate radii of curvature for each lens, diaphragm surface, etc.; d1, d2, . . . indicate the thickness of the lenses, diaphragm, etc. or the distance between surfaces thereof; nd1, nd2, . . . indicate the refraction index of each lens with respect to the d-line (λ=587.56 nm); and υd1, υd2, . . . indicate the Abbe number with respect to the d-line (λ=587.56 nm) of each lens. Lengths are indicated in units of [mm] and angles are indicated in [degrees].
Each aspheric surface shape above is expressed by equation [1], where Z is the distance from the vertex of the lens surface to the optical axis; c(=1/r, where, r=radius of curvature) is paraxial radius of curvature at the vertex of the lens surface; h is the height from the optical axis; K is the constant of the cone; A4 and A6 are the fourth and sixth aspheric coefficients, respectively; and the travel direction of light is positive.
As described, in the imaging lens of the first embodiment, the focusing lens group and the anti-vibration lens group are respectively configured by 1 lens, facilitating reductions in the size and weight of these movable lens groups and enabling further reduction of the size of the optical system overall. In particular, satisfaction of the conditional expressions above suppresses the distance that the focusing lens group and the anti-vibration lens group move, facilitating further reduction of the size of the optical system and enabling further improvement of the imaging performance of the optical system. Further, the imaging lens of the first embodiment uses, as necessary, lenses having an aspheric surface and cemented lenses, whereby favorable imaging performance can be maintained by fewer lenses.
The first lens group G21 includes, sequentially from the object side, a negative lens L211, a positive lens L212, a positive lens L213, a negative lens L214, a positive lens L215, a negative lens L216, and a positive lens L217. The positive lens L215 and the negative lens L216 are cemented.
The second lens group G22 is configured by a negative lens L221.
The third lens group G23 is configured by a positive lens L231. The surface of the object side of the positive lens L231 is aspheric. The third lens group G23 is moved along the optical axis, from the image plane IMG side toward the object side, whereby focusing from the state of infinity focus to the focus state for the minimum object distance is performed.
The fourth lens group G24 includes, sequentially from the object side, a front group G24F having a negative refractive power, an intermediate group G24M having a negative refractive power, the aperture stop AS, and a rear group G24R having a positive refractive power. The aperture stop AS suppresses the occurrence of aberration, which causes deterioration of the imaging performance of the optical system. The front group G24F includes, sequentially from the object side, a negative lens L241 and a positive lens L242. The negative lens L241 and the positive lens L242 are cemented. The intermediate group G24M is configured by a negative lens L243. Both surfaces of the negative lens L243 are aspheric. The intermediate group G24M has a function as the anti-vibration lens group. In other words, the intermediate group G24M is moved in a direction that is substantially perpendicular to the optical axis (shifting of lens center), whereby blur that occurs when the optical system is subject to vibration consequent to handshake, is corrected. The rear group G24R includes, sequentially from the object side, a negative lens L244, a positive lens L245, a positive lens L246, and a negative lens L247. The negative lens L244 and the positive lens L245 are cemented.
Various values related to the imaging lens according to the second embodiment are indicated below.
Among the values for each of the embodiments, r2, . . . indicate radii of curvature for each lens, diaphragm surface, etc.; d1, d2, . . . indicate the thickness of the lenses, diaphragm, etc. or the distance between surfaces thereof; nd1, nd2, . . . indicate the refraction index of each lens with respect to the d-line (λ=587.56 nm); and υd1, υd2, . . . indicate the Abbe number with respect to the d-line (λ=587.56 nm) of each lens. Lengths are indicated in units of [mm] and angles are indicated in [degrees].
Each aspheric surface shape above is expressed by equation [1], where Z is the distance from the vertex of the lens surface to the optical axis; c(=1/r, where, r=radius of curvature) is paraxial radius of curvature at the vertex of the lens surface; h is the height from the optical axis; K is the constant of the cone; A4 and A6 are the fourth and sixth aspheric coefficients, respectively; and the travel direction of light is positive.
As described, in the imaging lens of the second embodiment, the focusing lens group and the anti-vibration lens group are respectively configured by 1 lens, facilitating reductions in the size and weight of these movable lens groups and enabling further reduction of the size of the optical system overall. In particular, satisfaction of the conditional expressions above suppresses the distance that the focusing lens group and the anti-vibration lens group move, facilitating further reduction of the size of the optical system and enabling further improvement of the imaging performance of the optical system. Further, the imaging lens of the second embodiment uses, as necessary, lenses having an aspheric surface and cemented lenses, whereby favorable imaging performance can be maintained by fewer lenses.
The first lens group G31 includes, sequentially from the object side, a negative lens L311, a positive lens L312, a positive lens L313, a negative lens L314, a positive lens L315, a negative lens L316, and a positive lens L317. The positive lens L315 and the negative lens L316 are cemented.
The second lens group G32 is configured by a negative lens L321.
The third lens group G33 is configured by a positive lens L331. The surface of the object side of the positive lens L331 is aspheric. The third lens group G33 is moved along the optical axis, from the image plane IMG side toward the object side, whereby focusing from the state of infinity focus to the focus state for the minimum object distance is performed.
The fourth lens group G34 includes, sequentially from the object side, a front group G34F having a negative refractive power, an intermediate group G34M having a negative refractive power, the aperture stop AS, and a rear group G34R having a positive refractive power. The aperture stop AS suppresses the occurrence of aberration, which causes deterioration of the imaging performance of the optical system. The front group G34F includes, sequentially from the object side, a negative lens L341 and a positive lens L342. The negative lens L341 and the positive lens L342 are cemented. The intermediate group G34M is configured by a negative lens L343. Both surfaces of the negative lens L343 are aspheric. The intermediate group G34M has a function as the anti-vibration lens group. In other words, the intermediate group G34M is moved in a direction that is substantially perpendicular to the optical axis (shifting of lens center), whereby blur that occurs when the optical system is subject to vibration consequent to handshake, is corrected. The rear group G34R includes, sequentially from the object side, a negative lens L344, a positive lens L345, a positive lens L346, and a negative lens L347. The negative lens L344 and the positive lens L345 are cemented.
Various values related to the imaging lens according to the third embodiment are indicated below.
Among the values for each of the embodiments, r1, r2, . . . indicate radii of curvature for each lens, diaphragm surface, etc.; d1, d2, . . . indicate the thickness of the lenses, diaphragm, etc. or the distance between surfaces thereof; nd1, nd2, . . . indicate the refraction index of each lens with respect to the d-line (λ=587.56 nm); and υd1, υd2, . . . indicate the Abbe number with respect to the d-line (λ=587.56 nm) of each lens. Lengths are indicated in units of [mm] and angles are indicated in [degrees].
Each aspheric surface shape above is expressed by equation [1], where Z is the distance from the vertex of the lens surface to the optical axis; c(=1/r, where, r=radius of curvature) is paraxial radius of curvature at the vertex of the lens surface; h is the height from the optical axis; K is the constant of the cone; A4 and A6 are the fourth and sixth aspheric coefficients, respectively; and the travel direction of light is positive.
As described, in the imaging lens of the third embodiment, the focusing lens group and the anti-vibration lens group are respectively configured by 1 lens, facilitating reductions in the size and weight of these movable lens groups and enabling further reduction of the size of the optical system overall. In particular, satisfaction of the conditional expressions above suppresses the distance that the focusing lens group and the anti-vibration lens group move, facilitating further reduction of the size of the optical system and enabling further improvement of the imaging performance of the optical system. Further, the imaging lens of the third embodiment uses, as necessary, lenses having an aspheric surface and cemented lenses, whereby favorable imaging performance can be maintained by fewer lenses.
The first lens group G41 includes, sequentially from the object side, a negative lens L411, a positive lens L412, a positive lens L413, a negative lens L414, a positive lens L415, a negative lens L416, and a positive lens L417. The positive lens L415 and the negative lens L416 are cemented.
The second lens group G42 is configured by a negative lens L421.
The third lens group G43 is configured by a positive lens L431. The third lens group G43 is moved along the optical axis, from the image plane IMG side toward the object side, whereby focusing from the state of infinity focus to the focus state for the minimum object distance is performed.
The fourth lens group G44 includes, sequentially from the object side, a front group G44F having a negative refractive power and a rear group G44R having a positive refractive power. The front group G44F is configured by a negative lens L441. Both surfaces of the negative lens L441 are aspheric. The front group G44F has a function as the anti-vibration lens group. In other words, the front group G44F is moved in a direction that is substantially perpendicular to the optical axis (shifting of lens center), whereby blur that occurs when the optical system is subject to vibration consequent to handshake, is corrected. The rear group G44R includes, sequentially from the object side, a negative lens L442, a positive lens L443, a positive lens L444, and a negative lens L445. The negative lens L442 and the positive lens L443 are cemented. Furthermore, the positive lens L444 and the negative lens L445 are cemented.
Various values related to the imaging lens according to the fourth embodiment are indicated below.
Among the values for each of the embodiments, r1, r2, . . . indicate radii of curvature for each lens, diaphragm surface, etc.; d1, d2, . . . indicate the thickness of the lenses, diaphragm, etc. or the distance between surfaces thereof; nd1, nd2, . . . indicate the refraction index of each lens with respect to the d-line (λ=587.56 nm); and υd1, υd2, . . . indicate the Abbe number with respect to the d-line (λ=587.56 nm) of each lens. Lengths are indicated in units of [mm] and angles are indicated in [degrees].
Each aspheric surface shape above is expressed by equation [1], where Z is the distance from the vertex of the lens surface to the optical axis; c(=1/r, where, r=radius of curvature) is paraxial radius of curvature at the vertex of the lens surface; h is the height from the optical axis; K is the constant of the cone; A4 and A6 are the fourth and sixth aspheric coefficients, respectively; and the travel direction of light is positive.
As described in the imaging lens of the fourth embodiment, the focusing lens group and the anti-vibration lens group are respectively configured by 1 lens, facilitating reductions in the size and weight of these movable lens groups and enabling further reduction of the size of the optical system overall. In particular, satisfaction of the conditional expressions above suppresses the distance that the focusing lens group and the anti-vibration lens group move, facilitating further reduction of the size of the optical system and enabling further improvement of the imaging performance of the optical system. In particular, in the imaging lens of the fourth embodiment, the fourth lens group, which includes the anti-vibration lens group, is configured by 2 lens groups, further simplifying the configuration of the optical system overall. The imaging lens of the fourth embodiment uses, as necessary, lenses having an aspheric surface and cemented lenses, whereby favorable imaging performance can be maintained by fewer lenses.
Furthermore, according to the present invention, configuration of the front group by a single lens element enables effective suppression of aberration at risk of occurring.
As described, the imaging lens according to the present invention is useful for digital cameras, video cameras, etc. and in particular, is suitable for imaging apparatuses equipped with a contrast detection autofocus mechanism.
Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.
The present document incorporates by reference the entire contents of Japanese priority document, 2011-110768 and 2011-110769 filed in Japan on May 17, 2011.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-110768 | May 2011 | JP | national |
| 2011-110769 | May 2011 | JP | national |
