JP4992102B2 - Variable magnification optical system and imaging apparatus - Google Patents

Description

  The present invention relates to a variable magnification optical system. ) And an imaging apparatus including the same.

In recent years, with the spread of personal computers, digital cameras that can easily capture images are becoming popular. Along with this, a smaller digital camera has been demanded, and further downsizing of the photographing lens system has been demanded. On the other hand, since the number of pixels of the image pickup device tends to increase year by year, high optical performance corresponding to the increase in the number of pixels of the image pickup device is required for the photographing lens system. In addition, since digital scaling for general-purpose digital cameras is also demanded for zooming of images, particularly optical zooming with little image degradation, a zoom lens system of a type that is compact, high-performance, and capable of zooming optically is conventional. It has been proposed (see, for example, Patent Documents 1 and 2).
JP 2001-296476 A JP 2003-107350 A

  In terms of downsizing, there is a demand not only for simply reducing the size of the taking lens system, but also for downsizing considering the lens barrel configuration. However, this is not taken into consideration in the miniaturization of the conventionally known zoom lens system, and it is difficult to meet the recent miniaturization of digital cameras. For example, Patent Documents 1 and 2 propose a negative, positive, and positive three-group zoom lens system that is relatively small and has a zoom ratio of about 3 times. The conversion is insufficient.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a compact and high-performance variable magnification optical system that also considers downsizing of the lens barrel configuration, and an imaging apparatus including the same. There is to do.

In order to achieve the above object, a variable magnification optical system according to a first aspect of the present invention is a variable magnification optical system for forming an optical image of an object on a light receiving surface of an image sensor so that the magnification can be changed. In order, the first lens group having the negative power, the second lens group having the positive power, and the third lens group having the positive power, or the first lens group and the second lens. And a third lens group, and a fourth lens group. The zoom ratio is 2.5 or more, and at least the first lens group and the first lens group in zooming from the wide-angle end to the telephoto end. The second lens group and the third lens group are configured to move . In the second lens group, a positive lens is disposed closest to the object side. The following conditional expressions (1) , (2), and ( 3a) is satisfied.
1.8 <| f1 / fw | <3… (1)
0.34 <fw · tanωw / (L1-L2) <0.5 (2)
1.85 ≦ Np (3a)
However,
f1: focal length of the first lens group,
fw: Focal length of the entire variable magnification optical system at the wide-angle end,
ωw: Half angle of view at the wide-angle end,
L1: Relative movement of the first lens unit in zooming from the wide-angle end to the telephoto end (the image side is positive),
L2: Relative amount of movement of the second lens unit during zooming from the wide-angle end to the telephoto end (the image side is positive),
Np: refractive index with respect to d-line of the positive lens located closest to the object side in the second lens group,
It is.

In the variable magnification optical system according to a second aspect of the present invention, in the first aspect , the second lens group is disposed in order from the object side, a cemented lens including a positive lens and a negative lens, and a rear side of the cemented lens. And the lens arranged behind the cemented lens satisfies the following conditional expression (4).
| ΦL / φ2 | <0.4… (4)
However,
φL: refractive power of the lens arranged behind the cemented lens in the second lens group,
φ2: refractive power of the second lens group,
It is.

A variable magnification optical system according to a third aspect of the invention is characterized in that, in the first or second aspect of the invention, the following conditional expression (5) is satisfied.
-0.5 <(Tw1−Tt2) / fw <0 (5)
However,
Tw1: The distance from the surface vertex of the first lens unit on the image plane side to the image plane at the wide-angle end,
Tt2: Distance from the object-side surface vertex of the second lens group at the telephoto end to the image plane,
fw: Focal length of the entire variable magnification optical system at the wide-angle end,
It is.

Imaging device of the fourth invention is characterized by comprising a first to third variable magnification optical system according to any one of the invention.

According to the present invention, in a variable power optical system having a variable power ratio of 2.5 times or more, which includes a negative lens first lens group, a positive power second lens group, and a positive power third lens group in order from the object side. Since the first and second lens groups are configured to satisfy predetermined conditions, a compact and high-performance variable magnification optical system that takes into account the miniaturization of the lens barrel configuration, and an imaging apparatus including the same Can be realized. If the imaging device according to the present invention is used in devices such as digital cameras and portable information devices, it can contribute to thinning, light weight, compactness, low cost, high performance, high functionality, etc. of these devices. it can.

  Hereinafter, a variable magnification optical system, an imaging apparatus, and the like embodying the present invention will be described with reference to the drawings. An imaging apparatus according to the present invention is an optical apparatus that optically captures an image of a subject and outputs it as an electrical signal, and constitutes a main component of a camera used for still image shooting and moving image shooting of a subject. is there. Examples of such a camera include a digital camera; a video camera; a surveillance camera; an in-vehicle camera; a videophone camera; a doorphone camera; a personal computer, a mobile computer, a mobile phone, a personal digital assistant (PDA), These peripheral devices (mouse, scanner, printer, etc.), and cameras built in or externally attached to other digital devices can be mentioned. As can be seen from these examples, it is possible not only to configure a camera by using an imaging apparatus, but also to add a camera function by mounting the imaging apparatus on various devices. For example, a digital device with an image input function such as a mobile phone with a camera can be configured.

  The term “digital camera” used to refer to the recording of optical still images, but digital still cameras and home digital movie cameras that can handle still images and movies simultaneously have been proposed. In particular, it has become indistinguishable. Therefore, the term “digital camera” means a digital still camera, digital movie camera, or web camera (whether an open type or a private type, a camera that is connected to a network and connected to a device that enables image transmission / reception). , And the like, including those directly connected to a network and those connected via a device having an information processing function such as a personal computer.) It is assumed that all cameras having an imaging device including an imaging device or the like for converting a video signal as an electric video signal are main components.

  FIG. 15 is a schematic cross-sectional view showing a schematic optical configuration example of a camera CU (corresponding to a digital camera, a digital device with an image input function, etc.). An imaging device LU mounted on the camera CU is a zoom lens system ZL (magnification as a photographing lens system) that forms an optical image (IM: image plane) of the object in order from the object (namely, subject) side. ST corresponds to an optical system, ST: stop, and parallel plane plate PT (optical filters such as an optical low-pass filter and an infrared cut filter disposed as necessary; corresponding to a cover glass of the image sensor SR) And an image sensor SR that converts an optical image IM formed on the light receiving surface SS by the zoom lens system ZL into an electrical signal, and a digital camera and a portable information device with an image input function (that is, portable It is a part of a camera CU corresponding to a small and portable information device terminal such as a telephone or a PDA. When a digital camera is configured with this image pickup device LU, the image pickup device LU is usually arranged inside the body of the camera. However, when realizing the camera function, it is possible to adopt a form as necessary. is there. For example, the unitized imaging device LU may be configured to be detachable or rotatable with respect to the camera body, and the unitized imaging device LU may be detachable with respect to a portable information device (cell phone, PDA, etc.) or You may comprise so that rotation is possible.

  The zoom lens system ZL includes a plurality of lens groups, and the plurality of lens groups moves along the optical axis AX and performs zooming (ie, zooming) by changing the lens group interval. In the first and fourth embodiments to be described later, the zoom lens system ZL has a negative, positive, positive, and negative four-component zoom configuration, and in the second, third, fifth to seventh embodiments. The zoom lens system ZL has a negative / positive / positive three-component zoom configuration. In any of the first to seventh embodiments, the first to third lens groups GR1 to GR3 constitute a moving group, and in the first and fourth embodiments, the fourth lens group. GR4 constitutes a fixed group. Note that the photographing lens system used in the imaging device LU is not limited to the zoom lens system ZL. Instead of the zoom lens system ZL, another type of variable magnification optical system (for example, a variable focal length imaging optical system such as a varifocal lens system or a multiple focal length switching type lens) may be used as a photographing lens system. .

  The optical image to be formed by the zoom lens system ZL passes through an optical low-pass filter (corresponding to the parallel plane plate PT in FIG. 15) having a predetermined cutoff frequency characteristic determined by the pixel pitch of the image sensor SR. By doing so, the spatial frequency characteristic is adjusted so that the so-called aliasing noise generated when converted into an electrical signal is minimized. Thereby, generation | occurrence | production of a color moire can be suppressed. However, if the performance around the resolution limit frequency is suppressed, there is no need to worry about the occurrence of noise without using an optical low-pass filter, and the display system where the noise is not very noticeable (for example, the liquid crystal of a mobile phone) When a user performs shooting or viewing using a screen or the like, it is not necessary to use an optical low-pass filter in the shooting lens system. Therefore, in an imaging apparatus that does not require an optical low-pass filter, if the exit pupil position can be appropriately arranged, it is possible to achieve downsizing of the imaging apparatus and camera by shortening the back focus. As the optical low-pass filter, a birefringence low-pass filter, a phase low-pass filter, or the like can be applied. Examples of the birefringent low-pass filter include those made of a birefringent material such as quartz whose crystal axis direction is adjusted to a predetermined direction, and those obtained by laminating wave plates that change the polarization plane. Examples of the phase type low-pass filter include those that achieve the required optical cutoff frequency characteristics by the diffraction effect.

  As the imaging element SR, for example, a solid-state imaging element such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) sensor having a plurality of pixels is used. The optical image formed by the zoom lens system ZL (on the light receiving surface SS of the image sensor SR) is converted into an electrical signal by the image sensor SR. The signal generated by the image sensor SR is subjected to predetermined digital image processing, image compression processing, etc. as necessary, and recorded as a digital video signal in a memory (semiconductor memory, optical disk, etc.), or in some cases via a cable. Or converted into an infrared signal and transmitted to another device.

  In the imaging apparatus LU shown in FIG. 15, the zoom lens system ZL performs reduction projection from the enlargement subject to the reduction-side imaging element SR, but a display element that displays a two-dimensional image instead of the imaging element SR. If a zoom lens system ZL is used as a projection lens system using a liquid crystal display element (for example, a liquid crystal display element), an image projection apparatus that performs enlargement projection from the image display surface on the reduction side to the screen surface on the enlargement side can be configured. That is, the zoom lens system ZL of each embodiment described below is not limited to use as a photographing lens system, but can also be suitably used as a projection lens system.

  1 to 7 are lens configuration diagrams respectively corresponding to the zoom lens systems ZL constituting the first to seventh embodiments, and the lens arrangement at the wide angle end (W) is shown in an optical section. In each lens configuration diagram, the surface with ri (i = 1, 2, 3,...) Is the i-th surface counted from the object side (the surface with ri marked with * is an aspheric surface). In addition, the axial upper surface interval with di (i = 1, 2, 3,...) Is a variable interval that changes during zooming among the i-th axial upper surface interval counted from the object side. In each lens configuration diagram, arrows m1, m2, and m3 indicate the movement of the first lens group GR1, the second lens group GR2, and the third lens group GR3 during zooming from the wide-angle end (W) to the telephoto end (T), respectively. The arrow on the most image side indicates that the fourth lens group GR4 and the plane parallel plate PT are fixed in zooming. In any of the embodiments, a stop ST is disposed between the first lens group GR1 and the second lens group GR2, and the stop ST is zoomed together with the second lens group GR2 (arrow m2). It has become.

  The zoom lens system ZL of the first and fourth embodiments includes, in order from the object side, a first lens group GR1 having negative power (power: an amount defined by the reciprocal of the focal length), a stop ST, It consists of a second lens group GR2 having a positive power, a third lens group GR3 having a positive power, and a fourth lens group GR4 having a negative power, and zooming is performed by changing the distance between the lens groups. It has a four component zoom configuration. The zoom lens systems ZL of the second, third, fifth to seventh embodiments have, in order from the object side, the first lens group GR1 having negative power, the aperture stop ST, and positive power. The zoom lens includes a second lens group GR2 and a third lens group GR3 having positive power, and has a three-component zoom configuration that performs zooming by changing the distance between the lens groups. The lens configuration of each embodiment will be described in detail below.

  In the first embodiment (FIG. 1), each lens group in the negative / positive / positive / negative four-component zoom configuration is configured as follows. The first lens group GR1 includes, in order from the object side, a biconcave negative lens whose image side surface is an aspheric surface and a positive meniscus lens convex on the object side. The second lens group GR2 includes, in order from the object side, a cemented lens including a biconvex positive lens and a biconcave negative lens, and a positive meniscus lens having a double aspheric surface and convex to the image side. An aperture stop ST that moves together with the second lens group GR2 during zooming is disposed on the object side of the second lens group GR2. The third lens group GR3 is composed of only one double-sided aspheric and biconvex positive lens. The fourth lens group GR4 includes only one negative plano-concave lens that is concave on the object side, and is joined to the plane parallel plate PT on the image side surface. In zooming from the wide-angle end (W) to the telephoto end (T), the first lens group GR1 moves U-turn from the image side to the object side after moving toward the image side, and the second lens group GR2 monotonously toward the object side. The third lens group GR3 moves monotonously to the image side, and the fourth lens group GR4 is fixed at the zoom position with respect to the image plane IM.

  In the second embodiment (FIG. 2), each lens group is configured as follows in a negative / positive / positive three-component zoom configuration. The first lens group GR1 includes, in order from the object side, a biconcave negative lens whose image side surface is an aspheric surface and a positive meniscus lens convex on the object side. The second lens group GR2 includes, in order from the object side, a cemented lens including a biconvex positive lens and a biconcave negative lens, and a positive meniscus lens having a double aspheric surface and convex to the image side. An aperture stop ST that moves together with the second lens group GR2 during zooming is disposed on the object side of the second lens group GR2. The third lens group GR3 includes only one positive meniscus lens that is aspheric on both sides and is convex on the image side. In zooming from the wide-angle end (W) to the telephoto end (T), the first lens group GR1 moves U-turn from the image side to the object side after moving toward the image side, and the second lens group GR2 monotonously toward the object side. The third lens group GR3 moves monotonously to the image side.

  In the third embodiment (FIG. 3), each lens group is configured as follows in a negative / positive / positive three-component zoom configuration. The first lens group GR1 includes, in order from the object side, a biconcave negative lens whose both surfaces are aspheric surfaces, and a positive meniscus lens convex on the object side. The second lens group GR2 includes, in order from the object side, a cemented lens including a biconvex positive lens and a biconcave negative lens, and a positive meniscus lens having a double aspheric surface and convex to the image side. An aperture stop ST that moves together with the second lens group GR2 during zooming is disposed on the object side of the second lens group GR2. The third lens group GR3 includes only one positive meniscus lens that is aspheric on both sides and is convex on the image side. In zooming from the wide-angle end (W) to the telephoto end (T), the first lens group GR1 moves U-turn from the image side to the object side after moving toward the image side, and the second lens group GR2 monotonously toward the object side. The third lens group GR3 moves monotonously to the image side.

  In the fourth embodiment (FIG. 4), each lens unit is configured as follows in a negative / positive / positive / negative four-component zoom configuration. The first lens group GR1 includes, in order from the object side, a biconcave negative lens whose image side surface is an aspheric surface and a positive meniscus lens convex on the object side. The second lens group GR2, in order from the object side, a biconvex positive lens, a cemented lens composed of a biconvex positive lens and a biconcave negative lens, a positive meniscus lens having a double aspheric surface and convex to the image side, A diaphragm ST that moves together with the second lens group GR2 during zooming is disposed on the object side of the second lens group GR2. The third lens group GR3 is composed of only one double-sided aspheric and biconvex positive lens. The fourth lens group GR4 includes only one negative plano-concave lens that is concave on the object side, and is joined to the plane parallel plate PT on the image side surface. In zooming from the wide-angle end (W) to the telephoto end (T), the first lens group GR1 moves U-turn from the image side to the object side after moving toward the image side, and the second lens group GR2 monotonously toward the object side. The third lens group GR3 moves monotonously to the image side, and the fourth lens group GR4 is fixed at the zoom position with respect to the image plane IM.

  In the fifth embodiment (FIG. 5), each lens group in the negative / positive / positive three-component zoom configuration is configured as follows. The first lens group GR1 includes, in order from the object side, a biconcave negative lens whose image side surface is an aspheric surface and a positive meniscus lens convex on the object side. The second lens group GR2 includes, in order from the object side, a biconvex positive lens, a cemented lens composed of a positive meniscus lens convex on the object side and a negative meniscus lens concave on the image side, and a double-sided aspheric surface convex on the image side. The aperture stop ST that moves together with the second lens group GR2 during zooming is disposed on the object side of the second lens group GR2. The third lens group GR3 is composed of only one double-sided aspheric and biconvex positive lens. In zooming from the wide-angle end (W) to the telephoto end (T), the first lens group GR1 moves U-turn from the image side to the object side after moving toward the image side, and the second lens group GR2 monotonously toward the object side. The third lens group GR3 moves monotonously to the image side.

  In the sixth embodiment (FIG. 6), each lens group is configured as follows in a negative / positive / positive three-component zoom configuration. The first lens group GR1 includes, in order from the object side, a biconcave negative lens whose image side surface is an aspheric surface and a positive meniscus lens convex on the object side. The second lens group GR2 includes, in order from the object side, a biconvex positive lens, a cemented lens composed of a positive meniscus lens convex on the object side and a negative meniscus lens concave on the image side, and a double-sided aspheric surface convex on the image side. The aperture stop ST that moves together with the second lens group GR2 during zooming is disposed on the object side of the second lens group GR2. The third lens group GR3 includes only one positive meniscus lens that is aspheric on both sides and is convex on the image side. In zooming from the wide-angle end (W) to the telephoto end (T), the first lens group GR1 moves U-turn from the image side to the object side after moving toward the image side, and the second lens group GR2 monotonously toward the object side. The third lens group GR3 moves monotonously to the image side.

  In the seventh embodiment (FIG. 7), each lens unit is configured as follows in a negative / positive / positive three-component zoom configuration. The first lens group GR1 includes, in order from the object side, a biconcave negative lens whose image side surface is an aspheric surface and a positive meniscus lens convex on the object side. The second lens group GR2 includes, in order from the object side, a biconvex positive lens, a cemented lens composed of a positive meniscus lens convex on the object side and a negative meniscus lens concave on the image side, and a double-sided aspheric surface convex on the image side. The aperture stop ST that moves together with the second lens group GR2 during zooming is disposed on the object side of the second lens group GR2. The third lens group GR3 includes only one positive meniscus lens that is aspheric on both sides and is convex on the image side. In zooming from the wide-angle end (W) to the telephoto end (T), the first lens group GR1 moves U-turn from the image side to the object side after moving toward the image side, and the second lens group GR2 monotonously toward the object side. The third lens group GR3 moves monotonously to the image side.

  As described above, each of the embodiments includes at least the first lens group GR1 having negative power and the second lens group GR2 having positive power in order from the object side, from the wide-angle end (W). At the zooming up to the telephoto end (T), at least the first lens group GR1 and the second lens group GR2 move, and the zooming ratio is 2.5 or more. In such a variable magnification optical system, if the first and second lens groups are configured so as to satisfy a predetermined condition, a high performance and small variable magnification optical system that also considers a reduction in the size of the lens barrel structure, An imaging apparatus including the above can be realized. If the imaging device is used in a digital device such as a digital camera or a portable information device, it can contribute to thinness, lightness, compactness, low cost, high performance, high functionality, and the like. The conditions for obtaining such effects in a balanced manner will be described below. The zoom ratio is preferably 2.5 times or more, and more preferably 2.5 to 3 times.

It is desirable to satisfy the following conditional expressions (1) and (2).
1.8 <| f1 / fw | <3… (1)
0.34 <fw · tanωw / (L1-L2) <0.5 (2)
However,
f1: focal length of the first lens group,
fw: Focal length of the entire variable magnification optical system at the wide-angle end,
ωw: Half angle of view at the wide-angle end,
L1: Relative movement of the first lens unit in zooming from the wide-angle end to the telephoto end (the image side is positive),
L2: Relative amount of movement of the second lens unit during zooming from the wide-angle end to the telephoto end (the image side is positive),
It is.

  Conditional expression (1) defines a preferable condition range for the focal length of the first lens unit. If the upper limit of the conditional expression (1) is exceeded, the focal length of the first lens group becomes too large, resulting in an increase in the overall length and a decrease in power to be converged by the first lens group. growing. Therefore, it becomes impossible to obtain a small variable magnification optical system. On the other hand, if the lower limit of conditional expression (1) is exceeded, the focal length of the first lens group becomes too small, so that the negative distortion that occurs at the wide-angle end becomes too large, making it difficult to correct it.

  Conditional expression (2) defines a preferable condition range for the movement amount of the first lens group and the second lens group. If the upper limit of the conditional expression (2) is exceeded, the power of the first lens group and the second lens group becomes too strong, and it becomes difficult to correct the generated aberration. On the contrary, if the lower limit of conditional expression (2) is exceeded, the total length becomes too large, which is not preferable in terms of compactness.

  In a zoom lens system of a general digital camera in which the first and second lens groups are negative / positive, as shown by a movement locus m1a (two-dot chain line) in FIG. 16, from the wide angle end (W) to the telephoto end (T ), The first lens group GR1 moves U-turns, and the total length of the zoom lens system becomes substantially the same at the telephoto end (W) and the wide-angle end (T). On the other hand, in the variable magnification optical system satisfying the conditional expression (2), as shown by the movement locus m1 (solid line) in FIG. 16, even at the U-turn zoom solution, the zoom lens at the wide angle end (W) The total length of the system is reduced. For this reason, the activation time can be shortened, and a compact and simple lens barrel configuration can be achieved.

It is desirable that the positive lens is located closest to the object side in the second lens group, and that the positive lens satisfies the following conditional expression (3).
1.8 <Np (3)
However,
Np: refractive index with respect to d-line of the positive lens located closest to the object side in the second lens group,
It is.

  Conditional expression (3) defines a preferable condition range for the refractive index of the positive lens closest to the object in the second lens group. When the lower limit of conditional expression (3) is exceeded, the curvature of the lens surface becomes strong, resulting in large aberrations that are difficult to correct. Therefore, the refractive index of the lens located closest to the object side in the second lens group is preferably 1.8 or more, and more preferably 1.85 or more. A lens located closest to the object side in the second lens group tends to have a particularly high error sensitivity. Therefore, if the refractive index of the lens is increased, the refractive power of the lens can be increased without increasing the curvature, so that the error sensitivity can be effectively suppressed.

  As in the first to third embodiments, the second lens group is composed of three lenses, a cemented lens composed of a positive lens and a negative lens, and a lens having weak power in order from the object side. Is preferred. Having a cemented lens in the second lens group is effective in achieving space saving and chromatic aberration correction. In a normal zoom configuration in which the first and second lens groups are negative / positive, the second lens group bears the main zooming power, and the second lens group has strong power as it becomes compact. become. Since strong aberration occurs at this time, the second lens group is composed of four or more positive, positive, negative, positive and positive, negative, negative, and positive lenses in a general zoom configuration that is compact. The On the other hand, if the second lens group is composed of only three lenses, that is, a positive and negative cemented lens and a weak power lens as described above, it is possible to reduce the size while satisfactorily correcting aberrations. Furthermore, if the conditional expression (3) is satisfied, the occurrence of aberrations in the second lens group can be suppressed, which is more effective.

  In the first to seventh embodiments, the second lens group includes a plastic lens having weak power, and both surfaces thereof are aspherical surfaces. Thus, it is preferable to include a lens having weak power in the second lens group, and it is more preferable that at least one surface (preferably both surfaces) of the lens having weak power is an aspherical surface. Due to the aspherical surface of the lens having a weak power, it becomes possible to satisfactorily correct the aberration generated in the second lens group. In addition, by configuring the lens having weak power with a plastic lens, it is possible to reduce the cost and improve the performance by using an aspherical surface. Since the plastic lens is suitably arranged at a low sensitivity, it is preferably arranged at the most image side in the second lens group. Therefore, in each embodiment, the plastic lens is disposed on the most image side in the second lens group. Even if a plastic lens is used, since the power of the lens is weak, it is possible to suppress aberration fluctuations and lens back fluctuations when the temperature changes. In addition, as in the first to third embodiments, the second lens group includes three lenses, a cemented lens including a positive lens and a negative lens, and a lens having weak power in order from the object side. In this case, it is more preferable that at least one surface (preferably both surfaces) of the lens having the weak power is an aspherical surface. The aspherical surface of the lens having weak power in the second lens group can effectively correct the aberration generated in the cemented lens having strong power on the object side.

From the above viewpoint, it is desirable that the lens having weak power in the second lens group satisfies the following conditional expression (4). When the lens having the weak power is a plastic lens, it is more desirable to satisfy the following conditional expression (4).
| ΦL / φ2 | <0.4… (4)
However,
φL: refracting power of the lens having weak power in the second lens group,
φ2: refractive power of the second lens group,
It is.

  Conditional expression (4) defines a preferable condition range for the refractive power of a lens (preferably a plastic lens) having a weak power in the second lens group. The plastic lens has a large linear expansion coefficient and refractive index temperature dependency with respect to temperature change as compared with the glass lens. In order to suppress the influence of the linear expansion coefficient and the refractive index temperature dependency, it is necessary to reduce the refractive power of the plastic lens to reduce the sensitivity to temperature. Conditional expression (4) defines the conditions for that purpose. If the upper limit of the conditional expression (4) is exceeded, the refractive power of the plastic lens becomes too strong, so that the fluctuation of aberration due to the plastic lens at the time of temperature change becomes large and performance degradation becomes unacceptable.

It is desirable to satisfy the following conditional expression (5).
-0.5 <(Tw1−Tt2) / fw <0 (5)
However,
Tw1: The distance from the surface vertex of the first lens unit on the image plane side to the image plane at the wide-angle end,
Tt2: Distance from the object-side surface vertex of the second lens group at the telephoto end to the image plane,
fw: Focal length of the entire variable magnification optical system at the wide-angle end,
It is.

  Conditional expression (5) defines a preferable condition range for the moving method of the first lens group and the second lens group. In a digital camera, its activation time is one of the appeal points for the user. In order to speed up the start-up time of the camera, it is necessary to shorten the time from the storage state (for example, retracted state) of the variable magnification optical system to the set state (zoom position set state). In a zoom lens system of a general digital camera in which the first and second lens groups are negative and positive, a condition for that is to shorten the distance from the first lens group to the image plane. If the first and second lens groups are moved so as to satisfy the conditional expression (5), the image plane IM from the first lens group GR1 at the wide angle end (W) as shown by the movement locus m1 in FIG. The distance Tw1 can be shortened. As a result, it is possible to shorten the startup time of the camera. If the upper limit of conditional expression (5) is exceeded, the position of the first lens group GR1 at the wide-angle end (W) is arranged closer to the object side than the position of the second lens group GR2 at the telephoto end (T). Become. Therefore, the first lens unit is not desirable because it is too far from the image plane. On the other hand, if the lower limit of conditional expression (5) is exceeded, the power of the first lens group and the second lens group becomes too strong, which makes it difficult to correct the generated aberration, which is not desirable.

  The zoom lens system ZL constituting each embodiment uses a refractive lens that deflects incident light by refraction (that is, a lens that deflects at the interface between media having different refractive indexes). However, the usable lens is not limited to this. For example, a diffractive lens that deflects incident light by diffracting action, a refractive / diffractive hybrid lens that deflects incident light by combining diffractive action and refracting action, and a refractive index distribution that deflects incident light by a refractive index distribution in the medium A mold lens or the like may be used. However, it is desirable to use a homogeneous material lens having a uniform refractive index distribution, because the refractive index distribution type lens whose refractive index changes in the medium increases the cost of its complicated manufacturing method. Further, in the zoom lens system ZL constituting each embodiment, a diaphragm ST is used as an optical element in addition to the lens. However, a light flux restricting plate (for example, a flare) for cutting unnecessary light as necessary. A cutter) or the like may be arranged as necessary.

  Hereinafter, the configuration of the zoom lens system embodying the present invention will be described more specifically with reference to construction data and the like. Examples 1 to 7 listed here are numerical examples corresponding to the first to seventh embodiments, respectively, and are optical configuration diagrams showing the first to seventh embodiments (FIGS. 1 to 7). 7) shows the lens configurations of the corresponding Examples 1 to 7, respectively.

  Tables 1 to 14 show construction data of Examples 1 to 7, and Table 15 shows values corresponding to conditional expressions of Examples. In the basic optical configurations (i: surface number) shown in Table 1, Table 3, Table 5, Table 7, Table 9, Table 11, and Table 13, ri (i = 1, 2, 3,...) Is The radius of curvature (mm) and di (i = 1, 2, 3,...) Of the i-th surface counted from the object side are between the i-th surface and the (i + 1) -th surface counted from the object side. , And ni (i = 1, 2, 3,...) And νi (i = 1, 2, 3,...) Are optical elements located at the shaft upper surface distance di. The refractive index (Nd) and Abbe number (νd) of the material with respect to the d-line are shown. Further, the axial top surface distance di that changes during zooming is a variable air distance from the wide-angle end (shortest focal length state, W) to the middle (intermediate focal length state, M) to the telephoto end (longest focal length state, T). , F, FNO respectively indicate the focal length (mm) and the F number of the entire system corresponding to each focal length state (W), (M), (T).

The surfaces marked with * in the data of the radius of curvature ri are aspherical surfaces (aspherical refractive optical surfaces, surfaces having a refractive action equivalent to aspherical surfaces, etc.), and represent the following aspherical surface shapes: It is defined by the formula (AS). Table 2, Table 4, Table 6, Table 8, Table 10, Table 12, and Table 14 show aspheric data of each example. However, the coefficient of the term without description is 0, and E−n = × 10 ⁻ⁿ for all data.
X (H) = (C0 · H ² ) / {1 + √ (1−ε · C0 ² · H ² )} + Σ (Aj · H ^j ) (AS)
However, in the formula (AS)
X (H): Amount of displacement in the optical axis AX direction at the position of height H (based on the surface vertex),
H: height in a direction perpendicular to the optical axis AX,
C0: paraxial curvature (= 1 / ri),
ε: quadric surface parameter,
Aj: j-order aspheric coefficient,
It is.

  FIGS. 8 to 14 are aberration diagrams corresponding to Examples 1 to 7, respectively. (W) is the wide angle end, (M) is the middle, and (T) is the infinite focus state at the telephoto end. Aberration {from left to right, spherical aberration, astigmatism, distortion, etc. FNO is the F number, and Y ′ (mm) is the maximum image height (corresponding to the distance from the optical axis AX) on the light receiving surface SS of the image sensor SR. }. In the spherical aberration diagram, a solid line d represents spherical aberration (mm) with respect to the d line, and a broken line SC represents an unsatisfactory sine condition (mm). In the astigmatism diagram, the broken line DM represents the meridional surface, and the solid line DS represents each astigmatism (mm) with respect to the d-line on the sagittal surface. In the distortion diagram, the solid line represents the distortion (%) with respect to the d-line.

The lens block diagram of 1st Embodiment (Example 1). The lens block diagram of 2nd Embodiment (Example 2). The lens block diagram of 3rd Embodiment (Example 3). The lens block diagram of 4th Embodiment (Example 4). The lens block diagram of 5th Embodiment (Example 5). The lens block diagram of 6th Embodiment (Example 6). The lens block diagram of 7th Embodiment (Example 7). FIG. 6 is an aberration diagram of Example 1. FIG. 6 is an aberration diagram of Example 2. FIG. 6 is an aberration diagram of Example 3. FIG. 6 is an aberration diagram of Example 4. FIG. 6 is an aberration diagram of Example 5. FIG. 10 is an aberration diagram of Example 6. FIG. 10 is an aberration diagram of Example 7. FIG. 2 is a schematic diagram illustrating a schematic optical configuration example of a camera equipped with an imaging device. FIG. 6 is a schematic diagram showing movement trajectories of first and second lens groups during zooming.

Explanation of symbols

CU camera LU imaging device ZL zoom lens system (variable magnification optical system)
GR1 1st lens group GR2 2nd lens group GR3 3rd lens group GR4 4th lens group ST Aperture PT Parallel plane plate SR Image sensor SS Photosensitive surface IM Image plane AX Optical axis

Claims (4)

A variable magnification optical system for forming an optical image of an object on a light receiving surface of an image sensor so as to be variable,
In order from the object side, the first lens group having negative power, the second lens group having positive power, and the third lens group having positive power, or the first lens group and the third lens group Consists of four groups, a second lens group, the third lens group, and a fourth lens group,
The zoom ratio is 2.5 times or more,
At least the first lens group, the second lens group, and the third lens group move in zooming from the wide-angle end to the telephoto end,
In the second lens group, a positive lens is disposed closest to the object side,
A zoom optical system characterized by satisfying the following conditional expressions (1) , (2) and (3a) ;
1.8 <| f1 / fw | <3… (1)
0.34 <fw · tanωw / (L1-L2) <0.5 (2)
1.85 ≦ Np (3a)
However,
f1: focal length of the first lens group,
fw: Focal length of the entire variable magnification optical system at the wide-angle end,
ωw: Half angle of view at the wide-angle end,
L1: Relative movement of the first lens unit in zooming from the wide-angle end to the telephoto end (the image side is positive),
L2: Relative amount of movement of the second lens unit during zooming from the wide-angle end to the telephoto end (the image side is positive),
Np: refractive index with respect to d-line of the positive lens located closest to the object side in the second lens group,
It is. The second lens group includes, in order from the object side, a cemented lens including a positive lens and a negative lens, and a lens disposed behind the cemented lens,
Said joining rearwardly arranged lens of the lens, the following conditional expression (4) according to claim 1, wherein the variable magnification optical system, characterized by satisfying the;
| ΦL / φ2 | <0.4… (4)
However,
φL: refractive power of the lens arranged behind the cemented lens in the second lens group,
φ2: refractive power of the second lens group,
It is. The zoom lens system according to claim 1 or 2, wherein the following conditional expression (5) is satisfied:
-0.5 <(Tw1−Tt2) / fw <0 (5)
However,
Tw1: The distance from the surface vertex of the first lens unit on the image plane side to the image plane at the wide-angle end,
Tt2: Distance from the object-side surface vertex of the second lens group at the telephoto end to the image plane,
fw: Focal length of the entire variable magnification optical system at the wide-angle end,
It is. An imaging apparatus comprising the variable magnification optical system according to any one of claims 1 to 3 .

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