1. Technical Field
The present invention relates to a zoom lens system, an imaging device and a camera. In particular, the present invention relates to: a zoom lens system that has a high resolution, high capability of compensating curvature of field, and a short overall optical length at the time of non-use; an imaging device employing this zoom lens system; and a camera employing this imaging device.
2. Description of the Background Art
In the prior art, a large number of optical instruments have been developed that form an image of a photographic object onto an image sensor through a lens and then acquire the object image as an image. Recently, products such as digital still cameras and digital video cameras are spreading. Then, with the increase in the number of users, desire on these products is also growing. Among various types of these products, optical instruments having a zoom ratio of approximately three are comparatively small and still have an optical zoom function. Thus, these types are spreading remarkably widely as digital cameras of compact type or stylish type.
In the digital cameras of compact type, for the purpose of the property of easy carrying, further size reduction of the instruments is desired. In order to achieve the further size reduction of the digital cameras, the lens arrangement need be adopted such that the overall optical length (the distance measured from the top of the most object side lens surface of the entire lens system to the image surface) at the time of non-use should be reduced while lens elements that extend out relative to the main body by means of a multi-stage lens barrel at the time of use could be accommodated into the main body.
Meanwhile, as zoom lens systems suitable for digital still cameras of compact type, a large number of zoom lens systems of three-unit construction have been proposed that, for example, in order from the object side to the image side, comprise a first lens unit having negative optical power, a second lens unit having positive optical power, and a third lens unit having positive optical power.
In such a zoom lens system of three-unit construction, in zooming (magnification change) from a wide-angle limit to a telephoto limit, the air space between the first lens unit and the second lens unit decreases monotonically, while the air space between the second lens unit and the third lens unit varies, and while the third lens unit is fixed or moved.
Focus adjustment in the zoom lens system of three-unit construction is performed by moving the first lens unit or the third lens unit in the optical axis direction. In particular, from the perspective of size reduction of the entire optical instrument, in many cases, the focus adjustment is performed using the third lens unit which is less heavy, so that focusing onto the photographic object is achieved ranging from the infinity to a short distance. In contrast, when the focus adjustment is performed using the first lens unit, the first lens unit is larger than the third lens unit and hence requires a large size motor. This causes a tendency of size increase in the entire optical instrument.
The third lens unit having positive optical power has the effects of compensating curvature of field and bringing into a telecentric state the incident light onto the imaging surface. Further, in many cases, the third lens unit is constructed from one or two lens elements having a small outer diameter, and hence can be driven at a high speed using a small size motor. Thus, when the third lens unit is adopted as a lens unit for focus adjustment, an optical instrument is realized that has a reduced size and permits rapid focusing.
The first lens unit and the second lens unit move in parallel to the optical axis along a cam groove formed in a cylindrical cam. In the cam groove, a groove for zooming and a groove for the time of non-use are connected to each other. The groove for the time of non-use reduces the interval between the lens units and moves all the three lens units to the image sensor side. This configuration reduces the overall optical length at the time of non-use. In this case, if the thickness of each lens unit could be reduced, the overall optical length at the time of non-use would be reduced further.
As such, in the prior art, design has been performed such that the zoom lens system should have the above configuration where the size is reduced in the part relevant to focus adjustment and in the entire lens system at the time of non-use, so that the overall optical length of the digital still camera has been reduced.
For example, Japanese Laid-Open Patent Publication No. 2005-134746 discloses a three-unit zoom lens, in order from the object side to the image side, comprising: a first lens unit having negative optical power that is composed of a negative-powered lens having an aspheric surface and a positive-powered lens; a second lens unit having positive optical power; and a third lens unit having positive optical power. In this three-unit zoom lens, the most object side negative-powered lens of the first lens unit is provided with a high refractive index, so that the lens thickness in the periphery part is reduced in a state that curvature of field at a wide-angle limit is compensated. This reduces the thickness of the entire first lens unit and hence the size of the optical system.
Further, for example, Japanese Laid-Open Patent Publication No. 2005-084597 discloses a three-unit zoom lens that, in order from the object side to the image side, comprises a first lens unit having negative optical power, a second lens unit having positive optical power and provided with a diaphragm, and a third lens unit having positive optical power, wherein in magnification change, the first lens unit moves relatively in a direction approaching to the second lens unit, while the second lens unit monotonically moves to the object side, and while the third lens unit moves to the object side and then moves reverse to the image side, and wherein when the object distance is infinity, the position of the third lens unit at a wide-angle limit is located on the object side relative to the position at a telephoto limit. In this three-unit zoom lens, a condition is set forth concerning the focal length of the first lens unit in such a manner that the compensation of curvature of field and the size reduction of the optical system can be achieved simultaneously.
Nevertheless, in the configuration of the three-unit zoom lens disclosed in Japanese Laid-Open Patent Publication No. 2005-134746, the positive-powered lens on the image side of the first lens unit has a low refractive index and still is a spherical lens. This causes a problem of insufficiency in the compensation of curvature of field.
Further, in the configuration of the three-unit zoom lens disclosed in Japanese Laid-Open Patent Publication No. 2005-084597, for the purpose of size reduction, the focal length of the first lens unit is set up rather short. Nevertheless, in this case, although the diameter of the lens can be constructed comparatively small, when the first lens unit is composed of two lenses, the optical power becomes excessive in the object side lens. Further, the thickness of the image side lens also increases for the purpose of compensation of chromatic aberration. This causes a problem of increase in the overall optical length at the time of non-use.