1. Field of the Invention
The present invention relates to a zoom lens system and an image pickup apparatus including the zoom lens system, which are suitable, in particular, for an image pickup apparatus using a solid-state image pickup element, such as a video camera, an electronic still camera, a broadcasting camera or a monitor camera, or an image pickup apparatus such as a silver-halide film camera.
2. Description of the Related Art
In recent years, an image pickup apparatus using a solid-state image pickup element, such as a video camera, a digital still camera, a broadcasting camera, or a monitor camera, as well as a silver-halide film camera has high performance, and the entire apparatus is downsized. Then, a photographing optical system used for the image pickup apparatus is required to be a high-resolution zoom lens system having a short total lens length, a compact size, and a high zoom ratio. As one of the zoom lens systems which meet the requirements, there is known a so-called rear focus type zoom lens system, which performs focusing by moving lens units other than a first lens unit on the object side.
In general, a rear focus type zoom lens system has a smaller effective diameter of the first lens unit than other type of zoom lens system that performs focusing by moving the first lens unit, so as to realize a small size of the entire lens system easily. In addition, macro photography can be performed easily. Further, because a small and light lens unit is moved, small drive force of the lens unit is sufficient so that a quick focusing can be realized. As the rear focus type zoom lens system, there is known a zoom lens system including, in order from the object side to the image side, a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power, and subsequent one or more lens units. Among them, there is known a five-unit zoom lens system including five lens units which have, in order from the object side to the image side, positive, negative, positive, negative, and positive refractive powers (U.S. Pat. No. 7,177,092). In addition, there is known a six-unit zoom lens system including six lens units having positive, negative, positive, negative, positive, and negative refractive powers in order from the object side to the image side (U.S. Pat. No. 6,373,639).
In general, in order to obtain a zoom lens system having a downsized entire system while having a predetermined zoom ratio, refractive power (optical power, or inverse number of a focal length) of each lens unit constituting the zoom lens system needs to be enhanced to reduce the number of lenses. However, such a zoom lens system has much aberration variation accompanying zooming, and it is difficult to obtain high optical performance over the entire zoom range. In particular, it is difficult to correct various aberrations such as chromatic aberration at a telephoto end. In order to obtain a high zoom ratio and downsize the entire lens system while obtaining good optical performance in the five-unit zoom lens system or the six-unit zoom lens system described above, it is important to appropriately set refractive power and lens configuration of each lens unit and a move condition of each lens unit in zooming. In particular, it is important to appropriately set refractive power of the first lens unit and move conditions of the first, third, and fourth lens units in zooming. Unless those structures are set appropriately, it is difficult to obtain a zoom lens system having a small entire system, a wide angle of field, a high zoom ratio, and high optical performance.