1. Field of the Invention
The present invention relates to a wide-angle zoom lens system having an angle-of-view exceeding 80° at the short focal length extremity for use mainly in an SLR (Single Lens Reflex) camera.
2. Description of Related Art
A wide-angle zoom lens system having an angle-of-view exceeding 80° at the short focal length extremity typically employs a retrofocus lens arrangement in which a negative lens group and a positive lens group are provided in this order from the object. Generally, a retrofocus lens arrangement provides a long back focal distance which satisfies the structural conditions to place a quick-return mirror in the SLR camera; and furthermore, each lens group needs to have a strong refractive power in order to maintain the compact (shorter) overall length of a wide-angle zoom lens system. However, if a strong refractive power is provided in each lens group, it is difficult to correct aberrations, such as distortion, field curvature, and astigmatism, at the short focal length extremity.
In recent exchangeable lenses for SLR cameras, there has been demand for high-speed focusing to cope with autofocusing operations. In a conventional focusing system for a wide-angle zoom lens system, a first-lens-group telescoping system in which the first lens group telescopes (advances and retracts) is generally employed. In this respect, in order to achieve an autofocus lens which can perform a high-speed focusing operation, the focusing lens group is desirably lighter (a reduced weight).
In Japanese Unexamined Patent Publication Nos. 2000-131611 and 2000-131612, distortion which occurs in the first lens group cannot be sufficiently corrected by the second lens group and therebehind, so that distortion still remains at the short focal length extremity. Moreover, focusing is performed by the first lens group, which is unsuitable for high-speed focusing.
In Japanese Unexamined Patent Publication No. H08-248312, distortion also remains at the short focal length extremity; and furthermore, in regard to focusing, Embodiments 1, 3 and 5 disclose focusing lens groups of three lens elements, Embodiment 7 discloses a focusing lens group of two lens elements, and Embodiment 4 discloses a focusing lens group of one lens element.
Compared to an arrangement in which the first lens group is moved in the optical axis direction to carry out focusing, Embodiments 1, 3 and 5 achieve a zoom lens system in which the lens diameters are reduced and a high-speed focusing operation is possible. However, the reduction of the lens diameters and the extent by which a high-speed focusing operation can be performed in Embodiments 1, 3 and 5 are insufficient compared to Embodiments 4 and 7. In Embodiments 4 and 7, a high-speed focusing operation is sufficiently possible with respect to the number of lens elements, and to the location of the focusing lens group. However, the moving space (a traveling distance in the optical axis direction) within which the focusing group is driven (moved) is small (short), so that the minimum photographing distance becomes longer. In addition, in Embodiment 7, a convex lens element is provided as the most object-side lens element, so that the lens diameter of the convex lens element is large.