1.  Field of the Invention
The present invention relates to a rear-focusing telephoto lens system which is used in a single lens reflex (SLR) camera, a video camera and the like. 
2.  Description of the Related Art
In a telephoto lens system having a smaller F-number of 2.8 or less and an angle-of-view of about 26.degree.,  the number of lens elements tends to be larger due to a smaller F-number, the overall length thereof becomes longer due to being a telephoto lens system, and further the diameter of lens elements tends to be larger due to an angle-of-view of 26.degree.  which is relatively wider for a telephoto lens system.  Consequently, the weight of the lens system also tends to be heavier. 
On the other hand, an auto-focusing (AF) camera is required to have a lighter (less heavier) lens group to be moved upon focusing (hereinafter, a focusing lens group) in order to attain quick movement of the focusing lens group and to reduce a load imposed on a power source.  For the purpose of satisfying these requirements, a lens system, such as rear-focusing and inner-focusing type lens systems, in which a part of lens groups is arranged to be moved for focusing, is employed.  In this regard, a rear-focusing type lens system is suitable for a lens system having a relatively larger angle-of-view, while an inner-focusing type lens system is not. 
Various rear-focusing telephoto lens systems are taught, for example, in the U.S. Pat.  No.  4,426,137,the Japanese Unexamined Patent Publication (JPUEPP) No.  Sho-58-199311, JPUEPP Sho-60-166912, JPUEPP Sho-60-201313, and U.S. Pat.  No.  4,812,027.  However, in these lens systems, the rear lens group in a Gauss type lens system is made movable as a focusing lens group.  Upon the rear lens group being moved, a height of incidence on a large-curvature concave surface of the object-side surface of the rear lens group varies, and an angle of incidence varies accordingly, so that fluctuation of spherical aberration becomes larger. 
As an improved type of the above described lens systems, in U.S. Pat.  No.  5,172,274, the applicant of the present invention has disclosed a telephoto lens system.  This telephoto lens system has acceptable optical performance; however, an aperture stop is provided in the rear lens group (focusing lens group), because the distance between the front and rear lens groups is shorter upon photographing an object at a closer photographing distance.  An arrangement in which the aperture stop with a variable aperture moves upon focusing makes the structure of a lens barrel complicated.  Furthermore, since a rear lens frame is divided by the aperture stop, it becomes difficult to maintain precision on decenteration. 
Still further, more improved types of the above described lens system are disclosed in U.S. Pat.  No.  5,640,277 and U.S. Pat.  No.  5,627,686.  However, a large number of lens elements are required, and the structure of a lens barrel is complicated, since the aperture stop is arranged to move together with the focusing lens group.