1. Field of the Invention
The present invention relates to a vari-focal lens or vari-focal zoom lens, which requires slight focus adjustment as it varies the focal length, unless focused on infinity.
2. Description of the Related Art
Vari-focal lens or vari-focal zoom lens is defined as a lens whose focal plane slightly moves as the focal length varies. Therefore, the position of the focusing lens group (the amount of movement of the focusing lens group from a reference position) that is necessary for bringing an object at the same object distance into focus varies as the user varies the focal length. In AF (autofocusing) cameras, the amount of movement of the focusing lens group from a reference position at each focal length has only to be determined by taking into account the focal shift as the focal length varies, namely, the amount of movement of the focusing lens group at each focal length has only to compensate for a slight deviation of the focal plane from an in-focus position thereof as the focal length varies. Even in conventional MF (manual focusing) SLR cameras, even if the focal plane slightly deviates from an in-focus position thereof after the user varies the focal length, he or she has only to rotate the distance-scale bearing ring (focusing ring) provided on the lens, so that sharp object images can be viewed through the viewfinder. Therefore, there is practically no problem in focusing of the vari-focal lens.
However, in SLR cameras, since the distance-scale bearing ring needs to be provided thereon with a distance scale so that it can be seen by the user, the camera has to be sharply focused on an object at a distance indicated by the distance scale at each focal length. In other words, when the camera is to be focused on an object at the same distance, the camera has to be sharply focused on the object at different focal lengths by moving the focusing lens group by respective different amounts of movement by rotating the distance-scale bearing ring rotating by the same amount (angle) of rotation as the focusing lens group from, e.g., the infinity photographic position thereof. Accordingly, in this case where the SLR camera is used with a vari-focal lens, the user is required to manipulate the vari-focal lens as if it were a zoom lens with which the focal plane is substantially still even if the focal length varies. In this respect there is no practical difference between vari-focal lenses and zoom lenses. In the present specification and claims, the terms "zoom" and "zooming" will be herein after used so as to mean "vary" and "variation" of the focal length of the vari-focal lens.
Although various types of vari-focal lenses have been proposed and developed, not only are the structures thereof generally complicated, but also it is difficult to remove mutual interference (influence) between focus cams (e.g., cam grooves or slots) for varying the amount of movement of the focusing lens group per unit angle of rotation of the distance-scale bearing ring in accordance with a variation of the focal length, and zoom cams (e.g., cam grooves or slots) for driving a power varying lens or lenses in order to carry out a zooming operation.