1. Field of the Invention
This invention relates to optical lens systems for cameras and other optical devices, and, in particular, to high performance zoom lens systems that produce a high quality image over a full zoom range of focal lengths and are capable of being provided with an extremely large zoom ratio.
2. Description of Related Art
General Background of the Invention. The use of zoom lens systems for all types of photography, such as broadcast television, high definition television (“HDTV”), advanced television (“ATV”), video camcorders, film cinematography and still photography has become increasingly popular. As the use of zoom lens systems has increased, the demand for wider ranges of zooming capability, i.e. large zoom ratios, has also increased. For example, the zoom lens systems used in broadcast television have steadily increased in zoom ratio capability over the years to a maximum of about 101 to 1 at present but there is a demand for a still larger zoom ratio. While the focal length range of a conventional zoom lens system may be increased by the use of a drop-in extender or other multiplier, such as a broadcast television zoom lens system with a focal length range of 8.9 mm to 900 mm being increased to 17.8 mm to 1800 mm to increase the telephoto capability, this does not change the zoom ratio of about 101 to 1. Moreover, for broadcast television zoom lens systems there are somewhat different requirements for “studio” (indoor) or “outside broadcast” (outdoor) use concerning the focal length range and acceptable “f” numbers, whereby it has become conventional practice to employ two different zoom lens systems for indoor and outdoor broadcast television uses to maximize the capabilities for both types of uses.
Further, in addition to the demand and desirability of using zoom lens systems with wider ranges of focal lengths, such lenses must retain superior optical characteristics and performance that previously has been accomplished only by using separate objective lenses of different fixed focal lengths or zoom lens systems with a limited zoom ratio. As the zoom ratio increases, the difficulty in providing a high performance optical system with superior characteristics and performance also increases. Even most previously available zoom lens systems of a limited zoom range have one or more undesirable limitations such as the inability to focus adequately over the entire focal length range, the inability to focus on close objects, the lack of adequate optical performance over the entire focal length range and focus distance, the cost, the large size for the limited zoom range achieved and the like.
Still further, as the zoom range of a lens system increases, generally the length and weight increases whereby the difficulty in maintaining the lens and camera steady also increases. Therefore image stabilization also becomes an issue for the design of a practical zoom lens system having a large focal length range and zoom ratio.
Moreover, as the focal length range of a zoom lens system increases, generally the focusing problems also increase. Although close focusing at long focal lengths of the zoom range is not absolutely necessary, it is required at lesser focal lengths. In the past, continuous focusing over a considerable conjugate range from infinity to objects at a very short distance such as about 8 feet or less has been difficult to achieve. Further, the problem of “breathing” of the final image (where the perceived size changes as the focus distance is changed) at shorter focal lengths must be minimized to avoid, for example, one person disappearing from the scene as the focus is changed to another person at a different distance from the lens. These focus performance requirements, including maintaining the quality of the final image, tend to increase substantially the weight and cost of the zoom lens system unless the size can be minimized and performance maximized by the overall lens design, including glass selection.
Background Information Concerning Zooming. As discussed above, zoom lens systems with a wide-range of focal lengths are very desirable in numerous photographic applications, including broadcast television, cinematography and video and still photography. One standard zoom lens system used in these applications has a four-group PN(P or N)P structure, where P stands for a group of at least one lens element wherein the lens group has positive power, N stands for a group of at least one lens element wherein the lens group has negative power, and the groups are identified consecutively from the object space toward the image space, as is conventional. The front positive group is often called the focusing group because it can be moved for focusing the zoom lens system at any focal length position without the need to refocus for any other focal length of the zoom lens. The second negative group is the variator, and it induces significant magnification change during zooming. The third group, which can in general have either positive or negative power, is the compensator, and it is movable to insure that the image plane remains stationary. It also can provide some of the magnification change to effect zooming. The final positive fourth group is often called the prime lens group because it forms a sharp image.
This basic zoom lens system is suitable for zoom ratios of 50:1 or even more. As the zoom ratio is extended to about 100:1, however, the variator is required to change its object magnification to such an extent during zooming that aberrations become impracticably large and difficult to correct. In addition, at such large zoom ratios there is a very large change in entrance pupil location during zooming, and this tends to make the front group very large and difficult to correct. Another problem derives from the fact that, to reduce the aberration change that results from a large change of magnification, it is desirable that the variator have reduced optical powers. Weaker optical power, however, also increases the lens travel and length of the optical system. For a narrow field-of-view this would not be a problem, but, for a wide field-of-view, large motions lead to an increase in the principal ray heights at the rear portion of the lens system. Since the requirements for either the front or the rear of the lens system can be satisfied, but not simultaneously, this results in no ideal place for the aperture stop. If the stop is placed near the front of the lens, the front lens element diameters, and resultant aberrations, are reduced, and if the aperture stop is placed nearer to the rear part of the lens system, the rear lens diameters and resultant aberrations are decreased.