Source: {"pile_set_name": "USPTO Backgrounds"}

1. Field of the invention:
The present invention relates to a vari-focal lens system for cameras and more specifically to a vari-focal lens system for video cameras.
2. Description of the prior art:
In the recent years, compact and light-weight video cameras are offered at low prices, and demand for domestic video cameras is rapidly increasing. In order to further develop the demand for domestic video cameras, it is necessary to design compacter video cameras which are ligher in weight and manufacturable at lower cost. Under these circumstances, it is important to design compact, light-weight lens systems manufacturable at low cost.
At present, zoom lens systems to be used in video cameras for domestic use are mainly designed for a zoom ratio of 6 and an F-number on the order of F/1.2 to F/1.6. This is because the above-mentioned specifications are suited for obtaining high efficiency for design and meeting the practical needs.
As examples of the conventional zoom lens systems for video cameras, there are already known the lens systems disclosed by Japanese Unexamined Published Patent Application No. 102208/58, Japanese Unexamined Published Patent Application No. 153913/58 and so on. However, these zoom lens systems comprise a vari-focal unit consisting of three lens components and a relay lens system consisting of seven to eight lens components, or is composed of fourteen to fifteen lens components in total, whereby these lens systems have a common defect that they comprise a large number of lens components, require high manufacturing cost, have large diameters and are heavy in weight. Further, the zoom lens system disclosed by Japanese Unexamined Published Patent Application No. 110112/61 is so designed as to compose a vari-focal unit of two lens components and a relay lens system of three lens components so that the zoom lens system can be composed of eight lens components in total and manufactured at low cost.
However, this zoom lens system adopts extremely high curvature on the surfaces of the cemented lens components arranged in the vari-focal unit and the relay lens system, and requires high manufacturing cost contrary to the purpose thereof. Furthermore, the zoom lens system disclosed by Japanese Unexamined Published Patent Application No. 198813/62 is so designed as to consist of six lens components in total by composing a vari-focal unit of a single lens component and a relay lens system of three lens components. However, this zoom lens system has a low vari-focal ratio, and is usable only at the wide position and tele position thereof.