Stage lighting instruments and other similar luminaries are used to provide controllable illumination of persons and/or objects. Among the controllable parameters are beam size and intensity of a light beam used for illumination. Another controllable parameter is the optical focus of a field stop, aperture, or image pattern generator used in a spot light or image projector. Two ways of controlling beam size include the use of an iris diaphragm having an aperture of adjustable diameter, and the use of a zoom lens system having variable magnification power. A beam size iris controls beam diameter by acting as a field stop having an aperture of adjustable diameter, and varies the size of the beam by cutting off light rays which do not pass through the aperture thereby decreasing the total amount of light which reaches the illuminated area. A zoom lens system controls beam diameter by changing the divergence angle of a bundle of light rays entering the lens system thereby concentrating or deconcentrating the light beam such that the total amount of light reaching the illuminated area is not changed. A zoom lens, therefore, varies the energy density in a projected spot of light as the diameter of the spot is varied, while a beam size iris varies the diameter of the projected spot without altering the energy density thereof.
Two-lens systems (two groups of lenses, each group having one lens) afford variable magnification and focus, such as those shown in U.S. Pat. No. 3,594,566 to Kneisley, and U.S. Pat. No. 4,519,020 to Little. A triplet system (three groups of lenses, one lens in each group) is disclosed in U.S. Pat. No. 5,029,992 to Richardson. Other lighting projectors featuring adjustable focus optical systems include U.S. Pat. Nos. 2,076,240 and 2,114,963 to Levy, U.S. Pat. No. 2,650,292 to Strong, U.S. Pat. No. 2,909,652 to Pratt, U.S. Pat. No. 2,950,382 to Hatch, U.S. Pat. No. 4,101,957 to Chang, U.S. Pat. No. 4,232,359 to Leon, U.S. Pat. No. 4,338,654 to Logothetis, U.S. Pat. No. 4,462,067 to Altman, U.S. Pat. No. 4,739,456 to Little, U.S. Pat. No. 5,2295,056 to Peck, and U.S. Pat. No. 5,404,283 to Yantz et al. Examples of four group lens systems include U.S. Pat. Nos. 5,282,089; 5,223,979; 5,579,172; 5,583,700; and 5,606,459.
As recognized by Tejima in U.S. Pat. No. 5,283,599, image projectors using a zoom lens as the projection lens must necessarily effect a zoom adjustment in which the focal position is kept constant, even if the zooming is carried out by the zoom lens system and a back focus adjustment is made to keep the image pattern generator coincident with the focal position obtained by the zoom adjustment. A typical system utilizing five lens groups disclosed in U.S. Pat. No. 3,192,829 to Yamaji maintains focus over a wide range of magnifications by moving three of the lens groups with respect to the remaining two fixed lens groups. Some other zoom lens systems designed for projection applications are disclosed in U.S. Pat. No. 3,267,803 to Macher and Klemt, U.S. Pat. No. 4,063,800 to lizuka et al, and U.S. Pat, No. 4,118,108 to Muszumanski. However, the restriction in the above references that the image be focusable throughout the range of variable magnification tends to limit the range of magnification available from a particular zoom lens design.
The development of high quality projection lens systems having variable magnification has, for some time, been impeded by limitations as to cost, size, and particularly the overall length of the lens system. Whereas generally the ratio of overall length (from the front edge of front lens to the back edge of back lens) to longest focal length is usually 1.5.times. or greater, it has become desirable to develop a very short zoom lens system approaching 1.0.times. length and having about an 8:1 range of magnification change (zoom ratio), which can be produced inexpensively and withstand the rigors of high power density and portability.