ABSTRACT
Many of the images produced by photographic, cinematographic and video systems are magnified by projection for final assessment and viewing. Many different types of projection lenses are required, leaving aside the cases of projection printing and enlarging. The imaging requirements differ from those of conventional camera lenses. Very often, long focus, narrow angle lenses or short focus, wide-angle lenses are used to suit the projection conditions. The heat problems of projection mean that projection lenses almost always use air-spaced elements, as the cement layer of doublets may suffer thermal stress. This arrangement does give additional surfaces and separations to improve optical performance. For cine projection a particular design is available in focal length increments so as to suit the projection conditions and fill the screen. Slide projection conditions may require a long focus lens or a wide-angle lens, the latter especially for back-projection. Zoom projection lenses normally do not have the required range, so fixed focal length designs are used.
