In many complex technological fields the primary source of useful information lies not in books or periodicals but rather in the patent literature. Nowhere is this truer than in the field of zoom lens design, where in-depth discussion in books is very scarce but patents number several thousand. Several different types of zoom lenses that are commercially important today have roots in designs that are decades old, and the detailed path of development is publicly documented in the patent literature. These zoom types include the positive-negative type, which has been used extensively in compact 35mm and APS point-and-shoot cameras; the negative positive type, which was originally developed for 35mm SLR cameras and has been widely adopted for use in digital still cameras; and the positive-negative-positive type, which has been used in many applications ranging from 35mm still photography to cinematography to ultra-wide range TV camera objectives. Many innovations have fueled the development of these designs, including the use of aspheric surfaces, plastic lens elements, the use of abnormal partial dispersion glasses to achieve apochromatic correction, and the use of a variety of focusing techniques. Given that the patent literature is the best source of information on zoom lenses, there are a number of reasons that a designer or perhaps a product engineer would want to study this information. First and foremost is to gain a very broad historical understanding and appreciation for the technology. Next is to discover what has been done before by others in order to avoid re-inventing it, and also to increase the likelihood that a new design will be a genuine improvement over past efforts. This alone can save countless hours of work and potentially millions of dollars in development costs. Just as important as finding out has been done is finding out what hasn't been done. This can alert a designer to potentially over-ambitious specifications on a project. It can also aid in the process of creating new and valuable intellectual property. Another highly useful benefit of patent study is that it provides an almost limitless supply of starting points for new design projects. This is especially important in zoom lens design, where the final performance is more heavily dependent on the starting point than simpler fixed focal length lenses. Starting points can also be constructed from numerous different patent designs. For example the negative variator in a PNP type design can be inserted from dozens of different sources into the same basic starting point in order to find the optimum variator form.