Another example of Nikon ingenuity in lens design, the unique GN Auto-Nikkor reduces flash exposure to automatic accuracy and simplicity. The GN has a special scale on which you can preset the flash guide number of the film you are using, from 32 to 320. This simple adjustment activates a precision linking system that couples the focusing mechanism to the lens diaphragm. Once the guide number is set, the diaphragm adjusts automatically as you focus, assuring correct flash exposure at all subject distances. This method of flash automation is superior to computer flash units in several ways. One; the lens aperture changes as you focus, giving you greater depth-of-field at close distances. Two; the flash duration is constant and always within the capacity of color films, avoiding the risk of reciprocity failure which results from extremely short flash durations and may cause faulty color reproduction. And, of course, the GN provides automation with any flash unit. With guide number scale disengaged for non-flash photography, the GN Auto-Nikkor couples to Nikon and Nikkormat meter systems. Its fine optical quality and speed make it an excellant choice for either application.