The GN Auto Nikkor is designed to simplify flash photography. As its name GN (guide number) suggests, the lens employs the guide number coupling system for automatic diaphragm adjustment. With any ordinary lens, aperture size must be adjusted for each different shooting distance with flashbulbs or non-automatic mode with an electronic flash. Aperture size for each shooting distance needs to be calculated, using the guide number equation with the flash unit that you are using if the GN Nikkor is not employed. When the GN control is disconnected, the lens acts like an ordinary compact 45mm standard lens (sometimes referred to as a “pancake” lens due to small size). By disengaging the GN coupling device, the lens can be coupled with camera exposure meters for full aperture exposure measurement. The 7-bladed polygonal diaphragm, designed to form an almost circular aperture around the most commonly used f/8 setting, reduces the effects of light diffraction to the minimum. It can be stopped down to f/32. The lens is particularly popular due to two major qualities 1) Exceptional sharpness and 2) Exceptional compactness.