This lightweight GN Auto Nikkor performs automatic flash exposure through a guide number coupling system. Once the flash guide number is set, the diaphragm is coupled to the focusing mechanism. Thus the diaphragm is automatically set for correct flash exposure. There is no need to adjust the diaphragm with each change in focus. This coupling is disconnected when the flash is not used. Because of its focal lenght, speed and compactness, the versatile GN Auto Nikkor can also be used for general photography.