The GN was interesting idea. It was made to use with a flash; you enter the guide number (GN) on the side, lock the aperture ring and then the aperture automatically adjusts to perfectly expose your flash photos. Genuis!This lens is in very clean condition. It looks like it was used very little. The glass is clean and scratch-free. All of the controls are smooth.The lens comes with the rare HN-4 shade. The shade shows a little paint wear but is still in good condition. I think that the shade was mated to the lens after it had been used a bit because the lens is beautiful.