My very first Zeiss product... while cleaning and sorting old stuff in my house, I found this old light meter, I used in the sixties when I learned photography from my father. He told me what an aperture, a shutter speed or depth of field were when I was a young kid and how to meter (my first cameras did not even have a built in meter). Studio photographers still use a modern equivalent like the one I describe in my wiki: http://www.techniphoto.com/wiki/index.php?title=Gossen_Digipro_F but most photographers don't know anymore what this is. Basically, you point it at the subject you want to shoot, you select the ISO sensitivity with one of the wheels, then you turn the other wheel to put the needle in the small circle. At that point you can read the aperture/shutter speed combination you'll need to take a properly exposed picture. This meter is from the legendary Zeiss manufacturer, top end German brand. They are still making among the best lenses in the world, but that's another story. The last time I used this was like 40 years ago !