An early model of the Sangamo Weston light meter

Used by Pro photographers and amateurs too, to arrive at the correct exposure for every given lens aperture set on the dial. This model is calibrated to use their Weston film speeds, not the later ASA and DIN or today's ISO speeds but it does have the advantage of a selenium cell unit, not a battery, which works as well today as when it was marketed, well over 60 years ago now. With their unique Invercone fitted you can measure the light from the subject position , which in portraiture can be a big asset. They went out of favour when cameras began to have a built in exposure meter, as they all do today.