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Photographic equipment light meters.
Cameras tended to come in two broad categories: those with a fixed lens and limited ability to make adjustments, and those which allowed many adjustments of shutter speed, focal length etc. These adjustments had to be made manually.
In order to get the correct setting for the brilliance of the light, photographers carried a light meter which measured the brilliance and gave a guide what to set the shutter speed and lens aperture (f stop) to for optimum exposure for the film speed.
The film speed (sensitivity) and the brilliance of the subject and the shutter speed were all combined to consider the aperture of the lens.