Optima Ia takes 24x36 negatives on 135mm film. It has f2.8 45mm Agfa Color-Agnar lens. The shutter, 1/30-1/250, operates automatically when you press the shutter release. When you press the shutter lever halfway down a mechanical system sets the correct aperture value and speed from the electromechanical part of the meter. A red signal dot in a viewfinder turns green when the right aperture/shutter speed combination is achieved. The camera weighs 400g and measures 121x82x68 mm. This system has no battery. Required energy is produced by a selenium cell and focusing is manual. A selenium meter is a light-measuring instrument based on the photoelectric properties of selenium. The most common use of such light meters is measuring the exposure value for photography. The electric part of such a meter is an electromagnetic measuring instrument which is connected to the anode and cathode of a selenium photo cell that produces more or less electric power when exposed to more or less light.