Old photometer J. Decoudun Paris exhibitor, 1886. One of the most sought photometers in excellent condition and in its original box. The special feature of this photometer is measuring the exposure time through the lens. Photographer holds the gauge against the focusing screen of the camera and look through the viewing window to the center button until the light passing through the camera and the meter is hardly visible. Turning the front center button, one of the sixteen holes is placed in the slot of vision. Each hole has a letter from A to P, and can be seen on the back of the meter. Each hole allows a different amount of light passing through the instrument.