Diaphot Iconta circular slide rule light meter pocket watch 1930 Germany

This small round slide rule is pretty unusual since the value is determined by sight. It has a see-through part which by rotating gets darker in order to find the correct value. This corresponds with a reading for camera setting. It is from ca. 1930 and is shaped like a pocket watch and comes in the original leather case (printed on both sides). It has the Diaphot name as well as the Iconta brand which later became Zeiss Ikon. unfortunately it has suffered a little from age and use and the insides disc are a bit loose not turning correctly, which is reflected in the price. It measures ca 6 cm or 2 1/4 inches in diameter. Check the video on how to work your vintage man tool : slide rule operation instructions!