 The lens was of 38 mm focal length and with a maximum aperture of .A ring around the lens optic itself was used to set the film speed (ISO 25 to 400), which was indicated on a small window on the front of the lens assembly; also there, but below the lens optic itself, was the cadmium sulfide (CdS) photoresistor for the light meter.The location of this, inside the filter ring of the lens, meant that the meter would function accurately even with filters fitted to the lens.