	Built-in light metering with SLRs started with clip-on selenium cells meters. One such meter was made for the Nikon F which coupled to the shutter speed dial and the aperture ring. While the selenium cell area was big, the add-on made the camera look clumsy and unattractive. In order for built-in light metering to be successful in SLR cameras, the use of Cadmium Sulfide Cells (CdS) was imperative.