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.