The ExpoDisc Portrait White Balance Filter is a calibrated, white-balancing filter with a warm-toned bias optimized for portraiture. The ExpoDisc Portrait White Balance filter is also effective for reducing the bluish cast on overcast days or at high noon; reducing the excess blue from flash and flash-fill; as well as warm high altitude shots. ExpoDisc is an easy to use custom white balance solution that eliminates the inconvenience and inconsistencies of other methods. It is a neutral diffusion filter that gathers ambient light from 180 degrees and passively transmits 18% of it through to your camera's light meter. The resulting "gray frame" accurately represents the average colorcast of the light in which the photograph is taken. ExpoDiscs do not screw in. They lack front threads and were designed to be pushed-on over the lens threads in order to facilitate a quick white balance.