For the canon eos 1d mkiii and 1ds mkiii digital slr cameras the anti fog eyecup eg is useful in cold weather to prevent users breath from condensing on the eyepiece lens its also handy in hot humid weather to prevent fogging from evaporation around the users eyes or when stepping from a cool environment into a hot one please note this eyecup will not fit the canon eos 1dmkiimkiin or 1dsmkii cameras