The B+W 49mm KR-1.5 Skylight Filter is a delicate reddish-yellow conversion filter that absorbs violet & some blue plus a minimal amount of green, but it passes yellow & red undiminished; the colour of the light becomes &8220;warmer” by 15 Mired. That is mostly the optimal amount for correcting the light blue cast in the shadows under a blue sky. It also absorbs UV radiation & hence it provides better sharpness. With colour films and/or developing processes that generally yield results that are too &8220;cool, " this filter can be kept on the lens at all times. With this filter, skin tones in particular will be rendered more naturally.