This B&W Bay 6 KR12 Reddish-Brown Color Conversion Multi-Coated (MC) Glass filter allows the use of a tungsten-balanced film in daylight without a strong blue cast. It effectively absorbs the excess of daylight blue that is otherwise superimposed on all the colors.
But, the KR12 filter is not only useful as a "savior in cases of need", or for making good use of an otherwise useless remnant of film, because it can also be used effectively for overcoming extreme contrasts that are greater than a daylight reversal film can handle. A reversal film balanced for tungsten light has a flatter gradation curve, adding a possible high contrast control.
Multicoated to minimize reflection at the filter surfaces which reduces flare and ghosting.
This filter is specific to lenses with Rollei's exclusive Bay 6 filter size.