dmmartindale@watcgl.UUCP (Dave Martindale) (08/23/84)
Awhile ago someone asked what the difference was between 1A and 1B skylight filters. I just purchased a Hoya 1B filter and the data sheet that comes with it attempts to explain the difference. They show a transmittance vs. wavelength graph for 1A and 1B filters. Both filters attenuate the ultraviolet in about the same manner, and their transmittance rises to about 94% in the deep blue region at 450nm wavelength. The 1A filter then has a dip in its curve, dropping to 90% transmittance, at 500nm (blue-green), then it rises again to about 97% by 570nm (yellow-green) and remains there for yellow and red. The 1B also has a dip to 90% transmittance, but it occurs at about 540nm (green). It is back to 96% transmittance by about 610nm (orange-red). So, basically, both filters remove UV light. The 1B is trying to remove some excess green, and the 1A is trying to remove excess blue-green but has little effect on green. The data sheet says "The recently introduced Skylight (1B) filter has the new property of spectral transmission and is an ideal filter for outdoor photography. The peak absorption of the Skylight (1B) is only in the range which responds most strongly to the green spectrum in color film's sensitivity. With this new filter, the blue-green tinges can ideally be compensated, which was unavailable by the conventional skylight (1A)." (I'm not sure quite what that says, but there it is.) To further muddy the picture, the Kodak Wratten (gelatine) 1A filter has the dip in its transmittance located at 550nm, and a shape to the curve that looks more like Hoya's 1B curve than their 1A curve. Maybe Hoya is now getting right what Kodak had all along? But I would have expected Hoya's 1A to copy the Kodak 1A as closely as possible. Anyway, you can now go off and make your own partially-informed judgement!