Why doesn an orange filter make the blue sky black?
A. Second, if this chart is right, then an orange filter should make the blue sky black, or Zone 1. Why doesn’t this happen in practice? The easy answer is that the sky is not completely blue. The sky is blue, but it also contains red, orange and yellow light in smaller proportions. The same is true of any object that has a color. There are virtually no real-world objects that reflect excatly the wavelength that corresponds to their color. A red ball rarely, if ever, reflects only 700 µm light. Q. When are filter factors actually useful? A. Filter factors are useful on neutral (grey or white) subjects. Q. Can applying a manufacturer’s filter factor hurt my photography? A. Yes. Filter factors are useless when your subject contains large swathes of color that you are trying to eliminate and perfect greyscale reproduction (rather than relationship) is relatively unimportant. For example, grey cement or limestone buildings against a blue sky on a sunny day can cause tonal merger without a
