Originally Posted by
wickerprints
The problem with uneven density response as a function of wavelength is that you can clip your highlights in one or more channels without achieving adequate exposure in the other(s). For example, if the filter has a 10-stop density in the red and blue frequencies but, say, only 6 stops of density in green, then it becomes improbable to expose the sensor in such a way that would allow faithful capture of all three channels at once.
That's why ND filters are expensive--it is easy to make a material that blocks a lot of light, but not so easy to do it in a way that is roughly uniform across the visual spectrum.
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