Originally Posted by
konakoa
Second, the sensitivity of VC paper changes drastically with the color of the light. Green takes a a fair amount time to expose paper fully. Blue takes but fractions of a second. Variable contrast paper does not have an even response to the two colors of light. Adjusting the amount of blue and green will also have the added headache of simultaneously changing the basic print exposure time.
Ilford (and once upon a time, Kodak) solved this for variable contrast paper with their VC printing filters by building in varying amounts of neutral density into each filter. Every filter has slightly different amounts of neutral density so in use when a #2 is replaced with a #3, the underlying basic exposure for the paper is still the same.
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