Topic: Brightness and Saturation

[roy]

Hi Folks,

Here's a question that's been on my mind for a long time......

When I use a curve (e.g. master in RGB) which darkens part of an image, it also subjectively seems to increase the saturation.  (E.g. I has a sunset picture including the sunset light reflecting off a cliff-face.  An S-curve to add contrast to the sky and cliff has also made the colours in the cliff look unnaturally saturated).

I've tried the HSB colour-space, but then the subjective saturation seems to be reduced instead!

Any tips on how to adjust the brightness/contrast while keeping the (subjective) saturation similar?

all the best
Roy

[curvemeister]

Short answer - correct in Lab, where brightness and saturation can be changed independently.

Longer answer  - with the RGB composite curve, increasing contrast also increases color saturation.  This happens because of an inherent property of the RGB color space - making the red, green, and blue curve steeper draws out the differences between colors, increasing saturation.

HSB is a very interesting variation on this theme because moving the dark end of the Brightness curve decreases color saturation, while moving the light end increases it. So if you have an image where you want to make the image darker, and add more color, use HSB.