Originally posted by noblepa On B & W film, a red filter works by suppressing the blue light coming through the lens, but not the other colors. That is why it makes clouds really pop out. The blue sky is significantly underexposed, compared to the clouds. This makes the sky much darker.
Wouldn't the same thing be true in digital? I know that in PS, I can desaturate the image, removing all the color. But in addition to that, I want to reduce the brightness of the pixels that were originally blue, without changing the brightness of the other pixels. I'm unclear how to easily do that in PS (I use Elements 6).
Not sure about PSE 6, but most programs would allow several different ways of doing that, with lots more control over the results than a simple filter would provide.
One way would be to first alter the white balance of the shot (RAW), or similarly alter the color for JPEG. For instance, using a WB slider, use it to make the picture redder. Or with RGB controls, decrease the blue slider, increase the red. You get to control exactly how much redder you make the picture before desaturating it. The effect is the same as using a red filter.
Some programs provide something called a "channel mixer" which does a similar thing in principle, but lets you see the results in B&W the whole time. That's probably the most common way these days of doing what you describe. You can tell it to use only the red channel, or just the red and green channels in whatever ratio you want, or just enough blue to keep the sky from going completely black - whatever you want.
I've read that the most recent version of Photoshop (not sure about Elements) provides more controls still.
Quote: I've often thought it would be much easier, if I could just use a red filter, like on a film camera, to underexpose the blue in the scene.
You certainly *can* do this, but you lose the fine degree of control you can get by doing this in PP, so most people don't.