Saturate / Grayscale

R
Posted By
ronviers
Oct 28, 2007
Views
473
Replies
2
Status
Closed
Hi,
If I desaturate (shft-ctrl-u) an image and compare (using difference blend mode) it to the same image that I have done a mode>grayscale convert to, I get small differences. Is this normal? Should there be differences in a grayscale image compared to a desaturated image?

Thanks,
Ron
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grayscale

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MR
Mike Russell
Oct 28, 2007
If I desaturate (shft-ctrl-u) an image and compare (using difference blend mode) it to the same image that I have done a mode>grayscale convert to, I get small differences. Is this normal? Should there be differences in a grayscale image compared to a desaturated image?

Yes indeed. Not all monochromes are created equal, and this has been true from the earliest days of Photography, when blue was about the only light capable of registering on a photographic plate. A Hue/Sat adjustment is based on the HSB color space, which is based on a geometric conversion of RGB to a six sided pyramid, aka hexcone, with white at the center of the base, and black at the point. The brightness of a color value is equal to the distance along the central axis of the hexcone. For purposes of converting to grayscale, the important characteristic of this hexcone is that it gives equal brightness ratings to Red, Green, and Blue.

Photoshop’s conversion to grayscale is somewhat more sophisticated. It gives different weights to red, gree, and blue. Green is the brightest of the three, and blue is the darkest.

There are better ways to convert to grayscale. My favorite is one I learned from Dan Margulis’s books, and is based on using the channel mixer to manually adjust the weights of the different primaries. Another method, promoted by Russell Brown uses two HSB adjustment layers, one that desaturates, and the other which selectively changes hue.

Here’s a good article that says more:
http://www.swpp.co.uk/professional_imagemaker/monochrome.htm —
Mike Russell – www.curvemeister.com
R
ronviers
Oct 28, 2007
On Oct 28, 7:10 am, "Mike Russell" <
MOVE> wrote:
If I desaturate (shft-ctrl-u) an image and compare (using difference blend mode) it to the same image that I have done a mode>grayscale convert to, I get small differences. Is this normal? Should there be differences in a grayscale image compared to a desaturated image?

Yes indeed. Not all monochromes are created equal, and this has been true from the earliest days of Photography, when blue was about the only light capable of registering on a photographic plate. A Hue/Sat adjustment is based on the HSB color space, which is based on a geometric conversion of RGB to a six sided pyramid, aka hexcone, with white at the center of the base, and black at the point. The brightness of a color value is equal to the distance along the central axis of the hexcone. For purposes of converting to grayscale, the important characteristic of this hexcone is that it gives equal brightness ratings to Red, Green, and Blue.
Photoshop’s conversion to grayscale is somewhat more sophisticated. It gives different weights to red, gree, and blue. Green is the brightest of the three, and blue is the darkest.

There are better ways to convert to grayscale. My favorite is one I learned from Dan Margulis’s books, and is based on using the channel mixer to manually adjust the weights of the different primaries. Another method, promoted by Russell Brown uses two HSB adjustment layers, one that desaturates, and the other which selectively changes hue.
Here’s a good article that says more:http://www.swpp.co.uk/professional_imagemaker/monochrom e.htm —
Mike Russell -www.curvemeister.com

Hi Mike,
That was a very good article. There is a lot more to it than I thought.
To decide on a preferred method I will need a vision of how I want the image to look. Technically I like the method where the calibration is changed during the RAW conversion. But I will have no problem trying them all when it’s time.

Btw, the new Curvemeister is excellent. The GUI is beautiful – very Vista-like. I have only tried using the demo once to see if I could correct a portrait of myself. The image is a tough one though because of the way I shot it. I think it would be a good one for your class because the exposure and focus are correct only the colors are screwed up. I shot it using lights that were too warm. The problem I’m having is in maintaining neutrality across an otherwise white backdrop while getting the red out of the skin tones.

Thanks for the reply.

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