Practicing the craft: Monochromatic red and white striped lily

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Originally Posted by Mike Buckley

Ted,

My statement about losing two-thirds of the data comes directly from pages 9 and 10 of Vincent Versace's book, From Oz to Kansas. He explains that "Photoshop keeps all the color/luminance information in Lab and then translates it as necessary to ProPhoto RGB, CMYK, sRGB, Grayscale, etc. To see this, convert an RGB image to Lab and watch what happens in the Info panel. Applying the destaurate command discards the A (green-magenta) and B (blue-yellow) color channels, retaining only the Luminance (L) channel information. Two of three channels are lost, therefore, you have lost two-thirds of the original file's information, and the file size shrinks accordingly. Translating the file back to RGB returns it to the original file size, but what remains in the R,G, and B channels is only the L channel information. The same thing happens if you convert RGB to Grayscale; the file size is reduced by two-thirds."

I didn't check him out on that; I took his word for it. At the time his book was published, all references in it to Photoshop were to version CS6.

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Sorry, Mike, I don't use Adobe editing products at all so I can't try what he suggests, nor can I comment on this clarification.

Of course, it is obviously true that any RGB image can converted into a smaller single-channel image in several ways - but your earlier post did imply that just de-saturating an image would somehow shrink it, did it not?