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Telling RGB From True Grayscale

Technically, making a color RGB image look black and white is not the same as making it a true grayscale image.  By looking at the Channels panel in Adobe Photoshop, we can tell if an image is RGB or grayscale.  Referring to Figure 1A, the Channels panel has only one layer called Gray.  This is a true grayscale image.  In Figure 1B, the Channels panel has a composite layer and three individual layers.  A Red layer, a Green layer and a Blue layer.  This is a RGB image.  Please keep in mind that being a true grayscale image does not mean it is a better black and white image.  In fact many photographers, including myself, use color film for our black and white photography and convert our color images to black and white in the digital darkroom.  We do this so that we can retain the color information.

Grayscale Figure 1A.  Channels Panel for a Grayscale Image

RGB Figure 1B.  Channels Panel for a RGB Image

Why Not Convert to True Grayscale?

As discussed on the Understanding Light page, light has three characteristics: hue, saturation and luminance.  It is common to think of hue and saturation together as color.  If we do this, then we have color and luminance.  In traditional black and white photography, we discard color and record only luminance.  In black and white RGB photography, we keep both the color and luminance information.

In traditional black and white photography, we manage tone and contrast by many methods, including choice of film, choice of paper, lens filters, film development methods, dodging and burning, etc.  In the chemical darkroom, dodging and burning is commonly used to manage tone in select areas of the print.  In the digital darkroom, we have a number of ways of managing tone in select areas, such as selections and masks.  We also have an additional method not available in the black and white darkroom: managing tone by color.  In a color image consisting of a green field and a blue sky, I can use a number of Photoshop controls to target either the sky or field, or both, by color and then change the tone of the sky or field.

When creating black and white images in the digital darkroom, there is no need to give up the option of managing tone by color.  The Tone Management System methods presented here allow us to create black and white images without giving up this ability.  When we convert a RGB image to true grayscale, we permanently lose all color information in the image.  Thus, we give up the ability to manage black and white luminance by color.
 

Photoshop Preferences

Listed below are the Photoshop preferences I used when I devised the Tone Management System methods discussed here.  I list them here so if your results are different than mine, it may be due to a difference in these preferences.