Color spaces are abstract mathematical models that are commonly used to describe the way colors can be represented through numbers. In the photo industry, there are three RGB (Red, Green, Blue) color spaces that we use: (ProPhoto RGB, Adobe RGB (1998), and RGB, and we can think of them as being large, medium, and small, respectively.
Most current DSLRs and even point-and-shoot cameras support Adobe RGB and SRGB, which is often the camera's default color space. On a RAW workflow, these settings are irrelevant, but they are an important consideration when shooting JPGs.
For inkjet printing, Adobe RGB (1998) works great 99.9% of the time because it generally contains all the colors that the final output device (in this case a printer) can render. RGB monitors can display more colors than can be matched in print. Conversely, some CMYK colors (which stands for Cyan, Magenta, Yellow, and Black) cannot be matched on-screen.
Generally speaking, sRGB is the recommended color space to use when you have to upload images to a website or email them to a client. This is because sRGB is closer to the color space seen on most current monitors. Were we to instead choose Adobe RGB for online use, the images most likely would appear "washed-out or muted" on most browsers.
Why is ProPhoto RGB relevant? It offers the widest gamut of the three color spaces. Therefore, it is the preferred color space for workflows that require the preservation of the largest amount of color, as when heavy retouching is required. ProPhoto RGB is the color space that Adobe Lightroom uses when importing files into a catalog. Apple Aperture also uses a "wide color space", but does not define it as ProPhoto RGB.
"I want convert my images to the ProPhoto color space because I love the rich, saturated colors" is a common statement-and it is somewhat inaccurate. The truth is that converting to a larger color space does not change the colors at all. And converting to a smaller space will only clip any out-of-gamut colors.
Using a color space with an excessively wide gamut like ProPhoto RGB can lead to posterization, which is the continuous degradation of tones with abrupt changes from one to another.
Most digital workflows consist of a constant data downtrend, but there is no reason to worry if they are properly executed. Not only photographers can benefit from understanding color spaces, but also web designers, graphic designers, retouchers, and others.
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All images are copyright Eduardo Angel