Today’s Question: What does it mean for a color to be out of gamut, and how does it happen?
Tim’s Quick Answer: A color that is “out of gamut” is beyond the range for the current definition of available colors. One of the most common scenarios where we refer to colors being “out of gamut” relates to a situation where the printer, ink, and paper combination you’re using to print a photo can’t reproduce some of the colors defined within the actual image file for that photo.
More Detail: Ultimately, an out of gamut color is one that can’t be reproduced in a given context. In other words, when someone says that a color is out of gamut, it is always understood that it is out of gamut for a particular purpose. The color exists within our source image, for example, but can’t be reproduced with a particular type of output (such as printing).
When you’re applying adjustments to a photo, there are no colors that are out of gamut in that context. In other words, if you apply an adjustment it can only result in color and tonal values that are actually available. There is an underlying color space in use whenever applying adjustments, and that determines which colors are actually available for the image you’re working on.
If you think about tonal values I think it might be a little easier to understand the concept at work here. In a photo you are optimizing on the computer, the darkest value you can produce is black, and the brightest value is white. If you apply an extreme darkening adjustment to the photo, a large number of pixels may become black, but none of them can become darker than black. There are limits to the range of values available for a photo, you can’t produce values outside of that range.
If you print that photo, however, the darkest black that a specific printer, ink, and paper combination can produce might not be especially dark. And if the paper isn’t pure white, then the brightest value available for the print won’t be especially bright. In other words, the black in your source photo is too dark to print, and the white in your photo is too white to be represented accurately by the paper. That is essentially what “out of gamut” refers to.
While you can apply adjustments to your photos to produce any color or tonal value that is available based on the working color space in use by that software, that doesn’t mean you’ll be able to reproduce all of those tonal and color values with every possible method for producing output based on that photo.
In theory you could keep your adjustments modest so that, for example, you don’t end up with colors that are too saturated to be printed accurately. In practice you will generally want to optimize your photos so they are perfect from your perspective. Then you can determine if there are limits to how that photo can be printed through the use of soft proofing or print testing. That, in turn, may cause you to choose a specific print configuration, such as using glossy paper rather than matte paper in order to be able to produce a print with greater saturation.
In other words, colors are only out of gamut in the context of specific output, and since we tend to share our photos in a wide variety of ways that generally means we need to consider these gamut issues when producing output. When optimizing the photo on the computer, the focus is generally to produce a truly optimal source file, leaving issues of output to a slightly later stage of your workflow.