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What About ProPhoto RGB?

After spending some time last week discussing how sRGB and Adobe RGB differ, it seemed an opportune time to bring ProPhoto RGB into the mix. While the gamut of Adobe RGB is bigger than that of sRGB, ProPhoto RGB is really big. But is bigger necessarily better?

To answer that question, let's first look at just how big "big" is. ProPhoto RGB compared to sRGB, Adobe RGB, and a typical Epson printer gamutShown here is another diagram along the lines of those I used last week. In addition to the shapes outlining the gamut for sRGB and Adobe RGB, I have added a white line showing the extent of ProPhoto RGB's gamut as well as an indication of the typical gamut of an Epson desktop printer on glossy paper (the gamuts of matte papers tend to be somewhat smaller). ProPhoto is huge. While Adobe RGB covers about twenty percent more area than sRGB, ProPhoto RGB is about twice the size of sRGB. Better than fifty percent bigger than even Adobe RGB.

Indeed, ProPhoto is so big, it actually includes some colors that are beyond the range of human perception.

It is the only one of the three though that encompasses the entire gamut of the Epson printer shown. sRGB severely clips the Epson gamut in the cyan to green region (bottom left) and yellow-orange region (top). Adobe RGB can still clip some very saturated yellows but does cover all the greens, and green is a very important color being in the middle of the visual spectrum and very prevalent in nature. It seems tempting then to use ProPhoto RGB in order to not lose that area of yellow. But if we do, we have to accept the fact that we also will be encompassing colors we can't even see, never mind print.

Just as Adobe RGB and sRGB contain the same number of colors as dictated by the limitations of how they are represented in 8-bit and 16-bit mode, ProPhoto too has to live within what the numbers are capable of. 16.7 million colors in 8-bit and 281 trillion colors in 16-bit mode sure seem like a lot. But given how much bigger ProPhoto is than the alternatives, it may be worth considering whether we want to devote so many of them to colors that seem to serve no purpose. Even in 16-bit mode, this should give us pause for thought. That's a pretty high price to pay to not lose a few shades of yellow-orange.

But technology continues to advance, and just because we can't print much beyond Adobe RGB today doesn't mean we'll always be so color challenged.

Indeed, the advent of digital cameras allows us to easily capture colors that Adobe RGB can't represent. While you may be limited to sRGB and Adobe RGB when shooting, the use of RAW mode gives access to everything the sensor recorded. Selecting ProPhoto RGB in Adobe Camera RawAdobe Camera Raw (when using Photoshop, not Elements) allows you to select what color space you want to convert your RAW files to. In addition to sRGB and Adobe RGB, the list includes both Color Match RGB and ProPhoto. Color Match is a traditional color space with a gamut similar to that of sRGB used on early Macintosh computers and isn't of much interest today, but ProPhoto can come in handy.

Clipping on the same image in sRGB, Adobe RGB and ProPhoto RGBShown here are three versions of the same shot taken at Sylvan Lake in South Dakota's Custer State Park. I opened the RAW file in Adobe Camera Raw and turned on the highlight clipping option, then made three screen captures, each with a different color space selected. The bright red shows the portion in each where clipping would occur. As you can see, sRGB loses quite a bit, while Adobe RGB comes much closer. Only ProPhoto RGB is able to encompass all the colors without clipping. The pre-dawn sky itself is somewhat pink, but what matters here is the red clipping display, not the pink sky.

By converting your RAW files into ProPhoto RGB in Adobe Camera Raw you preserve more of what the sensor actually saw. Once you open them in Photoshop though, you need to decide what to do next. If you keep them in ProPhoto, you'll have files that in the future may yield better prints than if you settled for Adobe RGB. But the gamut of ProPhoto is so large that, unless you are careful, you may need to contend with banding and other artifact problems, even in 16-bit mode. A definite dilemma.

Some photographers have adopted a ProPhoto RGB workflow but so far I haven't gone that far. What I do do though is convert images to ProPhoto RGB if Adobe RGB shows excessive clipping. After opening such images in Photoshop, I then convert to Adobe RGB using Perceptual rendering intent which compresses the gamut rather than clipping it. Once the image is in my regular Adobe RGB working space I can then selectively adjust saturation to restore the image to how I want it to look.

In an 8-bit world, sRGB represented the best compromise between gamut and usability. Now that we can use 16-bit for most things, Adobe RGB rules the day. Once the world moves from 16-bit to 32-bit images, I will undoubtedly start using ProPhoto as my working space. For now at least, I'm being somewhat cautious. I use ProPhoto where it definitely makes sense to, and then retreat to the safety of Adobe RGB. As I gain more familiarity with ProPhoto, its charms are indeed alluring and it is possible I may adopt it completely before the era of 32-bit, but not yet.

For the curious, the shapes shown this week and last represent the two-dimensional gamut projections as graphed in LAB mode. LAB is a representation of color that is considered "perceptually uniform" in that the distance between any two colors is proportional to how different the two colors appear. As such, it is particularly useful for the purpose at hand since the area of each outline is therefore visually proportional to the colors each contains. Gamut is actually a three-dimensional function since, in additional hue and saturation represented in these two-dimensional shapes, brightness also plays a role. But the screen on my monitor is itself two-dimensional so I've limited myself to what I can show without resorting to special browser plug-ins and such.

By the way, don't even think of using ProPhoto RGB in 8-bit mode.

Update 6/11/2006 - I've been using ProPhoto RGB as my Photoshop working space for the past couple of months now. The color gamut captured by a digital camera can exceed Adobe RGB by a wide margin in some instances. I'm not sure if I'll ever get to print or otherwise show off my work with a gamut approaching that of ProPhoto, but if I ever can, at least I've got it now. And yes, everything is saved as 16-bit.

Update 9/21/2008 - Converting to Adobe RGB or another working space using Perceptual intent doesn't do what you might thing it does it turns out. Matrix-based profiles only support Colorimetric intent. More detail over here.

Date posted: January 29, 2006 (updated September 21, 2008)


Copyright © 2006, 2008 Bob Johnson, Earthbound Light - all rights reserved.
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Related articles:
Color Management: A Question of Intent
More Than a Bit of a Difference: 8-bit Versus 16-bit
Raw to Cooked with Adobe Camera Raw
The Great sRGB Versus Adobe RGB Debate
The Vast Uncharted Reaches of Color Space

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