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The Great sRGB Versus Adobe RGB Debate

As more and more photographers dip there toes in the developing trays of the digital darkroom (to stretch an analogy perhaps further than necessary), the choice of color space becomes a more frequent topic of conversation. But all is not necessarily as it appears since not all programs are color management aware.

Both sRGB and Adobe RGB are RGB color spaces. As such, they share the same basic structure. Both have three channels of data, one for red, one for green, and one blue. In 8-bit mode, both allow values from 0 through 255 for each channel since that's all that is possible in 8 bits. In 16-bit mode, both are bound by the limits of 16-bit data and provide for values from 0 through 65,535 for each channel value. Without regard to what the numbers mean in fact, there is no difference between sRGB and Adobe RGB.

But what do the numbers mean, and what is different between the two?

Adobe RGB is defined to have a wider gamut than is sRGB. That is to say, it uses the numbering scheme available to it to represent a wider range of colors. That's just the way Adobe specified it to be when they came up with it. Not more colors of course since both are limited by the same physical characteristics dictated by three channels of 8 or 16 bits.

It is a common misconception that Adobe RGB has more colors than sRGB since it is easy to assume that the palette of colors across the gamut of each is continuous, but it's not. Welcome to the digital world: zeros and ones. Digital creates the illusion of a continuous spectrum by the force of sheer numbers. Even in 8-bit depth with just 256 possible values in each channel, multiplying the possibilities of all three channels together yields 16.7 million total colors (256 times 256 times 256). That's a mighty big box of crayons.

Representation of colors spread over the gamut of sRGB Common misconception of the increased gamut of Adobe RGB

 
Left: The two outlines represent the gamut of colors available in sRGB and Adobe RGB. The dots show sample colors distributed over the gamut of sRGB.

Right: At first glance, the larger gamut of Adobe RGB should give you more colors (dots) than you got in sRGB to fill the expanded gamut, but such is not the case.

What Adobe RGB does have then is colors that are further apart from one another than are those in sRGB. The colors must be more widely spaced, since the quantity available is the same as in sRGB but they have to cover a wider gamut.

And since they do stretch to fill a different sized gamut, specific sets of red, green and blue values represent different colors in Adobe RGB than they do in sRGB. To cover that wider gamut in fact, specific values in Adobe RGB end up being used for more highly saturated colors. Take green for example. Since Adobe RGB can represent more saturated green colors yet both color spaces are limited to 255 for the most blazing green possible, 255 must get used for a more saturated green in Adobe RGB than in sRGB. The highest numbers have to represent colors at the edges of the gamut and that edge is further out in Adobe RGB.

As a corollary, to represent any given color, one would use lower numbers in Adobe RGB than in sRGB. You'd have to, since you'd need to reserve the really high numbers for those colors that are in Adobe RGB but lie beyond the gamut of sRGB. What this ends up meaning is that if one takes a file full of Adobe RGB data and mistakenly displays it as if it were sRGB, it would look somewhat unsaturated and washed out. Those lower Adobe RGB numbers represent more washed out colors when misinterpreted as sRGB.

This is precisely what happens in almost all web browsers and other common programs. Not being color management aware, they display everything as if it were sRGB, even when it isn't. This is why Adobe RGB often gets the unfounded reputation for being more washed out than sRGB. It's not the fault of the color space though. The blame lies in how the image gets displayed. In fact, the potential is exactly the opposite since Adobe RGB is capable of more saturated colors than sRGB.

One also often hears that working in Adobe RGB can promote more banding and other artifacts than can sRGB. The explanation goes that by having colors that are more widely spaced, any rounding errors or other problems that happen while manipulating the data in your favorite editing program will be more obvious. If you slip off the color you should be on and onto an adjacent one, you will be visually further away than would be the case in sRGB where the colors are closer together. This is true in so far as it goes, but neglects the obvious solution of working in a greater bit depth where more colors are available.

There are the same quantity of colors in Adobe RGB, but they are spaced further apart 16-bit mode offers a lot more colors in between the ones provided by 8-bit

 
Left: Adobe RGB doesn't provide additional colors to fill the expanded gamut, it simply spreads them further apart. The total number of colors remains unchanged.

Right: Working in 16-bit mode provides a heck of a lot more colors than does 8-bit, way more than is needed to overcome the wider spacing of colors.

As mentioned, 8-bit RGB provides for a total of 16.7 million colors. While that is quite a few, it pales in comparison to 16-bit RGB where there are 281 trillion possible colors. This doesn't give us even more saturated colors beyond the range of 8-bit; it gives us more colors in between each of the established 8-bit colors. So many more in fact that it utterly overwhelms any problem that may have resulted from the wider gamut. While it is always a good idea to do your editing in 16-bit mode, it is even more important when working in a wider gamut color space. After editing, you can optionally convert to 8-bit if needed since no further data loss can occur.

So which is better? As with many things, the answer is, "it depends." You have to step back to ask "better for what?"

If you are posting images on the web, sRGB is clearly better since Adobe RGB images will likely not display correctly. But if you are sending your images to a commercial printer they will prefer Adobe RGB or even CMYK. If you are printing at home, you can generally benefit from the wider gamut of Adobe RGB as well since current desktop printers with expanded ink sets can print colors beyond sRGB.

If you shoot in jpeg, your choice of color space in-camera can be important. If you use sRGB, you will be limited to colors that can be represented in sRGB. Even if you later convert to Adobe RGB, you won't end up with colors that were never captured. If you shoot in Adobe RGB, you'll need to contend with converting to sRGB at least for posting images to the web but you'll still have a master file that has the expanded gamut for other uses. If you shoot in raw, you can defer the choice of color space until you get your images onto your computer, but you generally can't ignore it completely. Many online photo printers deal exclusively in sRGB since that is the lowest common denominator. I'm guessing they have judged that keeping things simple minimizes the customer service and support issues they have to deal with, lowering their costs. But that doesn't mean you have to let them dictate how you shoot. Just be sure to follow their rules and convert things accordingly before you send them images to print.

Working in Adobe RGB can be at least marginally more complicated since so many programs default to sRGB. If you do work in sRGB, you can often ignore at least some aspects of color management and still have images that look acceptable. Working in Adobe RGB requires you to understand more how you are interpreting color throughout your workflow. If you're a regular reader here, you know that I've tried to do my part to help spread understanding in this area. It's not really all that complicated, but it sure can seem that way at first. A bit of time spent on learning about color management can yield much greater control of how your images end up looking.

Next week we'll look at ProPhoto RGB which has a much larger gamut than even Adobe RGB. But more of a good thing isn't always better. As we learned this week: it depends.


Date posted: January 22, 2006

 

Copyright © 2006 Bob Johnson, Earthbound Light - all rights reserved.
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Previous tip: More on Hue/Saturation in Photoshop Return to archives menu Next tip: What About ProPhoto RGB?

Related articles:
Color Management: Color Models, Color Spaces and Color Profiles
More Than a Bit of a Difference: 8-bit Versus 16-bit
What About ProPhoto RGB?
The Vast Uncharted Reaches of Color Space
 

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