Understanding colour profiles

Many of you, it seems, are learning that there's more than one way to colour a digital photo, and terms like sRGB and Adobe RGB appear to determine the ultimate appearance of your images. The only problem is that there's very little information available to help you make sense of these things, and so it's not at all clear what settings you should use -- or if it really matters at all. This week, let's take a look at the world of colour profiles.

What Is a Colour Profile?

Simply put, a colour profile defines the range of colours that can appear in a digital photo.

Here's the problem that colour profiles try to solve: Every device, whether it's a digital camera, a scanner, a computer monitor, a television, an ink jet printer, a colour laser printer, or a professional photo printer, has a somewhat different set of reproducible colours. This fact is evident when you try to print a picture that shows up with brilliant greens on your monitor and find that the photo that comes out of your printer has washed-out greens. Or you may see dark shadows on screen that lose all their subtlety and print out as one jet-black blotch.

To address this issue, colour profiles have been developed that apply consistency to the way images are displayed. In theory, a given colour profile uses a range of colours that are visible no matter what kind of monitor you have, for instance.

What Profiles Are Most Common?

While there are a lot of different colour profiles in use today, the most common two -- and the ones you will see almost exclusively on digital cameras and in photo-editing software -- are sRGB and Adobe RGB.

Hewlett-Packard and Microsoft designed sRGB. It includes the colour space used by most computer monitors, so that any colour that is allowed in the sRGB profile can be reproduced on any computer display. That's handy, and that's why sRGB is the only colour profile used by Web browsers. More to the point: Web browsers expect pictures to be rendered in the sRBG colour profile. If they're not, the images will be rendered inaccurately.

The alternative is called Adobe RGB. Championed by Adobe (obviously), this profile is optimized not for computer displays, but for printers; it offers a broader range of colours that better describe the capabilities of photo printers.

A handy way to think about this is to imagine that you have been given a huge box of crayons to colour your photos -- that's the entire range of colours that your camera's sensor can recognize. But neither sRGB and Adobe RGB can give you access to all the crayons; regardless of which profile you choose in your camera, some of those crayons are taken away.

You get roughly the same number of crayons with either profile, but each profile has a different range of colours available. Adobe RGB gives you a wider range of colours from lightest to darkest, but has fewer gradations of hue. sRGB has a narrower colour range available, but it provides more hues within that selection. So sRGB can show more colour detail within a narrow range of tones, while Adobe RGB provides a wider colour range across the total spectrum.

Which Colour Profile Should You Use?

Most digital cameras default to sRGB for a very good reason: It's used by Web browsers, home printers, and many commercial professional printers. That makes it a superb choice for almost everyone.

But the extra colour depth offered by Adobe RGB makes it a good choice when you want to do extensive editing before you print or share your photos, especially if you're shooting in RAW format -- since RAW has a lot more colour information than JPEG to begin with.

If you shoot in RAW and want to edit your photo on the PC, set your image editing program to use the Adobe RGB colour profile. And yes, despite the name, programs other than Photoshop also support Adobe RGB (Corel's Print Shop Pro, for instance). If your camera can shoot in RAW, it can probably tag your photos for the Adobe RGB colour space as well -- check your camera's menu system or user manual. If you can't set this in your camera, you can change the colour profile within your image editor before you start working on the photo.

One important reminder: Use your image editor to change the colour profile back to sRGB before printing. If you print an Adobe RGB image on a printer that uses the sRGB colour space -- and most do -- the results will be pale, washed out, and perhaps even a bit pixilated. The same is true if you share the photo on the Web.

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Dave Johnson

PC World (US online)
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