In-Camera Color Spaces
So you've been fumbling through the custom menu settings on your new dSLR or high end camera and you've found a setting called "Color Space" or something similar, with choices like "Adobe RGB" and "sRGB". What do these settings mean, and when are they used? Let's take a look and try to make some sense of this, because it can alter your images and frankly, can really foul things up if you don't know how to set this option and how to properly view/print the photos taken with the selected color space.
What is a color space
While I've covered color spaces and profiles in previous articles, a brief description of the term "color space" is probably worth summarizing here, if only briefly. A "color space" is like a language that describes what the red, green, and blue values in your images really mean. You might be tempted to think that knowing the RGB value of a pixel gives you it's exact composite color. Such is not the case, however, as different shades of red, green, and blue can be used as primaries which means that a particular RGB value can indicate an entirely different color on two different devices (or images). Tweaking the red, green, and blue primaries gives the ability to store images in a color space that better matches the device(s) that will reproduce the photos later. A monitor, for example, is capable of reproducing a different range of colors than a printer, so using a different color space for the monitor and printer will allow both of those devices to achieve closer to their full potential.
Visually, you can think of a color space as a container that holds all RGB values possible in an image from 0,0,0 to 255,255,255 including all combinations of vibrant, saturated colors in between. The larger the container, the more colors that can be reproduced but the further apart the RGB values become since they are spread across the entire container. The trick becomes trying to match the size/shape of the container that holds the image to the size/shape of the container that is used by the monitor/printer. This matching of containers (color spaces) is what color management is all about.
sRGB versus Adobe RGB
High end consumer cameras and dSLR cameras usually offer two choices for color space: sRGB and Adobe RGB. sRGB is what most PC's and monitors use and it will display reasonably well on emails and web pages without the need for any color management software (web browsers and the like do not offer color management). While sRGB is generally well matched for your average PC monitor, the "container" is rather small with this color space: it doesn't cover some of the more vibrant and saturated shades that might possible to capture with the camera and reproduce on your printer. That brings us to Adobe RGB. Adobe RGB is a larger color space than sRGB, meaning that the container is large enough to hold colors that would be "clipped" in sRGB space due to those colors being too bright/saturated to be reproduced in the smaller sRGB container. Shooting/storing images in the Adobe RGB color space will allow you to capture and therefore later reproduce vibrant, saturated colors like deep yellows, cyans, and magenta colors found in subjects like flowers, some clothing dyes, and other subjects with very deep and saturated color.
sRGB and Adobe RGB in practical use
By now, you're probably thinking, why even bother with sRGB if Adobe RGB can record a wider range of colors? Good question! The simple answer is that, unfortunately, the whole world is not yet ICC (color management) aware. By that I mean, sRGB is a good middle ground if you are placing images in a public venue such as the web or email, not knowing whether or not the recipient can "decode" the more specialized Adobe RGB color space. If he/she doesn't have color managed software, the Adobe RGB image will probably look washed out because it's "container" is not as well matched as sRGB to a standard monitor. Simply put, the use of Adobe RGB color space requires specialized software to view/print the resulting images accurately.
When using fully ICC aware software such as Qimage or PhotoShop, the software will know how to take the colors from the larger container (Adobe RGB) and map them properly into the smaller containers used by your monitor/printer. Since your monitor covers certain colors that your printer cannot print and vice versa, using a larger color space up front and then converting doesn't "penalize" either device and makes the most of your images.
If you have/use ICC aware software, there is a strong argument for using Adobe RGB in that it is a larger color space and can store a wider range of color. You can't get back what you didn't record in the first place! After all, if you are familiar with ICC aware software, you can easily convert from Adobe RGB to sRGB should you need to email someone some photos or upload photos to a web site, so using Adobe RGB doesn't mean that you can never use those photos for web/email display! In addition, when printing photos, your ICC aware software will know how to translate the wider range of colors that Adobe RGB color space offers so that they can be reproduced in print (provided you have an ICC profile for your printer and the paper you are using).
When using Adobe RGB, be aware...
Be aware that the sRGB/Adobe RGB selection on your camera applies to in-camera JPEG/TIFF images only. If you are shooting in raw mode, your raw images will not be altered or stored in any color space so the color space selection will not be a limiting factor: you'll choose the "converted" color space in whatever raw decoder you use to develop the raw images. Shooting in raw format really offers the widest gamut (color coverage) because raw images record data straight from the image sensor and that data covers an even wider gamut than the larger Adobe RGB color space! For frequent shooting of subjects with very vibrant and saturated colors, this can be important because there are likely some areas of color that your printer can reproduce that not even Adobe RGB can record. For example, most inkjet printers can reproduce some shades of yellow and cyan that are beyond the Adobe RGB color gamut. This is not normally an issue with "general" shooting, but can become a factor when shooting subjects that fill the frame with vibrant colors such as might be the case if you are shooting sunflowers in bright sunlight.
Also be aware that not all so-called ICC aware software can discern when your camera JPEG's have been stored with the Adobe RGB color space selection. The above mentioned Qimage and PhotoShop can automatically decode the color space properly, but many other photo editing and printing tools will not. The bottom line here is: make sure you check the software you are using to ensure that it is picking up the fact that your images are stored in Adobe RGB color space. If the software you are using opens the images in sRGB color space, you'll know that the software isn't properly decoding the embedded color space tag(s) in the images. In that case, you may need to manually assign the Adobe RGB color space to tell the software that the images are in that space. Unfortunately, camera manufacturers still aren't embedding the actual profile in the images even though doing so would only add about 500 bytes to the size of the file. What we are left with are a handful of programs that are smart enough to decode the proprietary embedded tags used by the manufacturers, so be careful when shooting in Adobe RGB color space that your software actually recognizes the photos as Adobe RGB photos!
The short story here is that I recommend using the Adobe RGB color space when shooting JPEG's or TIFF's with your camera if your camera offers the option AND you are familiar with color management and ICC profiles. Because the sRGB color space is smaller and cannot record as many colors in the vibrant and saturated range, it should be used only on more limited platforms such as specialized applications that do not (or cannot) make use of color management. For example, you may be forced to use sRGB color space if you must rely on super-fast or super-simple output that requires printing directly from the memory card using a printer that can print without a computer attached. In this case, the printer will likely not recognize Adobe RGB photos and will probably assume the photos are in sRGB color space. The result will be dull and inaccurate color if the standalone printer assumes sRGB color space but is "fed" Adobe RGB photos. So if you have the time, the software, and the know-how, Adobe RGB is the way to go unless you are shooting in raw mode, which gives you even more flexibility as the color space decision can be made later, when you develop the photos.
-- Mike Chaney