Adobe Lightroom and Photoshop are great tools for adjusting the colors, brightness and contrast of a photo, but there is a hidden gotcha - color space conversion. Sometimes things look great in Lightroom, but don't look so great when you export a JPEG to post on the web or send to your printer.
The problem is that colors aren't always the same - especially in the world of digital imaging. Colors are represented as values of red, green and blue where each component is expressed as a number. But the colors aren't absolute. Instead, they represent a percentage of the color. For instance, imagine a pure red color, expressed as 100% red, 0% green and 0% blue. You'll also often times see it expressed as 255 red, 0 green and 0 blue, but it's still really a percentage. Those values are telling the display device, such as your monitor, to display "as much red as you can." The problem starts there, because "as much red as you can" will be different on different devices. Some monitors can display a lot of red, where others might be very limited. So, what you see as a result can be quite different.
To further complicate the problem, we can see colors well beyond the range that most devices can display. Even if your monitor is giving you "as much red as it can" it's not going to be more red than you can see. The spectrum of colors we can see is well beyond most devices abilities to display.
Color scientists have addressed this issue by creating standard color spaces, and also the ability for individual devices to have their own custom color profile. The color space (or profile) defines the limits of the space's various colors relative to the visible spectrum. The three most common color spaces are sRGB, Adobe RGB 1998 and ProPhoto RGB. Of the three, sRGB is the smallest, meaning that its most vivid red will be less vivid than either of the other two. But, even the most vivid red in ProPhoto RGB is still less vivid than the most vivid red we can see.
When you're working with digital photos, you are working within a color space. The color values in your image only have meaning relative to the color space being used to interpret and display them. The most commonly used color space is sRGB, especially on the web. Most monitors can only display sRGB colors, so it's a good match. Most commercial photo printers can only print in the sRGB color space, but modern ink jet printers can usually print most or all of the larger Adobe RGB 1998 color space. As a photographer, you can choose to work in any of the available color spaces, so you can choose to include more colors in your photos if you want.
Choosing a working color space is an area that's often overlooked. At best, it seems that most tutorials gloss over the issue and suggest one of two approaches. One school of thought is to choose sRGB (the smallest color space) so that you're reasonably assured that your monitor can correctly display all of the colors in your image, and you can't edit what you can't see. The other school of thought is to use a very large color space, such as ProPhoto, so that you're not needlessly throwing away color information too early in the process. You can always convert to a smaller color space at a later time.
From my perspective, neither approach is always right. The best solution is to understand the ramifications and limitations of your choice. If you don't anticipate the problems that may occur during conversion, then you may be surprised at the results.
There are two main questions to consider. The first is the capability of your monitor. If your monitor can only display sRGB colors, then you can't see everything in you image, so you're working partially blind. The other consideration is the output destination. If you intend to post your photo online, then you'll want the end result to be in sRGB, but the conversion from a larger color space into sRGB can create problems. As you'll see in the video, the conversion from a large color space to a smaller one can result in significant clipping in one or more channels. The result is not just changing colors, but loss of detail. For instance, fine details in wispy clouds at sunset might be rendered as a block of solid color without any detail - probably not what you want.
Even if you only use Lightroom, you're not immune to the problem. In fact, it may be more of a problem since Lightroom always works in a very large color space. As you'll see in the video, you can't rely on the histogram in Lightroom to tell you how your image will look after you export it - and you may end up loosing details that you didn't expect.
This color space video tutorial explains why the problem happens and provides some techniques to avoid the problem. As I explain in the video, it's difficult to talk about things you can't see, and the nature of the video is that you won't be able to see the details and colors, so be sure to check out the photos and explanations below.
This is the photo I used in the video. If you move you mouse pointer on and off of the image you'll see the before and after versions. Notice how the before version (when you mouse pointer is on the image) lacks detail in the brightest areas of the clouds. If you can't see it change, it's because your monitor doesn't have a large enough color gamut to display all of the colors in the "after" version, so before and after look the same to you. Click here to see a larger version.
The photo below shows the sunset on a different night. Again, use the rollover feature to reveal the version using standard profile conversion. This image was processed a litle differently. I tried the trick using the Vibrance adjustment, but there were just too many areas that were out of gamut. I ended up using the red channel to paint back the overly saturated colors and enhance the details in those areas. Then, once the image was converted to sRGB I was able to add a Vibrance adjustment layer (in Saturation mode) to increase global saturation. However, I used the Blending Option sliders to block the increased saturation from the brightest red areas. That way, I was able to add back some saturation and avoid blowing out the brightest areas. The mouse-over version is a straight export to sRGB jpeg from Lightroom 3.6.