Do you know how color affects the exposure? Do you know why it is so easy to blow the red or blue channels when taking close up photos of flowers using automatic exposure? There are a couple of nice posts (this one and this one) on DPReview Nikon D1/D2/D100/D200 forum discussing this issue. At the end, you may also want to visit this article, in which Bruce Lindbloom discussed how to compute the luminance value from RGB values in different color systems (SRGB, Adobe RGB, etc).
Digital camera sensors have red, blue, and green sensing sub-pixels arranged in a special pattern (Bayer pattern) to capture three different colors. The final luminance value is calculated from the three color components. Not all colors are created equal. Red and blue colors do not contribute as much to the overall luminance values as the green does. So if the scene is dominated by red or blue colors, the camera may see the scene as too dark and try to boost the overall brightness of the scene to compensate. The consequence is blown red or blue channels. So, what is the fix?
The tips for avoiding blown red or blue channel
- Use RGB histogram if your camera has one: RGB histogram displays the histograms of three color channels separately. After taking one photo, check the RGB histogram of the image. If you see blown red or blue channels, you can dial in some negative exposure compensation to take another photo. If this makes the picture look too dark overall, you can always try to overcome by post-processing. It is not possible to recover if the details in the highlights are lost. Some also suggests changing white balance to avoid blown channel. I am not a fan of that but you can certainly try.
- If you have a camera without RGB histogram: First, try to understand how color affects the exposure on your camera and dial in the exposure compensation. You may get a good feel of how much exposure compensation is needed by practice. Second, use exposure bracketing. By taking pictures with different exposure compensation, you can always select the one with the perfect exposure on your computer. Most cameras have exposure bracketing function to take a series of photos at different exposures automatically with a press of shutter release.