Digital SLR cameras offer numerous functions including some that are not considered intuitive by everyone. For example, some first-time DSLR buyers wonder why their cameras blink in red or yellow over certain parts of an image in Playback display. This feature - called highlight/shadow detail alert or a similar term - may be set to "Off" by default but should be turned to the "On" position since it's useful for exposure evaluation.
Here's how it works. A blinking colored overlay on a very dark area, or on a very light-toned area, indicates a possible problem. If a bright area of the image blinks, the highlights are “blown out”: too bright to hold detail. When a shadow area blinks, it's “blocked up”: too dark to show detail.
Horse Blinking: This image - with excessively bright highlight areas - illustrates one of the two warnings provided by the highlight/shadow alert feature. Setting -2/3 compensation for the next shot produced a perfect exposure. (c) Peter K. Burian
Don't worry about warnings in regards to excessively bright highlights or excessively dark shadows if there is no important detail in those areas. In a portrait of a bridal party, you do want texture in the light-toned dresses and in the black tuxes. However, you probably don't expect (or need) detail in the bride's silver tiara or in the very dark bushes in a shaded background.
If the colored blinking indicates a loss of detail in an important light-toned area - such as the bride's dress - you should set minus exposure compensation (perhaps -2/3) and take the photo again. The next image will be darker; the camera should no longer provide the warning re: overexposure. When you note blinking over important dark areas of an image - such as a groomsman's very dark complexion - set plus compensation (perhaps +2/3) and re-shoot the photo. The next image will be lighter and the exposure of the dark area should be better
Exposure Statue: While displaying this image in Playback mode, the camera's highlight/shadow alert feature blinked over the very bright highlights and over the dark shadows under the umbrellas. Since neither area included important detail, I simply ignored the warning and was fully satisfied with the overall exposure. (c) Peter K. Burian
Be very careful when setting plus exposure compensation. That will lighten dark areas but the light-toned areas may then be excessively bright. Remember: it's quite easy to lighten moderately dark areas with imaging software. But it's impossible to fully "fix" excessively bright areas if the camera's sensor did not record any texture or detail. So use exposure compensation wisely and in moderation. You may need to take an important photo two or three times - with a different amount of compensation for each shot - to get an image with the optimal overall exposure.
NOTE: Peter Burian teaches two excellent online photography workshops - Mastering the Digital Camera and Photography - at BetterPhoto online digital photography school. Also see Peter's Pro BetterPholio website: www.peterkburian.com.