At first glance, the highlight overexposure alert might be considered by most new photographers to be indicative of the fact that they’re doing something wrong—that their exposure is inaccurate. But in fact, in much the same way devotees of Ansel Adams and his Zone System utilize a spot meter to determine the actual exposures based on the tones of specific elements in a scene, the modern DSLR user can employee blinking highlight alerts to ensure every other tone in a scene has fallen perfectly into place. After all, just because a highlight is blown out doesn’t mean it’s wrong. Some highlights should be blown out. As long as you limit the blinkies to the areas that should be blown out, you’ve created a perfect exposure.
First, though, you’ve got to be able to turn those blinking highlights on. Every camera is different, but here are a couple of generalities to point you in the right direction. In new Nikon DSLRs, look for the Highlights checkbox in the Playback display options menu. For Canon DSLRs, you’ll find the highlight alert option in the Playback menu as well. With it enabled, when you take a picture and it provides a preview for a few seconds after capture, any areas of blown out highlights will slowly blink black and white. I use this all the time when I’m trying to ensure a white background is pure white. Once I start to see it blinking black in the preview (represented here as red areas indicated by Lightroom’s overexposure alter), I know my exposure has arrived.
Beyond white backgrounds, though, are there really other real-world applications for the blinking highlight alert as it pertains to setting the correct exposure? Sure there are. Let’s say you’re photographing a portrait of a person in a white shirt. Should their shirt be blown out, all bright pure white? Probably not. Is it okay if just a hint of their white color blinks black and white, indicating the blown out detail? Yes, it probably is. And in fact, with a portrait subject, this is a pretty accurate method of establishing the correct exposure. No blinkies at all on that white shirt and you’re probably underexposed. Anything beyond the collar blinking, and you’re most likely overexposed.
What about with landscape photos? Should there be blinking highlights in landscapes? Absolutely. An entire sky full of clouds probably shouldn’t blink an alert, but the core of a cloud just might. Better still, a white subject like a waterfall shimmering in the sunlight very well might go pure white in the perfect exposure. But if too much stuff is blinking, your exposure is probably overcooked.
A blinking highlight represents clipping at the far right edge of the histogram. If only a select few pure white tones are clipping, you know you’ve employed the old "expose to the right" maxim which advocates for creating detail-rich images that lean toward overexposure rather than under. No matter your stance on the ETTR technique, blinking highlights will still work for you. If you don’t want your photo exposed to the right, you darn sure better not see any blinking highlights.
Sure, you can always check the histogram to see exactly where your tonal values fall, but the blinking highlight provides an even faster shortcut that’s just as useful since it gives you a baseline for your exposure. If only the select items that should be blown out are blown out, then you know you’ve got an exposure in which all other tones are going to fall nicely in line.
Oh, and one more thing. If you shoot RAW image files, you can be a little more off with this method and still have enough latitude to rescue your image during the raw conversion process. But by shooting RAW and capturing the ideal exposure where you’ve pushed every pixel as far to the right of the histogram as possible without losing any necessary highlight detail, you’ve got an image file that’s maximized the useful information and can be made to really sing.