Histograms and Exposing to the Right
When digital photography made its debut, probably the most misunderstood and misused – or unused – feature was the histogram - in my opinion, the most important feature of the playback display on that LCD. All SLR’s and most point and shoot digital cameras have a histogram display option. Some allow you to see each of the RGB channels.
Space doesn’t permit a detailed discussion of the subject but here's a quick synopsis of how to use this great tool for 85% to 90% of your shots:
The histogram is simply a curve that maps the distribution of tonalities in our image from detail-less black on the left to detail-less white on the right. The height of the histogram shows the relative quantity of specific tones with respect to one-another. There is no "standard" or "perfect" or "reference" histogram. It's just information. When the histogram climbs up the left or right side, we call that "blocking" or “clipping” respectively.
If underexposed (the histogram is far short of the right edge) your blocked tonalities can be recovered but you'll have more noise in your image. No, Photoshop, Lightroom, Aperture will not cure the problems (at the very best, not well). So, move the histogram to the right to maximize the signal to noise ratio in your image.
When we used transparency film, we avoided blowing out highlights and let the rest of our tonalities fall where they belonged. Dark was dark, light was light, neutral was neutral, etc. The same is true with digital capture - it's still photography after all. If your histogram shows crowding on the right side, you've probably blown away a lot of tonalities and rendered them detailess white. And, a lot of those tonalities are not recoverable - gone! So, here’s a simple way of keeping your image from having blown-out highlights: if you're shooting in JPEG mode, make sure your histogram doesn’t clip on the right. If your camera has the capability to show each of the RGB channels, make sure none of the individual channels clip. Let the shadows fall where they will.
Now, if you're shooting in RAW like most of us do, you have a little headroom. That means your histogram is not the "last word". RAW capture gives us about a 1/3 to 1/2 stop wiggle room so we can expose beyond the right side of our histogram. So, what is one to do? The answer lies in the "blinkies" - those irritating flashes of black we see in our LCD when we've overexposed. How many blinkies constitute 1/3 to 1/2 stop? That's an answer you'll have to determine for yourself. With practice, you'll be able to interpret both the histogram and the blinkies for a variety of image types and situations. I promise!
When you bring your image into your processing software, use the appropriate tools (exposure, recovery, fill light, etc) to adjust the image to your liking. By the way, using exposure alone will almost always not yield the best results. Use a combination of all the tools.
The histogram is a terrific tool. Used properly and intelligently, it’s taken a lot of guesswork out of exposure giving us more time to concentrate on the quality of our composition. So, shoot, peek at your histogram, watch the blinkies, adjust if necessary but, most of all, have fun.