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 and it means that at the exposure the image was taken, our image will have blocked shadows (no detail) and/or blown-out highlights (again, no detail). More importantly, information that has been blocked or clipped is lost. You didn't capture it or, more accurately, you rendered or assigned some tonalities to either detail-less black or white - gone! No, Photoshop will not bring it back (at the very best, not well).
When we used 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. So, here’s a simple way of keeping your image from having blown-out highlights: 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, to complicate the subject further, because of the linear characteristics of the sensor in our cameras, most of the detail or information in our image is contained on the right side of the histogram. So, not only do we want to avoid clipping but, to capture the maximum amount of information, we want our histogram to terminate as close to the right edge as possible. Again, if your camera shows individual RGB channels, be sure none of the channels clip.
So, summing it up - keep the histogram to the right (in digiphoto-talk, expose to the right) and avoid clipping. You move your histogram in one of two ways. If you’re shooting in a programmed mode (aperture or shutter priority or “program”), use exposure compensation to either increase or decrease exposure as needed then re-shoot. If you’re in manual mode, increase or decrease shutter speed. I usually shoot in aperture priority mode and adjust exposure compensation accordingly. The result will be an image with no blown out highlights and a high signal to noise ratio. In short, a optimally exposed image.
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.
Oh - what about the other ten percent or so of your shots? Shooting close-ups with all neutral tonalities; no bright spots? How about spot metering and bracketing plus or minus 1/2 stop and picking the one you like best? Shooting a fast moving stream? The most turbulent spots are detail-less white. You should expect a little clipping. As I said, it's still photography.
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, adjust if necessary but, most of all, have fun.