The ultimate tool for determining proper exposure
December 10, 2008
Correct exposure is critical, maybe even more so for digital capture than film, especially color negative film. That because the latitude (the ability to over or underexpose an image) is greatest with color negative film than for any other capture media.
Slide film has the least amount of latitude, especially on the overexposure side. Digital camera imaging sensors respond more like a hybrid of the two different kinds of color film: Overexposure wipes out image data but underexposure has more latitude, almost as much as film. The downside of digital underexposure is the inevitable creation of digital noise, especially in shadow areas that appears as "grain."
You are the ultimate arbiter of what is "correct" exposure and one way to objectively evaluate a particular image's exposure is by using your camera's histogram function. This capability is unique to digital photography and is a feature of all digital SLRs and even some point-and-shoot cameras. See your camera's User's Guide for information how to display a histogram on the LCD preview screen. Some cameras, those with Live View modes, even let you display it in real time before capture.
The image's histogram appears on the LCD screen as a graph displaying the photograph's range of brightness from highlight to shadow and shows light values in 256 steps. Zero is on the left size of the graph and represents pure black; 255 is on the far right-hand side and represents pure white or the famous shot of a "Polar Bear in a Snowstorm." In the middle are the mid-range values representing grays, as well as (you know) browns, and greens.
When viewing a histogram on an Olympus E3 and some other digital SLRs, the LCD screen superimposes the graph on top of the images it represents. In this histogram, there is a slight gap on the right-hand side indicating slight overexposure, which can be seen on the subject's forehead under the chart. ©2008 Joe Farace
You can also view a histogram of that same image file on computer software such as Adobe Photoshop. As you can see, the histogram is similar but different. The overexposure gap is gone and in fact that's what is shown on my monitor proving that the camera's LCD can only be used as guide for correct exposure.
On a typical photograph, all of a photograph's tones will be captured when the graph rises from the bottom left corner then descends towards the bottom right producing what statisticians call a "bell-shaped" curve because it's, well, shaped like a bell. If the histogram's curves starts out too far in from either side or the slope appears cut off, then data is missing or the image's contrast range may exceed the camera's capabilities to capture what you see with your eyes. While the classic histogram features a bell-shaped (aka Gaussian) curve, not every photograph fits this type of distribution. Dramatic images with lots of light or dark tones areas often have lopsided histograms but that doesn't mean they aren't good photographs.
Joe Farace is co-author of “Better Digital Available Light Photography” along with Pulitzer Prize winning photographer Barry Staver. It is published by Focal Press and is available in all the best bookstores, including Barnes & Noble and Amazon.com.