What’s the Correct Exposure?
The proper exposure is a perfect storm of shutter speed, aperture, and ISO setting that produces a pleasing result. Pleasing to whom, you might ask? You are the ultimate arbiter of what is “correct” but one way to objectively evaluate a particular image’s exposure is by using the camera’s histogram function. This capability is unique to digital photography and is a feature found of all digital SLRs and many point-and-shoot cameras.
The histogram appears as a graph showing a given photograph’s range of brightness showing 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, browns, and greens.
When viewing a histogram on some digital SLRs, the LCD screen may superimpose the graph on top of the images it represents. In the above histogram, there is a small gap on the right-hand side indicating slight overexposure, which can be seen on the subject’s forehead. You can also view a histogram using software such as Adobe Photoshop shown below. As you can see, the histogram is similar but different.
On a typical photograph, all of an image’s tones are captured when the graph rises from the bottom left corner, reaches a peak in the middle, then descends towards the bottom right creating 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 the data is missing and the image’s contrast range may exceed the camera’s capabilities or simply the exposure selected for that specific image.
Tip: 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 correctly exposed for that subject matter.
Do You Have The Blinkies? Another useful exposure tool found on some digital cameras is the highlight alert feature that my pal Barry Staver calls “blinkies.” When the highlight alert option is selected, the overexposed areas of your image is outlined with the “blinkies” or the entire affected areas blink making it easy to spot them. Some digital SLRs also offer a Shadow Alert but I find both of these features to bedistracting and don’t use them. Just because you’ve got “blinkies,” doesn’t mean the image is unacceptable. It’s up to you, the photographer, to be in control of the exposure.
