In a recent post about digital infrared photography I talked about the importance of obtaining the proper exposure, but as significant as it is for IR—and it may be more noteworthy—it’s crucial for every photograph that you make. That’s because proper exposure is a perfect storm of shutter speed, aperture, and ISO 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 to use the histogram function. The histogram is a graph showing a photograph’s range of light values in 256 steps. Zero is on the left size and represents pure black; 255 is on the far right-hand side and represents pure white or the famous “Polar Bear in a Snowstorm.” In the middle are the mid-range values representing grays, browns, and greens.
On a typical photograph, all of the 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 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.
When viewing a histogram, your camera’s LCD screen may superimpose the graph on top of the images it represents. In the histogram for my photograph of Mary above, 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, which is for the portrait of Pamela at left.
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.
Joe is co-author, along with Barry Staver, of the out-of-print “Better Available Light Digital Photography” that can be found on Amazon at bargain prices for used copies and new copies that vary from collector’s prices to affordable. Get the limited number affordable new ones while you can.