Today’s Post by Joe Farace
Whenever I’m teaching a workshop or seminar, no matter what the ostensible subject of the class may be, the topic of exposure almost always comes up. Ultimately my answer comes down to latitude: Exposure latitude is a measurement of how much an image created with either film or a digital imaging sensor can be overexposed or underexposed and still produce an acceptable result.
These days not every photographer started shooting photographs with film cameras but any understanding of how to obtain proper exposure is helped with just a little bit of information about how film capture worked and still works today.
How I Made This Shot: This image was captured at a event during SEMA 2008, when somebody thought having these cars surrounded by attractive models with a fog machine cranked up to eleven was a good Idea. Then camera used was a Canon EOS Rebel Xs that I was testing at the time for the former print edition of Shutterbug magazine. The lens used was the EF-S18-55mm f/3.5-5.6 IS kit lens at 18mm. Exposure was 1/80 sec at f/5.6 and ISO 800 and was slightly underexposed, creating a really lopsided histogram (see below.) The noise was removed by using the Dfine Photoshop-compatible plug in then coveted to Monochrome with Silver Efex Pro to create a nice (to me anyway) Film Noir mood.
Slide film has the least amount of latitude of the available film stock, especially on the overexposure side, while color negative aka print film has more latitude for underexposure, sometimes amazingly so. The imaging sensor in your DSLR or mirrorless camera seems to respond to exposure much like a hybrid of these two different kinds of color film: Any overexposure will wipe out the image data but underexposure has more latitude, almost as much so as film. The downside of underexposure is the inevitable creation of noise, especially in the shadow areas.
So what is the correct exposure for a a particular image? I believe that only you are the ultimate arbiter of what is “correct” and one way to evaluate a particular image’s exposure is by using your camera’s histogram function. Check your camera’s User’s Guide for information how to display a histogram on the LCD preview screen. Some cameras even let you display it in real time before capture.
Histograms appears on your camera’s LCD screen as a graph displaying the photograph’s range of brightness from highlight to shadow with light 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 browns and greens.
On an average photograph, all of a image’s tones will be captured and the graph will rise from the bottom left corner then descends towards the bottom right producing what statisticians call a bell-shaped (aka Gaussian) curve because it’s shaped like a bell. If the histogram’s curves starts out too far in from either side or the slope appears cut off, then some 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 curve, not every photograph fits this type of distribution. Dramatic images with lots of light or dark tones areas often have really lopsided histograms but that doesn’t mean they aren’t good photographs.
Along with photographer Barry Staver, Joe is co-author of Better Available Light Digital Photography that’s out-of-print but new copies are available for $21.87 or used copies for giveaway prices—around five bucks—from Amazon, as I write this. The Kindle price is really high for some reason. (Not Barry or me.)