3 minute read
How to Read a Histogram
Don’t risk under- or overexposing your photos. Learn to make use of handy histograms to ensure all of your images are bright and accurately exposed
One of the joys of digital photography is that, once you’ve made an image, you can instantly tell if the exposure is right by looking at the image on the back of the camera. However, only relying on this visual impression of a photo’s brightness may not always give you the right answer. The apparent brightness of a camera’s touchscreen is heavily influenced by the surrounding conditions. For example, images viewed in the dark often look brighter than they actually are, meaning you may have underexposed the image but don’t realize it yet. Conversely, in bright conditions, touchscreen images can look too dark, so you won’t always recognize overexposure.
It’s much better to judge a photograph’s brightness by looking at its histogram: a diagram that shows the distribution of brightness levels throughout the image. You can see a histogram on the back of your camera after an image has been created, in your favorite image-editing software, and (perhaps most usefully) in the viewfinder of your camera as you’re composing a picture. This is a great way to work and can give you an early indication if an exposure is truly looking too dark or too bright.
To turn on the camera’s histogram: press MENU OK and scroll down to the SET UP menu (the wrench icon). Choose SCREEN SET-UP > DISPLAY CUSTOM SETTING and tick the HISTOGRAM option.
Although histograms can look complicated at first sight, reading one is actually very simple. Brightness is plotted on the x-axis (horizontal), with absolute black on the left and absolute white on the right. Midtones lie in between. On the y-axis (vertical) is the relative number of pixels with a specific brightness value.
For a well-exposed, average scene containing some blacks, some whites, and an even range of tones between, you’d expect to see a histogram that reflects this: most of the peaks in the middle of the graph with some black and some white peaks at the ends. If the information in the histogram is skewed towards the right-hand side (white), then the scene is overexposed (aperture too wide, shutter speed too long, or ISO too high). Conversely, if it’s skewed to the left end (black), then the picture is likely underexposed (aperture too small, shutter speed too short, or ISO too low).
However, the above is only true for pictures that contain an average set of brightness tones in the first place. If you were making a picture of a white polar bear in a snowy landscape, you would expect the histogram to be bunched up to the right, since the scene does indeed contain predominantly white tones. Likewise, if you were photographing a black cat in a coal cellar, you’d expect a histogram skewed to the left.
The take-home message is this: when used in combination with some common sense, histograms are a powerful way of measuring the accuracy of an exposure and can save you the heartache of getting home to realize your favorite shot of the day is a badly exposed one.