If you like to play around with your digital camera’s buttons, you may have come across a graph of jagged lines while reviewing the photos on your camera’s memory card. What you’ve seen is a histogram – probably one of the most misunderstood – and most inherently useful – features of your digital camera.
Nearly every digital camera, from sophisticated SLRs to simple point and shoots, has a histogram feature. In addition, most photo editing software programs employ histograms in some fashion when adjusting the lighting of a photograph. So what is a histogram and what do all those jagged lines mean?
What a histogram is
A histogram is simply a graph that displays the range of tones within an image. The far left side of the histogram represents total black; the far right represents pure white. In between are 254 intermediate tones from black to white for a total of 256. Right in the middle is 18 percent neutral gray. In addition, the height of the bars tells you just how much of the photo contains that particular shade of light.
What a histogram tells you
First, a little background on exposure.
A light meter, like the one in your camera, will measure the light in a scene and choose an exposure that will render a standard 18 percent gray reference card as a mid tone gray. It represents a compromise of the many possible exposures that could be used to capture a particular scene.
The histogram of an image gives you, the photographer the ability to quickly evaluate what the camera did and whether you need to make any adjustments before trying again. For the most part, you are looking for a histogram that gives a fairly even distribution of tones from black to white. Except when you’re not. A little more about that in a minute.
First, let’s look at some images and their histograms. A few examples will give you an idea of how to interpret a histogram.
The photo above, which was taken during the “golden hour” at the end of the day, has a full range of tones from low to high. It may be a little heavy at the dark end, but exposing to open up the shadows may have overexposed the sky and lost the rainbow.
You can see the histogram of this shot, taken in the shady woods on a foggy morning, bunched toward the left side, indicating the predominant tones are mid-to-dark. It depends on what you want to accomplish with your image as to whether or not to adjust your exposure to lighten the tones and push the histogram to the right.
With some practice, you should be able to tell, at a glance, the quality of your exposure. Again, the rule of thumb is to shoot for an even distribution of tones across the histogram.
Exceptions to the rule
If you’re shooting in subdued lighting or at night, your graph lines will be bunched toward the left side of your histogram. This is simply because the predominant tones in your photo will be dark tones. Trying to correct your exposure to spread the tones across the entire range of the histogram will result in a greatly overexposed photo.
This histogram for a nighttime shot shows the tones really jammed to the left. There is really no need to adjust the exposure due to the subject matter and ambiance.
At the opposite end of the scale, a photo where the backdrop is predominantly bright or white will produce a histogram that is bunched up toward the right side of the graph. Again, this is because of the predominant tones in the photo. Correcting will result in an underexposed image.
What a histogram is not
A histogram is, unfortunately, not a magic cure all that guarantees all of your images from here on out will be perfectly exposed. There is really no one ideal curve to aim for when making an image.
Histograms will change depending on the lighting conditions and the type of photograph you are trying to create. Nighttime images will show the histogram bunched toward the left side of the graph, while high-key photos will show tones congregated to the right of the graph.
A histogram is merely a tool to give you information about the range of tones in a particular image. What you do with that information, whether to make exposure adjustments and try again, or keep your image as is, depends on the look that you are trying to capture.