As a photographer, you’ve probably heard, “Watch your histogram,” many times. Your response may have been, “Watch for what?” Like many artists, the technical explanation may be of no interest to you. You only want to know how a histogram helps you produce better images. So let’s start this discussion with what a histogram is in its simplest form: a small graphical display that typically looks something like this.
Histograms are found in digital darkroom software, on the LCD display of your camera, and sometimes in the camera’s viewfinder. (Note: if there is no histogram displayed on your camera, consult your camera user’s manual regarding how to turn it on.) In all those locations, it will resemble the example above; although, the light gray mound within the graph may be shaped differently.
The goal of most photographers is to correctly expose their images, and the histogram is a cheat sheet telling you how close you are to that goal. So how much technical knowledge do you actually need to effectively use histograms? Surprisingly, very little.
Cameras capture light reflected from the objects in the scene. That reflected light consists of two basic elements, color and brightness. An easy way to think of brightness is if you stripped the color from a scene leaving only black, gray and white tones, where would those tones fall on a gradient scale from solid black to solid white. A histogram is nothing more than a map of where the light tones in your image fall on the gradient scale shown below.
To understand how histograms work, let’s use an illustration. Imagine you could capture each particle of light in a scene and convert it to into a bead whose color matched the particle’s tone on the gradient scale above. You would have a pile of beads ranging in color from absolute black to absolute white with the majority being different shades of gray. In this pile of beads, some tones may have only one bead and others may have many beads depending on how many particles of light had that exact tone. You want to organize the beads to see how the light in the scene is distributed, so you take a poster board and mark one side “black” and the opposite side “white.” You then begin placing beads in a row across the bottom of the poster board in order of their tone from solid black to solid white. When you come to a bead that is the same tone as one already in the row, you place it above its match. When you are done, you will have created columns of beads all the same tone.
This is what a histogram does. It breaks the light in an image into its smallest parts and then graphs that light according to tone from the darkest to the lightest. Where there is a lot of one particular tone, the graph stretches up just as your columns were higher where you had many beads of the same tone. Where there is little of a particular tone, the graph stays low. Here are two other histograms that illustrate this point.
So which of the histograms shown above is a “correct” exposure? You may be surprised, but the answer is all of them. Despite being drastically different, they all have one key factor in common. They provide you with all the available detail and texture of the light in the image. It is then up to your artistic vision to determine how to present the final image. Do you want to have a dark, brooding feel to your image? Then you may want a histogram that has most of the graph shape to the left. Conversely, you may want to emphasize the light areas and want the graph shape to lean to the right? These are questions you must answer for yourself.
Then what is an “incorrect” exposure, and how does a histogram tell you that? In the simplest of terms, incorrect exposures fail to capture all the available detail and texture in a scene. This occurs when the photographer’s camera settings (ISO, aperture and shutter speed) do not capture some portion of the light reflected by the scene. In such cases, non-black areas are captured as solid black, underexposed, or non-white areas are captured as solid white, overexposed. Regardless of your artistic vision, those areas can never be anything but pure black or pure white. Your vision is limited by the loss of the detail and texture in those too dark or too light areas, and you have lost the ability to use those areas in a creative way.
How does the histogram solve this problem for you? How do you tell if your settings will result in overexposure or underexposure? Check the left and right “walls” of the histogram graph. As you learned in the illustration of the beads, the left side of the graph is pure black and the right is pure white. Ideally, the shape in the histogram graph should just touch the bottom right and left corners of the graph. If you see the shape climbing up the left or right wall of the graph, you have an incorrect exposure. The higher the shape climbs either wall, the more detail and texture you are losing. For example, the histogram below indicates an image that is substantially pure white with little texture or detail in the brightest parts. Before pushing the shutter in this scenario, you would want to adjust your ISO, aperture or shutter speed to bring the shape to the left until it only touches the bottom right corner of the graph.
There are, of course, exceptions to this rule. If you are photographing a solid black object, a histogram climbing the left side of the graph is acceptable. If you are photographing the sun, the histogram will climb the right wall. In those cases, you haven’t lost detail and texture, because there wasn’t any in the surface of the black object or in the bright disk of the sun.
In some cases, you will see a histogram that doesn’t come near to the bottom corners of the graph and instead is bunched in the middle of the graph. This tells you there are only gray tones in the image, no black or white. The image has little contrast between the darkest and lightest parts. It is not an incorrect exposure, but it is an exposure you may want to tweak to improve the image’s impact. You can either adjust the camera settings or use digital darkroom software to increase the contrast, if your artistic vision calls for it.
In summary, a histogram shows you how the light in your image is distributed across the spectrum from pure black to pure white. The key to getting usable exposures is to make sure the shape in the histogram only just touches the two bottom corners of the graph. How the shape is distributed across the graph is up to you and your artistic vision. Move the shape right to favor lighter tones. Move the shape left to favor darker tones. Stay off the walls of the graph, and you will have the texture and detail available to execute your vision in the digital darkroom.
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