You’ve probably heard many times as a digital photographer, “Watch your histogram!” Your response may be, “Watch for what?”…. or maybe even “What is a histogram?” If you are like many artists, the technical explanation is of no interest. What you want to know is how a histogram helps you produce better images. Let’s start with what a histogram is. In its simplest form, it is a small graph 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. In all those locations, it resembles the example above; although, the light gray mound within the graph may be shaped differently. (Note: if there is no histogram displayed on your camera, consult your camera user’s manual regarding how to turn it on.) The goal in photography is to produce an image that maximizes your ability to express your artistic vision, sometimes referred to as a “correct” exposure. A histogram tells you how close you are to that goal. How much technical knowledge do you actually need to effectively use a histogram? Surprisingly, very little.
Cameras capture light reflected from objects in a scene. That reflected light consists of two basic elements, color and brightness. Here’s an easy way to think of brightness: 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 the darkest (solid black) to the lightest (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 histograms, let’s use an illustration. Imagine you could capture each particle of light in a scene and convert it into a bead whose color is the light particle’s tone on the gradient scale above. You would have a pile of beads ranging in color from solid black to solid white. The majority being different shades of gray. Some tones may have only one bead in the pile and others many beads depending on how many particles of light had that exact tone. To be useful, the pile needs to tell you something about the light in the scene and how it is distributed, so you take a poster board and mark one side “black” and the opposite side “white.” Then you begin placing beads in a row across the bottom of the poster board in their tone order from solid black to solid white. When you come to a bead in the pile that is the same tone as one you’ve already placed in a row, you place it above its match in the next row. When you are done, you will have created columns of beads all the same tone.
Where there is a lot of one particular tone, the beads stretch up toward the top of the poster board. Where there is little of a particular tone, the bead column stays low. This is exactly what a histogram does. It breaks the light in an image into its smallest parts and graphs that light according to tone from the darkest on the left to the lightest on the right. The height of any part of the graph tells you how much of a particular tone exists in the scene. Here are two histograms that illustrate this point.
Keeping in mind how a histogram distributes the light in a scene, which of the 3 histograms below is the “correct” exposure? You may be surprised, but the answer is any of the 3 depending on the scene you are photographing and your artistic intent. If your scene is a landscape with even, consistent lighting and you want to capture it as is, the first histogram is correct. However, if your scene is a dark room with a faintly lit vase in the center and you want to capture it with a moody feel, the middle histogram is correct. The last of the three would be correct if your scene is a bright window in a moderately lit room, and you want to emphasize the brightness of the window.
So if all three above can be correct, you may be thinking, just what is an “incorrect” exposure? Cameras, as described above, capture light reflected from objects in a scene. If your camera as set is not able to capture some portion of the available light in the scene, the exposure is incorrect. In such cases, areas that aren’t black are captured as solid black or underexposed and areas that aren’t white as solid white or overexposed. Those areas have no visible detail or texture in the resulting image though they did in the scene. Once incorrectly captured, you can do nothing to modify those areas to fit your artistic vision. Below is the same image underexposed, correctly exposed and overexposed. The loss of detail and texture in the overexposed and underexposed versions is evident.
How does the histogram point out this problem for you? How do you tell if your settings will result in overexposure or underexposure? 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 of the histogram graph should just touch the bottom right and left corners of the graph indicating detail has been captured in all tones except a small portion in the solid black and solid white spots. If you see the graph shape climbing up the left or right walls 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 solid 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, rare 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.
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 touches the two bottom corners of the graph without climbing the sides 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.