Now, what happens when the brightness range exceeds the dynamic range of the sensor? Figure 11-7 shows a histogram for such a scene. The histogram shows significant concentrations at and near both edges, which tells you that you cannot capture the entire brightness range of the scene in a single capture. Here, it would seem that you have a limited set of unpleasant choices: #1) Expose for good shadow detail by giving sufficient exposure to move the histogram away from the left edge and thereby cause additional higher values to be clipped; #2) Expose for good highlight detail by reducing exposure enough to move the histogram away from the right edge and thereby cause additional lower values to be rendered pure black; or #3) Expose for the midtones and live with empty shadows and blocked highlights.
Figure 11-6. Histogram for capture underexposed by 2 f-stops
Figure 11-7. Histogram where the brightness of the scene exceeds the dynamic range of the camera
Fortunately, these aren’t the only choices. In the section below under the heading High Dynamic Range Images—The Extended Zone System for Digital Photography, we’ll see how to overcome those limitations and extend the range of the sensor by making multiple exposures at various settings and then, in the computer, integrating those several exposures into a single image. Not surprisingly, the decision of how to proceed depends on your desired interpretation of the scene. While the histogram cannot make the artistic decision, it provides you with the information with which to set the exposure to favor the shadows, the highlights, the midtones, or to make a series of captures that will record the entire brightness range of the scene.
Now that you understand the histogram, it’s time to get a feel for how the histogram on your camera’s display responds to changes in exposure, and in particular, increases in exposure as you approach overexposure. This time, let’s capture a sequence of exposures with the camera on a tripod or placed on a tabletop. Choose as your subject an evenly lit, uniform surface such as an evenly illuminated interior wall. Place the camera a foot or so from the wall, turn off autofocus, use maximum focal length if you are using a zoom lens, and focus at infinity. The idea is to have as uniformly lit a target as possible. Take a series of captures from grossly underexposed (histogram bunched at the left edge of the display) to grossly overexposed (histogram bunched at the right edge of the display) using one-half or one-third stop increments.
Scroll through the captures with the histogram display visible and you will quickly get a feel for your camera’s dynamic range, and the amount by which the right edge of the histogram moves with each increase in exposure. The dynamic range can be estimated by counting the number of exposures (how many one-third or one-half stop exposures) it takes to have the majority of pixels move from the left edge to the right edge. Note the sensitivity of the histogram to increases in exposure as the right edge of the histogram approaches clipping. Your understanding of the relationship between changes in exposure and the response of the histogram will serve you well in quickly setting exposures in the field. So far, we have been working with the luminosity histogram, which displays the perceived brightness of the scene with color information being weighted to take into account human color perception. The luminosity histogram is usually sufficient if the final output is to be black-and-white, generally referred to as “grayscale”. For more precise control, which is especially useful for high fidelity color output, you can view histograms for each of the red, green, and blue channels.
Most cameras allow the user to display the histogram for the three channels in one of a couple of formats: #1) In a three-channel composite similar to the one shown in Figure 11-8 or #2) in three distinct color channels similar to the one shown in Figure 11-9.
Both display formats convey the same information; a channel-by-channel depiction of the distribution of brightness in the capture. If your camera gives you a choice, pick the display that you find easier to read. At the risk of overselling, if you expect your final image to be rendered in color, it is best to view the three-color histograms rather than the luminosity histogram, because the overexposure—clipping—of any one of the color channels may result in a distortion of highlight colors. Even if the final image is to be rendered in black-and-white, it is helpful to capture all three channels of information for ease in making selections in your editing software and for maximum flexibility in converting the color capture into a grayscale image. Remember, every digital RAW capture contains the information from which a full color rendering can be produced, even if you intend the final product to be a black-and-white print.
Some cameras have a dedicated histogram button; other cameras permit the user to assign the histogram display to a button. If your camera has, or permits, the assignment of the histogram display to a designated button, you will save time and improve your images by making the assignment and using the histogram button to quickly review your exposures. As an additional aid, many cameras can be set to cause the display to indicate areas of underexposure and clipping by highlighting or flashing such areas in a contrasting color. The flashing display is particularly helpful when used with the histogram. If, for example, the histogram shows significant bunching at the right edge but the flashing display shows that the overexposed area is nothing but blank overcast sky, there is no advantage in reducing exposure. It’s best to ignore the apparent overexposure and see if the shadows need additional exposure. If, on the other hand, the flashing area contains important detail, a reduction in exposure or multiple captures (discussed below) may be the best way to proceed.
Figure 11-8. Three-channel composite histogram
Figure 11-9. Individual channel histograms
So, let’s prescribe a sensible sequence that will usually assure optimum exposure (this assumes the scene to have a brightness range within the sensor’s dynamic range):
Start with a test capture at the exposure suggested by the camera.
Review the histogram.
If the histogram is now biased toward the right, you have your exposure and can make your capture. If necessary, adjust the exposure based on the histogram (i.e., give additional exposure if there is room at the right edge of the histogram, give less if the histogram indicates highlight blocking).
Take a new capture.
Recheck the histogram to see if your image is underexposed on the left or clipped on the right, and also (if your camera has this feature) view the image display to check which subthreshold areas or overexposed areas are flashing.
Readjust if necessary.
If your captures consistently require you to apply highlight recovery in your RAW converter, you are probably overexposing; set your camera to reduce your exposures by one-third of a stop and try a few more exposures, using the same procedure outlined above. Repeat the process until you get consistently well exposed captures. Depending on your camera’s controls, it will probably be most convenient to remain in automatic exposure mode and use the overexposure/underexposure setting to bias the exposure for your current lighting and subject environment. Alternately, you can increase or decrease the exposure in manual mode. As long as the ambient lighting and scene conditions remain substantially constant, the same exposure bias should apply. Just be sure to view the histogram from time-to-time to be sure the lighting has not changed significantly. The histogram serves very much like a light meter in this respect, so use it that way.