Using Histograms to Diagnose Exposure
Problems
There are some exposure problems that Levels can’t fix. Some scans (or digital cam-
era images) have important tonal detail missing. Shadow detail might be missing
(the shadows are pure black), highlight detail might be missing (the bright areas are
pure white), or both. Just as film can be underexposed and overexposed, digital
camera images or scans can suffer from the same problems.
Often you can tell whether an image is badly exposed by simply looking at the
monitor, but its histogram will help you make a more confident diagnosis (see
Figure 14.7). Digital images that are over- or underexposed are said to show clipped
histograms. The term clippedsuggests that the pixels necessary for a correct exposure
have been clipped off the ends of the histogram by bad exposure.
The highlighted (bright) areas are clipped in the histogram. Notice that there are no
pixels at the left side of the graph. This often indicates overexposure. Adjusting the
sliders will not help because no image detail was captured in the highlights.
In most well-exposed and well-scanned images, some pixels are found at both ends
of the graph. In a clipped scan, there will be a spike of pixels. If a scan is clipped,
there is no way for editing software to fix it without hand-painting details on the
image or copying them from another image. Be careful not to confuse a clipped
image with one that intentionally contains large areas of pure black or pure white.
The histograms of both types of image will look the same.210 ABSOLUTE BEGINNER’S GUIDE TODIGITAL PHOTOGRAPHY
FIGURE 14.6
Move the sliders
closer together
to increase
contrast.