What Histograms Tell Us
The luminance histogram is a good indicator of underexposure and clipped highlights, and it's really as a guide to these two issues that histograms are most useful.
Clipped highlights are indicated when the histogram is cut off on the right. A histogram of a photo that doesn't contain clipped highlights dips down to meet the x axis somewhere on the right side of the graph. You can see this principle in practice in the histograms shown earlier in this article.
Generally, clipped highlights are undesirable because they indicate areas in the photo that lack detail, but there are exceptions. Some images contain specular highlightssmall hot spots on shiny surfaces such as metal. Specular highlights are usually unavoidable, but as long as they're not too large, they don't harm the image.
Even when you're using the Raw format, the histogram is generated from a JPEG file created from the Raw file. Raw files contain more highlight detail than JPEG files do, which means that the highlights are not necessarily clipped, even if the histogram (or the highlight alert) indicates that they are (see Figures 7 and 8). In practice, you have more leeway over exposure with Raw files than you do with JPEG files.
You can use the Highlight Alert feature (sometimes called "blinkies") on your camera to indicate which tones have been clipped, as shown in Figure 9. Check your camera's instruction manual for details.
Figure 7 Looking at this photo on the camera's LCD screen doesn't tell you whether the exposure is accurate.
Figure 8 The histogram is cut off on the right side, indicating that the image contains some clipped highlights.
Figure 9 The highlight alert reveals the clipped highlights (indicated in black). However, I wouldn't say that this image is overexposed, as the clipping occurs in the brightest part of the sky. In situations like this, it's best to use the Raw format, as it holds more highlight detail than JPEG files do. Normally you can retrieve detail that the camera indicates is clipped in your Raw processor.
If the histogram leans to the left, with a gap on the right side, it probably indicates underexposure (see Figures 10 and 11). The only exception is if the image contains a lot of dark tones and no bright highlightsin this case, such a histogram would be appropriate. If you see a histogram like this, and your subject is static, it's easy enough to take another photo with increased exposure just in case.
Figure 10 It's not easy to tell whether the exposure is correct in this photo just by looking at the image on the camera's LCD screen. To check the exposure, you need to look at the luminance histogram.
Figure 11 The histogram here indicates that the photo is underexposed. It leans to the left, with a large gap on the right side. The exposure needs to be increased by at least a stop.