Interpreting the histogram

One of the most difficult photo problems to correct in a photo-editing program is known as blown highlights in some circles and clipped highlights in others. In plain English, both terms mean that highlights — the brightest areas of the image — are so overexposed that areas that should include a variety of light shades are instead totally white. For example, in a cloud image, pixels that should be light to very light gray become white due to overexposure, resulting in a loss of detail in those clouds.

In Shooting Information display mode, areas that fall into this category blink in the image thumbnail. This warning is a great feature because simply viewing the image isn't always a reliable way to gauge exposure; the relative brightness of the monitor and the ambient light in which you view it affect the appearance of the image onscreen. Again, though, blinking highlights doesn't necessarily indicate that your exposure is off. If you have a dark subject against a very bright background, for example, you may not be able to properly expose the subject without creating at least some blown highlights in the background.

For a detailed analysis of the image exposure, check the histogram, the graph that appears to the right of the image thumbnail in Shooting Info display mode. By default, the histogram is set to Brightness Display mode and appears similar to what you see in Figure 4-11. In this mode, the histogram indicates the distributions of shadows, highlights, and midtones (areas of medium brightness) in your image. Photographers use the term tonal range to describe this aspect of their pictures.

Shadows Highlights -►

Figure 4-11: The standard histogram indicates the tonal range of your image.

The horizontal axis of the graph represents the possible picture brightness values, from the darkest shadows on the left to the brightest highlights on the right. And the vertical axis shows you how many pixels fall at a particular brightness value. A spike indicates a heavy concentration of pixels.

For example, in Figure 4-11, which shows the histogram for the wagon wheel image shown in Figure 4-10, the histogram shows that the picture doesn't contain a lot of highlight pixels or any pixels at the darkest end of the spectrum. To put it in photography terms, the image is a little lacking in contrast — most of the image is relatively similar in brightness, without strong highlights or shadows.

As with the highlight alerts, the Brightness Display mode of the histogram is provided to give you a way to gauge exposure that's a little more reliable than simply eyeballing the image on the monitor. Remember, if you adjust the brightness of the monitor or the ambient light affects the display brightness, you may not get the real story on exposure.

For information about the other histogram mode, check out the next section. For help with adjusting exposure, see Chapter 5.

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