Look at the image below (Figure 2). It is well exposed with no serious problems. The entire light range of this particular image fits within the histogram window, which means that it is not too light or too dark, and will take very little or no adjustment to view or print. It contains no more than four or five stops of light range.
Look at the left side of the histogram graph above (Figure 2), and see that it does not cram itself against the dark value side. In other words, the dark values are not clipped off on the left. This means that the camera recorded all the dark values in this image, with no loss of darker detail. Then look at the right side of the histogram graph, and note that it is not completely against the right side, although quite close. The image contains all the light values available. Everything in between, such as the blues and grays, are all exposed quite well, with full detail. A histogram does not have to cover the entire window for the exposure to be fine. When there is a very limited range of light, the histogram may be rather narrow.
The image in Figure 2 is a relatively bland image with smooth graduations of tone, so it makes a nice smooth mountain peak looking histogram graph. This will not happen all that often, since most images contain quite a bit more color information. Each prominent color will be represented with its own peak on the histogram graph. The most prominent colors will have higher peaks, while the less prominent will have lower or no peaks.
As we progress into images with more color or light information, we will see that the histogram looks quite different. Look at the image in Figure 3 below, which is one that far exceeds the range of the camera's digital sensor.
Notice that, overall, this image is dark and underexposed looking. The clouds are pretty well exposed, but the image is not very usable unless the clouds are the primary subject. See how the histogram above (Figure 3) is crammed to the left, effectively being clipped off there? There are no gradual climbs like on a mountain range, from valley to peak and back to valley. Instead, the image shows up on the left side in mid-peak. It is "clipped." (Remember that word) If this is confusing, refer to the histogram graph (Figure 1) at the very top of this article, and notice that it has unclipped peaks and valleys like a mountain range.
If you don't fully grasp this yet, do not worry. The most important thing to know is that when you see a histogram like in Figure 3 above, with part of the peak and valley clipped off on the left, THE IMAGE IS TOO DARK. This problem could be corrected on this image by using a neutral density filter on the sky, which would have compressed the light range enough that the image could be more fully recorded. This image above is even clipped a little on the highlight side (right). You can see why when you look at the rays of light shining between the clouds. The light is too bright, so it exceeds the light range of the sensor and is clipped.
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