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Understanding Your Digital Camera's Histogram (Continued)
By Darrell Young
Original article courtesy of
Image and Histogram Shape ... continues
Now look at a similar image below (Figure 4). In this image a larger aperture was used and more light was allowed in. We can now see the ground,
but, once again, the range of light is too great for the sensor, so it is now clipped off on the highlight side (right). The dark-side graph
value is not clipped; instead the graph extends right to the left dark-side edge but stops there.
This image in Figure 4 above shows more detail in the ground area, but it is not professional looking, and will win no awards. The range of light is
simply too great to be recorded fully. The clouds and light behind them are overly light, and that can be seen by the histogram's clipping on the right
side. The most important thing to remember with this image's histogram is that when you see a histogram graph that is crammed all the way to the right
and clipped, THE IMAGE IS TOO LIGHT. Overall, a great deal of the image in Figure 4 is recorded as pure white and is gone permanently. (It is "blown out")
Also notice in the Figure 4 above that there are few mid-range values, as represented by a big valley in the mid-range area. It has two peaks, the left
representing darker values and the right representing lighter values. There is no strong mid-range peak. Of course, other images will have multiple peaks
and be just fine. The important thing is that you prevent the image's light values from being clipped on the left or right. This is not always possible,
but do your best to try. In other words, if you try to center the histogram, your images will be better exposed. If you take a picture, and the histogram
graph is shifted way left or right, well, you can then retake it, exposing in the direction of the opposite light value.
If there is too much light to allow centering the histogram, you must decide which part of the image is more important, the light or dark values. Does
that make sense? You must expose for the highlights, or you will lose detail in the light areas. Which is more important, the dark areas, or the light areas?
How Does The Eye React To Light Values?
The camera, with its lenses, film, or sensor is only a weak imitation of our marvelously designed eye and brain combination. There are very few situations where
our eye cannot adjust to the available light range, and we can see well. So, as photographers, we are always seeking ways to record even a small portion of
what our eye and mind can see.
Since our eye tends to know that shadows are black, and expects that, it is usually better to expose for the highlights. If you see dark shadows, that seems normal.
We're simply not used to seeing light that's so bright that all detail is lost. An image exposed for the dark will look very weird because all highlight detail
is gone. Your eye can see a HUGE range of light in comparison to your digital sensor. The only time you will ever see light values that are so bright that detail
is lost is when you are looking directly at an overwhelmingly bright light, like the sun. So, in a worse case scenario, expose the image so that the right side
of the histogram graph just touches the right side of the histogram window, and the image will look more normal. (See the next image for a view of this)
This is no different than shooting with film, since we have always fought with only being able to record a limited range of light. But, with the digital camera
and its histogram, we can now see a visual representation of the light values, and can immediately approve of the image, reshoot it with emphasis on lighter or
darker values, or see that we must use a filter to capture it at all.
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