Digital photography offers certain benefits for thinking photographers that are only realized with a basic understanding of a metering feature called the "histogram."
Using the histogram in your camera's image viewing LCD will guarantee you a much higher percentage of well exposed images. It is well worth working to understand the histogram, and is not overly complicated. I will try to cover this feature with enough detail to give you a working knowledge of how to use the histogram to make better pictures. If you are deeply interested in the histogram, there is much research material available on the Internet. This article is only covering a small amount of the information that is actually available, but will present enough knowledge to improve your technique immediately.
Please note, we have an updated version of this article as well.
Nikon D2X rear view
A digital camera sensor can only record a certain range of light values. Remember how with film the light range was limited to only a few f/stops of range? With transparency film the limitation was about 3 or 4 "stops" of light, while with negative film it could go on out to 5 or 6 stops. Today's digital sensors seem to be able to record about 5 or so usable stops of light. Most will agree that the digital camera can record a little more than transparency film, and a little less than negative film. This will change, since some digital cameras are now offering extended range sensors.
Unfortunately, many of the higher contrast subjects we shoot can contain over 12-stops of light values. This is quite a bit more than it is possible to capture with film or a digital sensor today. It is important to understand how your digital camera records light, so that you can better control how the image is captured.
The histogram basically is a graph that represents the maximum range of light values your camera can capture, in 256 steps. (0 = Pure Black, and 255 = Pure White) In the middle of the histogram are the mid-range values that represent middle colors like grays, light browns, and greens. The values from just above zero and just below 255 contain detail.
The actual histogram graph looks like a mountain peak, or a series of peaks. The more of a particular color, the taller the peak. In some cases the graph will be rounder on top, or flattened. The left side of the histogram represents the maximum dark values that your camera can record. The right side represents the maximum white values your camera can capture. On either end of the histogram the light values contain no detail. They are either completely black, or completely white. The top of the histogram (top of mountain peak) represents the number of different colors, a value you cannot control, so it is for your information only. We are mostly concerned with the left and right side values of the histogram, since we do have much control over those. (Dark vs. Light)
So, basically, the histogram's left to right directions are related to the darkness and lightness of the image, while the up and down directions of the histogram (valleys and peaks) have to do with color information. I repeated this for emphasis! The left (dark) to right (light) directions are VERY important for your image making. If the image is too dark, the histogram will show that by clipping off the light values on the left, or, if too light, by clipping on the right. This will become easier to understand as we look at well exposed and poorly exposed images.
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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