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.
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.
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