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A digital photography histogram is a graph representing the distribution of brightness in an image. This frequency distribution chart can be calculated automatically by a digital camera or image processing program to provide feedback and information for the photographer. While not all photographers use histograms when they take and process images, they can be very useful tools for some. When real-time feedback in the field is important, photographers should check the technical specifications on camera models to find out if they have a histogram option.
In a photograph, the brightness of each pixel can be represented by a number ranging from zero to 255. The lowest score is assigned to pure black, while the highest goes to pure white. A computer can quickly process the pixels in an image to generate a histogram, a bar chart that shows the distribution of pixels in various ranges of this scale. The digital photography histogram can help a photographer determine if an image is overexposed, underexposed, or has other problems.
Simply looking at a digital photography histogram may not provide enough information about the image. Roughly speaking, too many pixels at one end of the scale or the other can indicate an exposure problem, such as overexposure with too many highlights, though this is not always the case. For instance, a picture of a snowy field would have lots of pixels in the high end of the range, creating a clustered bump at one end of the chart, while a picture shot at night would have a cluster at the opposite end. Photographers shooting in the snow would want to see a histogram with a marked peak on the high end of the scale.
Images with significant dark tones are known as low key, while high key pictures have lots of highlights and lighter tones. Using a digital photography histogram, a photographer may be able to tweak high or low key images to make them as clear and crisp as possible. The curve of the graph should be smooth, for instance, indicating an even distribution of tones with sufficient contrast. If it’s choppy, or has numerous peaks and troughs, the picture may have issues with contrast and could appear muddy or dull.
There is no ideal digital photography histogram, as the needs of individual images can be quite variable. Generally, the chart should be smooth, with a sufficient range of tones to provide contrast and variability that make the picture “pop” visually. One advantage to using histograms in the field is that photographers can see how adjustments to shutter speed and aperture change the overall exposure of the image. This can allow people to correct immediately if there’s a problem, rather than only spotting issues during processing, when it may be too late.