Each pixel taken with a digital camera is assigned a color and these colors range from 1 (black) to 255 (white). A histogram displays these colors in a graph by selecting and grouping them together. Histograms read from black to white, left to right--in other words the dark pixels show up on the left hand side and white pixels show up on the right hand side. Histograms quickly show whether the image is over or underexposed and allow you to make exposure adjustments while shooting. The most important information a histogram indicates is whether the shadow areas or highlight areas are being "clipped". If a shadow area is clipped it will contain noise and no detail or information. If a highlight area is overexposed or "blown out", it also contains no detail and often the information cannot be recovered. Clipping can be detected when the histogram bunches up against either the left or right side of the panel.
The most important information a histogram indicates is whether the shadow areas or highlight areas are being "clipped". If a shadow area is clipped it will contain noise and no detail or information. If a highlight area is overexposed or "blown out", it also contains no detail and often the information cannot be recovered. Clipping can be detected when the histogram bunches up against either the left or right side of the panel.
Digital Exposure: Histogram Example 1In this image (right), all of the information is contained within the panel, which means that it is a fairly well exposed image. Notice that the histogram does not extend all the way to the right edge of the frame so it might be a good idea to adjust the exposure compensation to a +.3 and take another image. This adjustment to the exposure will move the histogram closer to the right side of the frame.
Generally is it desirable to have the histogram spread from left to right to cover the entire dynamic or tonality range that the camera can handle. This is usually a range of about five stops. However this is only a guideline and there are many instances where this would not produce the desired result. For instance, if you are shooting in the snow or fog and the photograph is light in tonality, then you would not expect the histogram to extend to the left side of the frame, instead all of the pixels will be in a grouping more to the right hand side.
Use histograms as a tool or source of information to assist you in obtaining the best exposure and remember that there is no perfect histogram that applies to every image or situation.
Histogram Made Simple: Example 2Now letís look at this image on the right - a more complicated scene and histogram:
In this photograph, the great white heron is very light against a dark background. The camera's light meter will try to average this scene or make it as close to 18% gray as possible. In doing so it will lighten the dark background and darken the heron. But as you can tell from the spike along the right hand side of the histogram the heron is overexposed and detail will be lost in the feathers.
By watching the histogram I would know to underexpose this image, probably by -.3 or -.7. This change might cause the background to be slightly underexposed, but the white bird is the main subject. These adjustments are much more difficult to make when shooting wildlife, especially when the heron has a fish in his mouth. However, if the bird is overexposed it will not be a shot that can be kept or used.
It is important to set your camera so a histogram appears immediately after you shoot an image and then take a second to look at it to see where the exposure is falling. With practice, histograms are not mysterious or confusing and will assist greatly in getting a good exposure.
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