by Dave Montizambert Published 01/10/2008
Just as digital SLRs have multiple metering methods, so do photographers in an attempt to capture the best exposure possible. A very popular method of judging exposure is to look at the histogram displayed on the LCD image preview on the back of the camera.
When judging exposure using histograms, we have been taught to look at the histogram's shape and to check for clipping of the shadow and highlight - clipping is when either end of the histogram bunches up against the left and right ends respectively. Reading histograms is not a precise way to judge exposure since there is no perfect shape for a histogram; a histogram's shape is based on a collection of tones unique to each image. Really a histogram only gives us a general idea of exposure, kind of like licking your finger and sticking it out to read the wind as opposed to using meteorological instruments.
You should also keep in mind that histograms are calculated on a version of your image based on the in-camera processing algorithms used to create JPEGs and not the actual RAW capture data. There is more data available in the RAW file than in a processed JPEG file and so a histogram that clips at the shadow or highlight end on a JPEG file is not giving the real picture if you are shooting RAW. Also, looking at a histogram only, and not considering the image tones present, can lead to unfortunate images such as Image 01.
In this image the white seamless backdrop is over-exposed, burning-out to white. This backdrop and the subject's white coat cause the histogram to bunch up at the highlight side, and yet the main subject, actor Rich Reynolds posing as a lab technician, is under-exposed. A pure white, nodetail background is generally okay but an under-exposed subject usually isn't. The point is, if you are not using precise metering methods, then make sure that you compare the image tonal content from the actual scene and the LCD preview with the histogram to make sure that you at least have a serviceable exposure.
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