Histogram
A histogram is a graph illustrating whether an image is under, over, or well exposed. It illustrates exposure in the form of a distribution curve; underexposed left of center, overexposed right of center. After shooting an image, we check the histogram and use a combination of the visual image and the histogram.
We always shoot in manual mode. You can shoot in one of the auto modes, but you won’t have full control over exposure. Exposure can be thrown off by sunlight or any bright source of light in the composition.
ISO, Speed, f-stop
If the histogram is weighted to the left dark side, you need to bring in more light. You can raise the ISO, lower the speed, or lower the f-stop. The lower the number f-stop, the wider the iris. So if the image is underexposed and dark, you correct with a lower number f-stop that opens the iris. For an overexposed, too light image, you do the opposite with ISO, speed, or f-stop number (widen the aperture).
Basically, that is what lighting and exposure is about. With practice, you get to know what is throwing exposure off. For example, in this 2004 session, we photographed this portrait of the bride looking at herself in the mirror. The light from the flash reflecting off the mirror caused the iris to close and the bride’s image to be underexposed and dark. We might not have realized this unless we looked at the histogram.
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| Underexposed |
The curve is scrunched at the left dark side of the histogram. The colors show the red, green, and blue pixels ( RGB ).
Good Exposure
We retook the image, this time making sure the flash was not reflected back by the mirror. And the new image and histogram looks more balanced.
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| Good exposure |
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