Photography tips: How to understand histograms
Histograms can be daunting at first glance. But they’re not as confusing as they look – here’s our simple tutorial to make them work for you
What is a histogram?
Most digital cameras will have an analysis tool called a histogram – a graph of the tones of the image – on the preview screen. It is much easier to see and assess in strong light than the image itself. Some cameras can also show a separate histogram for each of the red, green and blue channels of your picture, but here you want the single graph that refers to the whole image.
How does it work?
An image is made up of pixels. Each pixel in a jpeg can have one of 256 tonal levels, where 0 is absolute black and 255 is absolute white. The extreme tones will have no detail.
Image-editing software such as Adobe Photoshop uses histograms to help amend pictures. Once you are familiar with histograms, you can assess the exposure and contrast of your photo, then make changes to it where needed.
Some cameras also feature an overexposure indicator. This flashes up the parts of your picture registering a tonal level of 255: the blown highlights. If there are just a few obvious bits – eg lightbulbs – then you can ignore them, but if key parts of the picture are flashing, you might want to consider reducing the exposure.
It takes some practice to read a histogram but once you know what you are doing you can use it as the basis for controlling your exposure – and you need never take over- or underexposed pictures.
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