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How To Read Your Histogram

by Brooke Tasovac (follow)
Blog (395)      Articles (166)     
It took me a while to realise what a histogram was, and how to access it on my camera. Since discovering it though, it’s improved the quality of my photos so much I can’t shoot without it anymore.

A histogram is a novice photographer’s secret weapon. Even though it looks complicated, it’s the key to understanding how your image is exposed, and whether it’s too dark or too light. You can set your camera to show a histogram reading after each photo is taken, and then check it to see how even the exposure is.

If you have a reading that shows no peaks at all in one of the sections, or a peak that reaches the top of the graph, this is a very clear indication that the photograph is probably going to be underexposed or very overexposed, and you should adjust one of the three exposure elements (ISO, shutter speed or aperture). This is called “clipping”, and it will mean that the dark or light areas will be missing detail — such as in the images below.

If you’re getting a reading that is skewed far left or far right, you need to check the LCD screen, as well as the histogram to see if the settings you are using are suitable. It is important to do this when shooting subjects that are white or black because you may find you get a skewed reading, even though the picture is properly exposed. The histogram can be fooled in these cases, as well as when a scene has strong contrasts, such as a burst of sunlight in an otherwise dark photograph.

I find using the histogram goes hand in hand with the Blink Function. This is when the highlights in a photo are displayed by “blinking” on the LCD screen after the photo is taken. I find this helps to determine whether the highlights are “blown out” (too bright), or in proportion with the rest of the scene. Generally, if the blinking areas are dominating the scene, the photo is going to be overexposed, but if there are only a few small areas and they aren’t on a subject’s skin, I’ll leave the settings as they are. The most important thing is that you’re not losing any detail in your highlights because of your settings.

When shooting in RAW, edits can be done later to fix exposure problems. The Levels tool is a function found in most Adobe editing programs that allows the histogram to be displayed for the original photo, which can be altered within each of the three sections to bring back any lost detail, and to even out the light across the entire photograph. This is only possible for slightly underexposed or overexposed photos though — photos where there are large sections that are pure white or pure black usually can’t be recovered, because all detail has been completely lost.

By using the Levels tool some of the material detail has been brought back to the wedding dress.

Black and White vs Colour histograms

Most histograms are in black and white, or grey and white. Some histograms, such as the one below, appear in colour, combining the three primary colour RGB channels — red, green and blue — to make up the other colour channels — magenta, cyan and yellow.

In a colour histogram, all of the pixels in each colour channel will be expressed as peaks from the shadows to the highlights, instead of simply being displayed as tones from black to white. Sometimes histograms can also be broken down into the individual red, green, or blue channels, such as in the image below.

The purpose of a colour histogram is the same as a regular histogram — to check that the colours aren’t too light or too dark, and to check on the white balance. Images that are spiking in one of the three RGB channels can sometimes indicate a colour cast.

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