This article over at Picture Correct is explaining what a histogram is and how you can easily learn how to use it.
I remember the first time I was trying to understand the histogram – it seemed so difficult and I just didn’t get it even though I tried to read as much about it as I could. But once you get it, it’s actually quite simple. Doing these 5 exercises will help you understand so much faster!
1. Put the lens cap on, take a photo, and look at the histogram. There should be a single line on the left of the graph, yes? If there had been all sorts of colors in your scene and you’re getting something too black or too dark, and if the lines of your histogram are mostly over on the left of the graph, then you’re losing detail and would need to make certain adjustments, such as decreasing the shutter speed, choosing a wider aperture, and/or increasing the ISO.
2.Point the lens at something white and fill the frame with it. If you don’t have anything white to use, turn your ISO up to something like 1600 or higher, then turn the shutter speed to a really slow setting. Give it a good 30 seconds and point the lens at the lightest color(s) available to you and take a photo. When you look at your histogram there should be a single line, or a very small bunch of lines, over on the extreme right of the graph. The camera interprets this as an overexposed image. If there had been all sorts of colors in your scene and you’re getting something too white or too washed out, and if the lines of the histogram are mostly over on the right of the graph, then you’re losing detail, once again. Adjustments you might want to make include increasing the shutter speed, choosing a narrower aperture, and/or reducing the ISO.
3. With your camera still trained on that light subject take a series of photos with ever faster shutter speeds. Look at the histogram for each respective image, and you should see the line or group of narrow lines gradually travel from the right side of the graph, over toward the left side. If you were training your camera on something white, then the images in the sequence should begin to look ever more grey.
4.Go hunting for objects with single colors, filling the frame with each object in turn, and then taking individual photos of these single colors. Photograph something red (filling the frame so your entire photograph is a mass of red), and there will be a narrow bunch of lines in this photo’s histogram slightly to the left of the very center of the graph. A photo that’s all yellow will have a bunch of lines further over on the right side of the graph, just over half way from the very center of the graph. Play about with taking photos different single colors, and their corresponding histograms should give you a better understanding of how the histogram is helping you to interpret individual colors in any given image.
5. Take photos of anything you like. Introduce a variety of colors into your photos and see the wild patterns of their corresponding histograms. If the majority of the lines are bulked over on the left of the histogram graph, it’s probably telling you that your image is too dark. Conversely, if the graph is mostly bulked over on the right side of the graph, then your photo is likely to be too bright and washed out.
Source: Picture Correct