Reading a histogram
If you've ever used a Levels or Curves adjustment layer, you may have noticed a graph in the area where you're making the adjustment. This graph is known as a histogram. The histogram basically shows you how many shadows, midtones, and highlights there are in the image.
In the example below, we have the Levels adjustment layer selected, and you can see the histogram in the Properties panel on the right:.
Screenshot of Adobe Photoshop CC
So what does the histogram tell us about this image? First, remember that every image has a mix of shadows, midtones, and highlights. The histogram simply counts the number of pixels for each tone and displays them as a graph with shadows on the left, midtones in the middle, and highlights on the right. Note that the histogram only tells you how much there is of each tone; it does not tell you where the tones are located in the image.
In the example above, we have a mix of different tones-the rocks and ground near the bottom of the image make up most of the shadows, the bridge and ocean make up the midtones, and the sky and clouds make up the highlights. If you look at the histogram, you can see that the image has a fairly even balance among shadows, midtones, and highlights.