HOW TO READ A HISTOGRAM
- Before starting this lesson, you should have a basic understanding and know how to operate your digital camera. Read the following lesson to get an idea on how a camera works.
- Have a basic understanding of Exposure - Aperture, Shutter Speed and ISO. Refer to Lesson #1: Understanding Exposure
- You must have a digital camera that displays a Histogram. Almost all modern digital camera have this feature.
What is a Histogram?
One of the best tools your DSLR camera provides is a histogram. A histogram is a graph that shows the distribution of an image brightness. It shows the maximum range of tones that your camera can capture. The left side shows the maximum dark tones (black) that your camera captures and the right side shows the maximum light tones (white) and the middle of the graph shows the midtones (colors like gray, blue, green, brown). The left and right end of the graph do not contain any detail and shows either pure black on the left end or pure white on the right end.
The histograph graph contains a peak or a series of peaks that represent the individual colors. The higher the peak represents more instances of a particular color. The left side of the histogram displays the shadows, the middle represent the midtones and the right side represents the highlights.
Good Exposure Histogram
Now let's look at examples of images with their corresponding histogram. The image below shows a properly exposed image and its histogram.
The histogram of the image in Figure 2 is able to fit the entire range of tones. Notice that there are no high peaks on both the left and right ends of the histogram. This means that the camera sensor recorded all of the dark tones without any loss of detail in the shadows because the left side of the histogram is not clipped. This also means that the camera sensor is able to record all the light tones without losing detail in the highlights because the right side of the histogram is not clipped.
So what exactly do I mean by Clipped? Let me give you another histogram example to show you what I mean by Clipping.
Over-Exposed Image and its Histogram
One of the things that you need to pay attention to in your photograph is losing detail in the shadows (the dark part of your photograph) and the highlights (the light side of your image). Clipping happens when the image loses detail on either the dark shadows and/or the light highlights of your image which renders parts of your photo as black or white. In the example image in Figure 3, the photo is overexposed and the light side of the image lost some detail because they were rendered as pure white. In the histogram, notice that the right side peak is clipped or cut off. This indicates overexposure of parts of your image.
If you compare the image in Figure 2 and Figure 3 and look at the corner area by the gutter of the house on the left, you will notice that the image in Figure 3 rendered the corners as pure white and lost the detail that it had in Figure 2. The corner actually disappeared because it was rendered as white. The texture of the wall also disappeared. In this case, no amount of post-processing will get those lost details back so if those overexposed areas are important to you and you did not intend to lose any of the highligh details, then you need to pay close attention to your histogram. See the comparison image1 and image2 below magnified at 100%.
Under-Exposed Image and its Histogram
Now let's take a look at a histogram of an under-exposed photo. Examine the image and the histogram in Figure 4 below.
Notice how the distribution of the peaks in the histogram are shown only on the left side. This indicates that the photo has mostly dark and shadow tones. Also notice how the left side peak was clipped at the left-most end of the histogram. This means that parts of the image was rendered as pure black and therefore the detail in the shadows in these areas are lost. Once again, because the detail was lost it would be nearly impossible to restore them even with post-processing software like Photoshop. To avoid these kinds of problems, it is best to be aware of what your histogram is telling you. If the details in both the shadows and the highlights are important to you, you need to examine the histogram.
A magnified portion of the image is shown in Figure 6 that shows you a comparison of a properly exposed photo and an underexposed photo. If you look at image 2, you will see the the details in the leaves are gone and you can only see the leaves as black silhouttes.