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When you’re first starting out, you might not be sure how to use your camera’s histogram. It’s a very underutilized tool. The histogram helps you to achieve the correct exposure for your photos. We’ll start with the basics and then get into how to use it.
Firstly, you will notice that your camera has different histograms. The primary histogram—a single black and white graph—is for luminosity. The other three are color histograms representing the red, green, and blue pixels in the image.
How to Read a Histogram
A histogram is a graphical representation of the pixels exposed in your image. The left side of the graph shows the dark information and the right side shows the light information. The middle portion of the histogram represents mid-tones.
By looking at the histogram you can tell if you have overexposed or underexposed your image. You may wonder why you don’t just look at the image on the camera’s LCD screen to see the exposure. Well, your camera’s LCD screen may not always be that accurate. It most likely has adjustable brightness, and the lighting conditions may affect what you see on the screen.
Figure 1 - Overexposed
The histogram’s horizontal (X) axis shows the luminance of the image, from pure black on the left to pure white on the right of the graph. Growth on the vertical (Y) axis indicates the relative quantity of light for the given luminance. As seen above, this histogram shows that the image is overexposed with the spike on the right of the graph. Conversely, Figure 2 shows the histogram of an image that is underexposed with the spike on the left edge of the graph.
Figure 2 - Underexposed
As you can see in the histogram below, this image has a balanced exposure shown by the arch in the middle of the graph, representing the mid-tones, that tapers off towards the right and left edges.
Figure 3 - Perfect Exposure
You may be thinking, that’s great but what do I do with this information? Take the data from the histogram, information from the scene itself and the desired final image–then fine-tune your exposure accordingly by adjusting the aperture, shutter speed, and ISO. Keep in mind, depending on the subject matter and background of the image, if both are light, the histogram may indicate that the shot is overexposed. Therefore, we need to be aware of the entire scene before we make any adjustments to exposure.
Compared to the human eye, a camera’s sensor is quite limited in its ability to collect information from a scene with a broad dynamic range, from extremely light and extremely dark areas. Dynamic range is the ratio between the lowest and highest values of density and light in the image. Due to its limited dynamic range, the camera may attempt to produce an image with the widest possible range of light and dark, potentially resulting in an image with pure white highlights or pitch-black shadows.
When using a histogram, you may have heard the term “clipping”. Clipping refers to the loss of tone and detail due to severely light or dark areas, which are represented on the histogram by spikes on the left or right edges. Unfortunately, the data lost is unrecoverable.
The histogram is a remarkable tool to have at your disposal. When reviewing your images, be sure to not solely rely on the image on the LCD screen. Rather, use your knowledge of the scene and the histogram data to get the best results. Don’t forget to watch out for clipping too. Happy shooting!