Understanding histograms in photography.

How To Use Histograms To Understand Image Exposure

A DSLR or mirrorless camera provides important information about the exposure of an image in real time. The camera can display what is called the histogram, and this gives the details about the image’s exposure. The exposure is how the image was captured by the camera sensor. What the histogram can tell is whether the light and contrast is balanced in the exposure. It can also show the tonal variations in an image, which is a way to understand brightness and contrast. There are comprehensive detailed studies about histograms in photography in print and on the Internet, but this article will focus on the basics.

Histograms Defined

Example of a basic histogram. It is a graphical representation of values that varies by height.

Histograms are a graphical display of data that provides a visual presentation. In an image, the histogram provides information regarding the exposure. The exposure is the captured image in the camera. The histogram can be viewed on the back of the camera’s LCD screen. It will look like a graph of bars of varying heights that represent values.

The histogram is shown at the back of a camera LCD screen on the upper right hand side. This does not display the full histogram, just a simple example of it. Photo editing software can display a more complete information of the image’s histogram. (Source Nikon)

In photography, the histogram displays information of the 3 primary colors in an image which are R(Red), G(Green) and B(Blue). The RGB levels are represented in the histogram and can be further examined in photo editing software like Photoshop.

Example of histogram information from Adobe Photoshop CC. Information about the RGB color channels can further be examined.

Why is the histogram important?

The reason is that it will give the photographer an idea about how their exposure is looking like, in terms of the brightness and colors. In an image exposure, there are three important elements aka Exposure Triangle. This consists of the aperture, shutter speed and ISO. These are the basic foundations of taking a photograph, going back to the days of film. With digital there is more leeway around it especially with digital sensors and in post production using editing software.

The histogram and the three color channels RGB. The graph shows that the R channel is shifting toward the shadows while the B channel is more towards the highlights. This can be adjusted in post.

What the histogram can tell us is whether the image is overexposed or underexposed, and whether the lighting and contrast in the image is balanced. When the graph shifts toward the right, the image is considered to be more toward overexposed. This means blown highlights which shifts brightness toward completely white. Highlights kill details in an image, so if the image is too overexposed, the details are lost in the highlights. The opposite is when the graph shifts toward the left. The image is considered to be more underexposed. This means more shadows which shifts contrast toward completely black. The details will be lost in the shadows or darker regions of the image.

This is an example of an overexposed image. The graph on the histogram shifts toward the right.

This is an example of an underexposed image. The graph on the histogram shifts toward the left.

This can be corrected by adjusting the exposure triangle. If the image is too underexposed, the photographer will need to adjust the aperture to allow more light or decrease the shutter speed and bump up the ISO. There is no absolute formula for getting a perfect exposure, but at least the correct exposure meaning the image is neither too dark (left shifting) or too bright (right shifting). Instead the image is more balanced if the graph is more toward the middle, also called the midtones. This means the image tones are more in the middle. This means there is more balance between the brightness/darkness (luminosity) and contrast in the image.

This is a more balanced image, the graph is toward the center of the histogram. Many retouchers will actually make further adjustments to this because it looks too flat. Some saturation and color channel adjustment can bring more vibrancy and life to the image. It will be considered a good exposure if it shows the details in the highlights and shadows without excessive clipping.

The histogram also shows what the color channel (RGB) is shifting towards. In the image’s color balance, this can be corrected. For example if the image is too strong toward the R channel, in post it can be adjusted to shift more toward blue. There techniques are used for color grading accuracy with the color space (e.g. sRGB, P3, etc.).

Provided the images were shot in RAW, more details can be recovered. It is best practice for image quality. Otherwise, compressed JPEG format will not be able to preserve as much detail in an image.

Summary

Using the histogram gives direct feedback from the camera to the photographer about their image exposure. The adjustments can be made in real time using the exposure triangle. What the photographer will learn from the histogram is whether the image has balance in lightness and contrast. It is recommended to shoot in the RAW format in order to recover as much details as possible.