At a workshop I once taught, an older fellow in the back of the room raised his hand. He said, “I just bought me a new toy and she’s a beauty! But I can’t figure out how to make the hysterectomy work?” Fortunately, my student wasn’t describing a lady of the evening. He was talking about how to pull up the histogram on his new Nikon digital camera. His confusion is not unusual. One of the first things new digital cameras ask about is the histogram…what is it, and how should they use it.
In this article, I will detail the basics of working with histograms.
At its simplest, a histogram is a graphical representation (such as a bar graph) of digital data (brightness values) in a given image. According to Adobe, a histogram:
“[I]llustrates how pixels in an image are distributed by graphing the number of pixels at each color intensity level. This can show you whether the image contains enough detail in the shadows (shown in the left part of the histogram), midtones (shown in the middle), and highlights (shown in the right part) to make a good correction.”
I like to think of a histogram as a very sophisticated light meter. It can help the digital photographer understand if an image is over or underexposed, and it can evaluate the quality of the light, for example, is the image flatly lit or is it a high contrast lighting situation? (Try doing that with your father’s old Soligar meter!)
Histograms illustrate how 256 possible levels of brightness are distributed in a digital image. The histogram’s horizontal axis represents the range of brightness from 0 (the shadows) on the left to 255 (the highlights) on the right. Think of it as a football field with 256 yard markers (0 to 255) upon which the team can stack pixels of the same brightness. Since these are the only values that can be captured by the camera, the horizontal line also represents the camera’s maximum potential dynamic range. In other words, the horizontal line (from left to right) represents increasing brightness in your image. The vertical axis represents the number of pixels that have one of the 256 brightness values. The higher the line goes (coming up from the horizontal axis,) the more pixels there are at that level of brightness. In other words, the vertical line represents an increasing amount of digital information from bottom to top.
Generally, if all you learn from this section of the article is that the histogram helps you to understand the tonal range of your image, you are ready to move on.