A histogram is a digital photography tool that you can use to find the perfect image exposure. This tool is available on all modern digital cameras. Even if you use a cell phone camera, there is an app for that (actually, there are many from which to choose).
What is a histogram?
In my post “Reading 256 Shades of Gray,” I discuss how digital images are captured by the camera’s image sensor using 256 tonal variations ranging from total black (0) to total white (255). The histogram is a graphical representation of the pixels exposed to these different tones in your image. Do you remember all the fun we had with graphs when we were in school? In this histogram, the horizontal axis represents the tonal variations and the vertical axis represents the number of pixels capturing each of the 256 tones. The darker tones are to the left and the lighter tones are to the right.
How do you use the histogram?
To capture the most detail in your image, it is important to set your exposure to avoid black shadows or completely white highlights. Obviously, if you are photographing bats in a cave or snowmen in a blizzard this will be unavoidable. However, for normally exposed images, the tonal range should be between the extremes of the histogram. When you are composing your picture and have the exposure set to center your light meter (you are using manual mode, right?), check your histogram to see if any pixels are “climbing the walls” (called clipping) and adjust your exposure to center the tonal distribution.
The images below illustrate the effect of exposure on the histogram. The first image is underexposed and the histogram is climbing the left wall. The second image is overexposed and the histogram is climbing the right wall. The third image is exposed correctly according to the light meter and the histogram is centered between the two extremes.
Notice that there is still a small amount of clipping in the example above, due to the high dynamic range of the image. In situations like this, it is usually better to underexpose the image slightly and to accept more clipping in the shadows. With post-processing software, it is sometimes possible to recover some of the details lost in the shadows. Once you blow the highlights, however, there is no going back.
Why expose to the right?
If you do not post-process your images in Lightroom, Photoshop, or some other post-processing software, then this section is not for you. Simply keep your histogram centered for the most satisfying exposure. If you DO post-process, then the “Exposing to the Right” (ETTR) technique will result in images that are more detailed. ETTR simply means setting your exposure so that your histogram moves as far to the right as possible before hitting the right wall (and blowing your highlights). The image in your camera’s viewfinder may look overexposed, but it will contain more details when adjusting the exposure in post-processing.
This works because the right half of the histogram represents twice as much data as the left half. Take two pictures of the same scene, but underexpose one and overexpose the other (without clipping either image). In post-processing, adjust each image to the exact same “correct” exposure and the originally overexposed image will look vastly superior to the underexposed image. Why, you may ask? It is because the ETTR image contains more data about the details.
Most cameras have a “highlight warning” feature that you can use in conjunction with the histogram to achieve the ETTR result. When enabled, these so-called “blinkies” make the overexposed highlights flash on your camera’s screen. Simply reduce your exposure until the “blinkies” stop and you will be as far to the right as possible on the histogram without clipping.
There is one caveat to ETTR of which you should be aware. The histogram (and highlight warning feature) is based on the jpeg image created by your camera’s processor. If you shoot in RAW, the RAW histogram can vary significantly from the one displayed on your camera. Through experience and/or testing , you will learn the amount of exposure correction that you need to apply to arrive at your desired RAW exposure.
What does the perfect histogram look like?
There is no perfect histogram. The histogram shape will depend a lot on the contrast of your subject. High contrast images, such as those above, often yield a “twin peaks” shape to the histogram. Low contrast images with a wide tonal range often appear as a bell shaped curve. The general rule is to avoid overexposure and blowing the highlights. However, highlights such as a bright, hazy, and cloudless sky do not contain many details and may not matter to your composition. Remember, the histogram is just another tool in your arsenal to help you achieve your artistic vision.
If you have any questions or comments, please start the discussion in the comments section below.