The Histogram - How to interpret it
Whether or not we completely agree with it, we as humans thrive on feedback. Positive feedback makes us feel good and makes us stronger, while negative feedback can help us improve ourselves and learn from mistakes. For years, photographers' only feedback came long after the photos were taken, once the film was processed and returned from the lab or darkroom. With the advent of digital, the privilege of instant gratification has been bestowed upon us. But often, simply looking at a photo only tells part of the story, especially on a tiny screen designed to be bright and saturated.
In digital photography, the best feedback for evaluating an image on the camera's screen is the histogram. The histogram is a graphical representation of our exposure, how bright or dark the image is. Learning to read this graph is actually quite straightforward.
First, let's get an idea of what we are looking at. The horizontal axis of the histogram runs from 0 to 255, each number represents a level of "lightness," with 0 being pure black, and 255 being pure white. The vertical axis of the graph shows numbers of pixels. To make it simple, the histogram shows how many pixels of each level of lightness there are in an image. A "bright," also known as high-key image, will have lots of pixels that are closer to white than they are to black, so the histogram will have the majority of data towards the right side of the graph. Conversely, a "dark" image, or low-key, will have more pixels to the left of the graph, closer to black.
Now that we have the basics, we can use this information to improve image quality, by being aware of any problems with exposure. When there are pixels in an image that are very dark (underexposed), they become pure black, numerically represented as 0 on the histogram. Any pixels with this value will have no detail whatsoever, and no amount of post-processing will save the data. This can result in pure black shadows, or dark colors without any detail whatsoever. This is known as "clipping." Clipping happens when the image has levels of brightness that are literally "off the charts" and cannot be rendered in the final file. When a portion of the histogram is pushed up against either side of the graph, it tells us that there is a part of the image that has no detail; it is either pure black or pure white. Once we capture an image, check out our histogram, and see this problem, we can then correct exposure to best capture the most amount of detail possible.
"An example of a good histogram. This image is slightly underexposed however. You can make this assessment because there is still some room left over on the right side, or highlights, of the graph. Since the majority of the graph is located in the middle, this image would be relatively low in contrast."
When shooting, it is always best to have the histogram as far over to the right side as possible, without actually clipping any data. Digital sensors perform best towards the right of the histogram, and will give you the best image quality that way. Even if you want to shoot a dark image, keep the histogram pushed to the right and then darken the image in RAW processing. This way, the shadows will be noise-free. It is important to remember that while the rule of digital photography is always "shoot to the right," be sure not to actually clip the histogram and lose image data.
Another item to keep in mind when working with a histogram is white balance. A histogram represents all of the pixels in an image. Since a digital image is composed of red, green, and blue pixel data combined, many cameras have the ability to show histograms for each color channel. One of the many benefits to shooting a RAW file is that the white balance is not applied to the image, and thus can be changed without any loss of quality in any RAW conversion software. However, it is still ideal to get the best white balance possible in camera. If the color is significantly off, this will cause one of the color histograms to be off, and since the main histogram is composed of the three color histograms, this will skew the data of the main histogram. In other words, shooting images that are significantly off-color in camera due to poor white balance will often result in a misleading histogram.
In short, the histogram is a feedback system, available on almost all digital cameras, that allows us an unbiased analysis into our exposure. This helps us avoid pure white skies and solid black shadows, saving time and frustration in post processing.
Joshua Lehrer | Website
Josh is a recent graduate of the Advertising Photography program at the Rochester Institute of Technology. His career started in the NJ/NYC area where he worked as a freelance photographer, writer, and consultant. He also worked as marketing coordinator for a large photography retailer. He currently resides in South Florida, where he continues to be heavily involved in the photography industry.