Your camera’s histogram is one of the most powerful tools you have for taking great photos. But what is it, and how can use you a histogram for photography?
A histogram is a graph that shows the distribution of tones in an image. It’s a visual representation of the brightness values in your photo, from pure black to pure white.
By understanding how to read a histogram, you can make sure your photos are correctly exposed, and avoid blown-out highlights or blocked-up shadows.
What Is a Histogram For Photography and How to Read It
A histogram represents the tonal range in photography mathematically. The tonal range is all tones between the darkest part and lightest part of your photo.
The histogram maps out the brightness in a given image in grayscale. Black is on the left of the histogram, while white is on the right. You can find all the shades of grey between these.
Every shade has a scale of brightness values. For a standard .jpg image, there are 256 different recorded values of brightness. “0” being pure black and “255” pure white.
A histogram maps out the given brightness values; each pixel from the image is assigned to a value. The number of pixels on the vertical (Y) axis determines how high the columns are depicted.
Red, green, and blue depict a histogram in color photography. Overlapping areas are shown in grey.
The image of the apple pie below has been correctly exposed. You can see that most of the pixels are away from the black and white values in the histogram.
The areas that look black in the image are technically shades of dark grey.
Most of the pixels are on the left-hand side of the histogram because it’s a dark and moody style image.
It’s not considered underexposed because there is a gap between the brightness values and the ends of the histogram. Neither columns touch the end.
Your photo is either underexposed or overexposed if you have pixels located on the very ends of your histogram. Unfortunately, you can’t recover these missing details in Photoshop or Lightroom. At this point, you should adjust your exposure and re-shoot.
Have a look below at these two food images with their respective histograms. One is overexposed, while the other is underexposed.
To know if an image has lost detail, you can also turn on the clipping warning in Lightroom. These small triangle icons in the upper corner of the histogram show clipping.
When these appear and change color, Lightroom is warning you of detail loss in the image. When it turns white, it means there is detail lost. This lost detail will also be indicated in the image as long as the clipping warning is on.
Red indicates areas that are blown out. Blue indicates areas where shadow detail has been lost. The information from your histogram or the clipping warnings in Lightroom are helpful. But, you also want to be making creative decisions in your photography. This means looking at an image and determining if a bit of clipping is affecting the exposure. For example, backlit images will often show highlight clippings.
In the picture of the cobbler on the left below, you can see the whole back of the image was blown out. This is because the image was backlit with window light. The important areas of lightness, such as the white plate, are properly exposed.
The dark image of the croissant has some areas where it has lost shadow detail, particularly around the edges of the props. Note out their matching histograms for comparison.
Look at your histograms in RGB to assess your exposure. This refers to the three channels of color: red, green and blue. The RGB histogram displays these colors separately.
A typical histogram represents these colors together. You don’t actually know if you’re overexposing or underexposing a specific color.
To get proper exposure, you should avoid under or overexposing any of the color channels. Assess each channel’s histogram and make sure the graph isn’t favouring one side.
Each color channel’s histogram will be different in every photo. What you’ll see will depend on the colors in the photo and how bright they are.
Referring to the RGB histogram is most important when shooting bright colored subjects. Check it when you’re shooting to ensure that you don’t blow out a color channel that doesn’t show up in the main histogram. This can result in an overexposed image.
When taking pictures, check the histogram on the back of your camera. Or check it out in Lightroom if you’re using it to shoot tethered.
The goal is to get your exposures right in camera. It’s not always possible to salvage an incorrectly exposed photo.
Photography is a blend of artistry and science. Learning as much as you can about the technical side can help you make better creative decisions.
Once you become confident with the histogram, you’ll stop struggling with exposure. You will have a lot more consistency in your photography.