Lesson 2: Reading Light – How Histograms Work | Photography Education

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What is a Histogram?

A histogram is an incredibly useful tool to understand as you learn your camera settings and strive for properly exposed images.  

Therefore, this lesson is devoted to explaining all about the graph you see both when you look at an image on the back of your camera display, and also when you load your images into whatever program you use for editing. I use Adobe Lightroom, which is what you see in the below picture, (histogram circled). The histogram is a colorful graph on the top right that shows how your image is exposed. The different colors represent things such as overall exposure, highlights, shadows, blacks, and whites. 

twin boys hugging with warm light

Interestingly, this graph may look a little different depending on the style of editing. However, the gray-colored shading represents overall exposure in the image, and that should be pretty well centered for a properly exposed image. If you notice the gray shading favoring one side or another, then the overall image is going to be either under or over-exposed. Having the gray area somewhat centered means that your exposure is balanced.

I tend to favor images that have a wide dynamic range (more on this in another post). In a nutshell, this means I like to have my histogram somewhat spread across the graph, where I have some darks very dark and some lights very light while still balancing exposure. This makes my images a little more contrasty, which fits my style.

The far-right portion of the graph represents highlights and whites in the image. The further the graph leans to this side the brighter the image. If your graph leans to the left, then your image will be darker and have deeper blacks and shadows.  

How a Histogram Works

The histogram represents your lights, darks, shadows, exposure, highlights, and everything in between. If you have ever done statistics at any point, one of the things you will be introduced to is a bell curve. A histogram is similar to that. So the thickest portion of the graph will represent the greatest number of points that fall into each category. That’s why a properly exposed image will have the gray shading mostly centered. It’s a balance between light and dark. 

According to the histogram, when you plot all your “light” points on a bell curve your image is properly exposed if the majority of your light points fall into the middle section, forming the “bell”. These represent your “neutral gray” area. This is why that part of the graph is shaded gray. Your dark points will be represented by lines falling off to the left and your bright points will be represented by lines on the right. At each end of the graph, you will have the extremities. Pure black on the left and then pure white on the right. The correct terminology for your darks are often referred to as your shadows, the lights are highlights and everything in between are mid-tones. For a properly exposed image, you will want the majority of your light in the middle with a few lines scattered on either side for dimension or range in the image. This will form a bell shape with the peak of the bell in the middle of the graph. If your bell peak is too far to the left then your image is on the dark side or under-exposed and if your peak is too far to the right then your image is too light or over-exposed.

How to Use a Histogram

If the bell is not “centered” you can make tweaks to the exposure of an image both in-camera and in post-processing later to make it “properly exposed”. However, for now, if you are shooting in auto your camera is making these tweaks for you. In auto, the camera attempts to bring the exposure to neutral gray or to center that bell curve as much as it can automatically. All I want you to do is to start looking at the graph and understanding what it is telling you as it pertains to your image. Notice also, in the picture to the left, that there are peaks of color and they are also plotted. Simply put, these are the colors in the picture and they are plotted on the histogram where they fall in the scheme of light versus dark. In fact, each small vertical line represents a pixel in your image. The taller the peak, the more that particular color is present in the photo.

Key Learnings
  • The graph you see after you take a picture on your camera or when you load an image into editing software on the computer is called the histogram
  • The histogram is a bell-shaped curve with black on the left and white on the right
  • When the bell is centered it symbolizes that the image is properly exposed

Note: To keep this training basic and easy to understand we are assuming you will want every image to be exposed evenly with your bell curve in the middle which, for the sake of this course, is how we are defining “proper exposure”. However, to be honest, that may not be entirely the case. Many people consider “properly exposed” somewhat subjective and can move their exposure based on what they like stylistically. Also, because the histogram is a measurement of the entire image, if you are going for very dramatic lighting, the bell curve theory may not hold true because of the sharp contrast you will have between your lights and your darks. In fact, in this case, you may have most of your points on the dark side so your histogram will be lop-sided towards the dark side. And yet that may be exactly how you want your image to look regardless of how the histogram reads. But for now, let’s assume properly exposed is when the bell curve is in the middle.

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