One of the first things you probably want to know after taking a photo is whether or not you got your intended exposure.
You might be tempted to look at the preview on your LCD screen, but when you’re shooting photos outdoors in the sun, those previews can be very deceiving, making your photos look much brighter than they actually are.
So, how do you know if you got the right exposure? Well, one way is to use the histogram.
What is the histogram?
If the word “histogram” brings back bad memories of boring lectures in math class, don’t worry: the histogram is pretty simple.
It basically just shows the distribution of light and dark pixels in your image.
Here’s an example, with each axis labeled:
The left side of the graph shows how many dark pixels you have in your image, while the right side of the graph shows how many light pixels you have in your image. So, in this example, the image mostly has dark to midtone pixels.
At first you may think that every image should have a balanced (i.e. bell curve) histogram, but this doesn’t always happen. It really depends on your image. For example, the histogram of a snow-covered landscape will mostly consist of bright pixels, so it’ll carry most of its weight on the right side of the histogram.
How to use the histogram
The histogram is best explained with a bunch of example photos, so here’s an unedited shot (along with its histogram) I took this past weekend, using auto exposure:
When I shot this photo, the exposure looked fine on my LCD preview, but because I was outside in the bright sun, the preview looked much brighter than it actually was. And, the histogram proves this: all the pixels are dark to midtoned. And, since I knew the scene was actually pretty bright with the sun shining on all those desert plants, I knew the photo was underexposed.
The first shot was exposed at 1/20 sec, so I stopped down half a stop to 1/15, and here’s the result:
Notice how the histogram is now more spread out, and moved closer to the right? This is because of the longer exposure. There’s still a little gap there on the right side, so I tried a slightly longer exposure of 1/13:
With this photo, the histogram is even more spread out, and has a bigger gap on the left, meaning there aren’t many dark pixels in the image. Look at the right hand side though: there are a few pixels on the verge of being overexposed (pure white).
A good rule of thumb is to get the histogram as close to the right as possible, without actually touching the right side.. but, this doesn’t always work!
After seeing the histogram of this photo out in the field, I thought this would be the perfect exposure, but I took one more shot just to be sure (at 1/10 sec):
It’s a little hard to see in this histogram, but there’s a black line going all the way to the top on the far right. That means the image has overexposed (white) pixels. If you look at the rest of the histogram and notice how it looks very compressed, you’ll realize that quite a few pixels are being overexposed.
So, the winning photo is the third one! It does still look a little underexposed to me, so I’d probably adjust the levels slightly in Photoshop as well.
How to enable the histogram on your camera
On most cameras, all you have to do to see the histogram is change the “view” on your image review screen (on my Canon XTi, I just hit the “display” button). If that doesn’t work, try looking through your camera’s menus or as a last resort just google your camera with “histogram”
Turn on highlight warnings to make your job easier!
Sometimes it’s hard to spot those blown out highlights on the histogram, but luckily there’s something that’ll make your life much easier: they’re known as highlight warnings. With these enabled, the blown highlights will “blink” on your image review screen, making it clear where you’re overexposing the shot.
They’re enabled by default on some cameras, but if they’re not on your camera, just do some more looking in your camera’s menus, try hitting the “info” or “display” button on the back, or as a last resort there’s always google.
Things to remember about the histogram
- The histogram shows you the distribution of light and dark pixels in your image
- Consider how bright the scene is, before you decide on the proper histogram
- A properly exposed image will not always have a balanced histogram
- Turn on highlight warnings to help you spot blown highlights
Stay tuned for a post on the RGB histogram!
Since the standard histogram combines all the color channels (red, green, and blue), it’s not always helpful in determining the proper exposure. Most cameras also have an option to see the histogram separately for each color channel: one for red, one for green, and one for blue. These are especially useful for photographing wildflowers.
About the Author: Steve Berardi is a naturalist, photographer, computer scientist, and founder of PhotoNaturalist. You can usually find him hiking in the San Gabriel Mountains or the Mojave Desert, both located in the beautiful state of California.