Photography 101 – How to Read a Histogram

Photography 101 – How to Read a Histogram

Monday, 28th June 2010

Written by: Tiffany Joyce

I happened to be driving through the “fashionable” part of Arizona a while back (Scottsdale, for those of you familiar with the state), and saw a professional photographer arranging a wedding party in front of a fountain. The photographer was holding up an external, hand-held light meter to judge the exposure settings to be used. My first thought was, “Huh! So there are folks out there who still use a light meter!” I thought about that as I drove, and thought about how light meters(*) have been rather displaced in the world of photography by the use of modern digital SLR cameras and their light metering capabilities.

In the past, a light meter was vital to set the correct aperture and shutter speed to get the right exposure the first time. Film cameras didn’t have the sensory capabilities to “suggest” the settings that should be used, nor did they have the capability to review a shot after it was taken. There was a high risk of wasting film and having a pile of too-dark or too-light snaps returned from the developer.
Where we would have used an external light meter before, to get the right aperture and shutter speed to correctly expose the shot, we now use the camera’s internal light meter and automated functions to guide us. We will shoot “test shots”, which we can then review and make adjustments as necessary. In my own work, I will take several test shots of a venue in order to ensure that I am getting the correct exposure, then adjust the ISO, aperture, and shutter speed until I am happy (more often than not I just shoot in Aperture Priority mode and let the camera dictate the shutter speed in accordance to the ISO I’ve chosen and the available light).

If you know how to read a histogram, it comes in very handy to assess the quality of the photograph that might not be apparent in the small display screen available on your camera. A histogram is a visual representation of the light and dark elements in your photograph, displayed in a graph form. Today’s D-SLR cameras have the ability to display the histogram right on the LCD for each photo taken. For Canon cameras, during playback simply press the display (“disp”) button to toggle between different viewing modes until you see the histogram. For Nikon cameras, during playback tap the four-way directional selector to the left or right.

What you’re striving for, in a correctly exposed photograph, is for the histogram to look something like a bell curve. Dark pixels (underexposure) show up to the left of the graph, and light pixels (overexposure) show up toward the right hand side. So, if your shot is neither predominantly dark, nor predominantly light, the pixels will register in the middle of the graph. Take, for instance, this photo of my daughter and her new puppy, Maddie:



This is straight out of the camera, with no adjustments made in post-processing. Using Photoshop CS3, I can see this picture’s histogram, which looks like this:

As you can see, this photo is pretty well exposed, with most of the pixels registering toward the center of the graph. However, the graph is trending toward the left, or underexposed, side of the histogram, with some dark elements registering on the extreme left hand side. So I would probably adjust accordingly by reducing the shutter speed just a little bit.

To show you an extreme example, take a look at this photo, one of the bracketed images used in my entry about HDR Learnings:



As you can tell, this shot is very overexposed. This is reflected in the photo’s histogram:



All of the pixels are jammed up on the right hand side of the graph, indicating that the photograph is overexposed.

I hope you’ve found this information to be useful! Let me know if you have any questions, and feel free to leave feedback in the comments.

(*The use of flash meters in a studio setting is a different, and interesting, subject – check out this excellent video from The Flash Centre.)

Photo credits (all): Tiffany Joyce.