Histograms are a much misunderstood species, but can be an incredibly useful tool for any photographer and Photoshop artist. In my previous tutorial about clearing a colour cast I have already shown one example of how you can use these graphs; there are countless other ways they make themselves invaluable. And it’s not just in Photoshop that histograms are a useful guide. They can also be found in the display options of all digital SLR cameras, and most point-and-shoot cameras, and can be a useful aid in achieving correctly exposed photos.
I use them constantly while I’m working in Photoshop, so before I write any more tutorials that use these I’d like to give a quick explanation as to what it is you’re looking at, and why they’re so helpful.
Quite simply, a histogram is a graph that shows the tonal range in your image. There are 255 possible brightness levels for each pixel in a standard digital image; a histogram gathers the luminance data from your image (luminance = brightness according to how your eye sees it, as opposed to technical brightness) and displays it as a bar graph.
The horizontal axis shows the possible brightness levels from 0 (absolute black) to 255 (absolute white), while the vertical axis shows how many pixels of each brightness level there are in your image.
In this example, most of the lines are grouped to the left of the middle, showing that the image itself will be fairly dark. It could be under-exposed, or could just be a dark subject.
This is the actual image, and the histogram (here it’s in a layer adjustment palette). As you can see, the photo is indeed very under-exposed.
Here’s a more evenly exposed photo:
And here’s the histogram:
Instead of being bunched up to the left of the histogram, the lines here are distributed across almost the entire tonal range. They trail off just before the right of the histogram, because the image doesn’t have any true whites. The graph peaks on the left because there are some very dark tones at the corner of the image, and in the centre of the flower.
Okay, so we’ve looked at an under-exposed image, and one that was more balanced.
Now let’s take a look at an image with a lot of bright tones:
I took this photo with a flash while facing into the sun. Dark tones didn’t stand a chance.
Here’s how this shows up on a histogram:
There are no lines at all to the left of the middle, and a huge peak in the absolute white zone, showing where the main body of the pixels are based. In this case it’s a purposeful over-exposure. If I was looking for a more even exposure, this histogram would tell me that I’d missed that by a long shot and that all my highlights were going to be blown out.
With histograms there’s no right or wrong. Quite simply, a histogram gives you the information you need about the lights and darks of your images, and lets you make informed decisions based on the knowledge it provides.
This goes as much for when you’re taking the photos as for when you’re editing them. You can activate the histogram display on your camera, then when the camera LCD shows you your last photo taken it will also show you a histogram of the image. With a little practice you can use this to see at a glance whether your photo was correctly exposed, or whether your darks or highlights are clipped. Using this information you can retake the photo with the needed exposure adjustments, and save yourself from some nasty surprises when you get your photos home to the computer.