When I was teaching Photoshop to my students at ITT Technical Institute, one of the features they grappled most with was understanding histograms. A histogram is basically a graph showing the tonal range of the photo you’re currently working with. You can access histograms within “Levels”. Let’s take a look at one. Below is a photograph of a hot air balloon over a forested area.

Hot air balloon photograph.

Now, here is the histogram for this image.

Histogram for the hot air balloon photo shown above.

Histogram for the hot air balloon photo shown above.

A histogram shows the tonal range of an image in 256 brightness values, from pure black to pure white.  Zero, at the far left, represents pure black.  Pure white is represented by 255 at the far right.  The black area in the graph represents the tonal distribution over the entire photo.  Most histograms look something like silhouettes of mountain ranges.  In this case, very little of the image is either pure black or pure white – only a tiny spike for each.  Most of this image falls in the darker tones, which we can see in the heavy greens and blues.

Here’s another example.

Moravian Tileworks in Doylestown, PA.

Moravian Tileworks in Doylestown, PA.

This is a relatively high-contrast image, with lots of black area and lots of white area.  Here is the histogram for this image.

The histogram here is commonly representative of a high-contrast image.

The histogram here is commonly representative of a high-contrast image.

Notice how our mountain range looks more like a “U” in this histogram.  This is typical for a high contrast image.

There really is no “perfect” histogram;  there is no real right or wrong distribution in a histogram.  If you’re tweaking the levels of your image, keep an eye on the histogram.  Too much of your distribution slammed to the outer edges means you’ve likely lost much of your image’s detail, while too much of a grouping in the middle will leave your image looking flat and washed-out.