We all know what a good histogram should look like, right? A “normal” scene should not be too heavily weighted toward the left side of the histogram—the shadow side—or else we risk underexposure. Likewise, a normal scene’s histogram shouldn’t spike heavily toward the right, the highlight side, or the image might be overexposed with blown-out highlights. (The exception, of course, is in scenes like a black cat in a black hat in a black room, or a white cat in a white hat in a white room—which should each have histograms weighted toward the left and right ends of the spectrum, respectively.)
Whatever the look of your image’s histogram, you can check it on the back of the camera’s LCD, find it in Photoshop or you can see it in Lightroom. It’s Lightroom that I’m discussing today, because it offers one simple twist: In the Develop module, you can click and drag on the histogram itself and modify the image directly.
The histogram appears in the top right of Lightroom’s Library and Develop modules. Helpfully, when you mouse over various areas in the histogram, the area represented by that portion of the chart is listed below. All the way to the left you’ll find Blacks, then Shadows, then Exposure (representing the midtones that especially define the overall exposure), then Highlights toward the right and Whites all the way at the right edge. Want to modify the whites in a scene? Simply click and drag on the far right area of the histogram. Pull to the right and the whites get whiter. To the left, they darken.
Each of these adjustments can be found in the Basic section of the Develop module, not far below the histogram. But because Lightroom’s developers recognize that different approaches to image editing can be useful in different circumstances, this click-and-drag approach is included, too.
Want to see where the black detail disappears into blocked-up shadows, or where the white detail becomes blown out? In the top left and right corners of the histogram, Lightroom displays a Triangle in a box. These are clipping warnings. The left one, when hovered over, displays a blue overlay on areas of pure black pixels, while the right triangle displays a red overlay in areas where the white pixels are devoid of detail—depicted in the accompanying image. Clicking on either triangle will maintain the colored overlays for the duration of your editing, so you can be sure to not sacrifice important detail at the ends of the spectrum. Option-clicking (on Mac, or Alt-clicking in Windows) on the histogram allows you to see which channel—Red, Green or Blue—has clipped.
All in all, grabbing hold of the visual representation of your image’s shadows, midtones and highlights as they manifest on the curves of a histogram is a very fast, simple and intuitive way to make adjustments to an image file.