In digital photography, clipping means that a pixel has no light/color information – it’s either pure white or pure black. In the case of black clipping, it means that a pixel is pure black.

The picture to the left shows an image in Adobe Lightroom with the clipping indicators on. All of the blue areas (mostly black curtains in the background, where very little light came back to the camera) have been clipped, and they’re simply black pixels.

Why does this happen?

The human eye is capable of perceiving a really high level of contrast. Go out on a bright sunny day, and you’ll see really bright things as well as really dark shadows. Sometimes you might have trouble making out the highlights or the shadows, but then your eyes will adjust automatically.

A camera doesn’t have that luxury, and a digital image is even more restricted. Digital images are often displayed as jpegs – a file type that gives you eight bits to describe the tone of a color (red, green, or blue). That ranges from 0 (complete absence of the color, pure black) to 255 (pure white) and a gradation of points in between. The brightest possible point is only 256 times brighter than the darkest possible point; in other words, the brightness can only double eight times, equivalent to 8 stops of exposure. Once a part of the scene is more than four stops darker than the average (the exposure you set the camera to) it’s going to come out… pure black.

One thing to note here is that your camera is capable of capturing a slightly larger tonal range. Modern digital cameras have a range of about 10 to 12 stops (1 to 2 stops on each end more than a jpeg image). If you shoot your pictures as jpegs, the camera picks an average exposure and drops the extra information at the ends (resulting in black and white clipping).

If you shoot in RAW, the camera saves this extra information and allows you to play around and pick the average exposure later in your post processing software (like Adobe Lightroom or Camera Raw). This is a good reason to shoot in RAW, especially under changing lighting conditions, because you’ll be able to tweak the exposure by about a stop or so without creating noise or digital artificats.