By eliminating the top of the head from the portrait, the composition is inherently simplified and the eyes become the singular center of interest. In fact, those eyes tend to move up higher in the frame by doing this, which is a good way to get them on one of those “Rule of Thirds” axes, and that also helps to make for an appealing portrait.
One type of composition that lends itself especially well to cutting off the top of a head is a horizontal composition with white space on either side of the subject’s face. I’ve noticed in my own experience that when I crop in close on horizontal portraits, they almost always become more interesting. The same often holds true with verticals, but with horizontals, it’s practically a sure thing.
The only problem with cropping off the top of a subject’s head is that almost without fail someone won’t understand why you cut off the top of the head. They assume it must be a mistake; after all, frequently the first thing we all learned when we picked up a camera was, “Make sure to get their whole head in the frame!” But here’s the thing: The human eye is smarter than we give it credit for. We know people have tops of heads. You can crop in close and, sure enough, the viewer’s brain will automatically fill in the missing details.
Sure, there are no hard-and-fast rules when it comes to cropping, but cutting off the top of a portrait subject’s head works surprisingly well in many situations. So, the next time it happens, make it purposeful rather than by accident.