In this age of technical wonder, when every cell phone carries a camera, we are all photographers. That means, of course, we are all also fair game to be photographed.

Henri Cartier-Bresson, the famous French photographer whose name is synonymous with “the decisive moment,” once explained those words: “There is a creative fraction of a second when you are taking a picture. Your eye must see a composition or an expression that life itself offers you, and you must know with intuition when to click the camera. … Oop! The Moment! Once you miss it, it is gone forever.”

Heeding Cartier-Bresson’s advice, we click away, compiling moment upon moment into ever-growing digital collections. As voluminous as these visual records grow, they are incomplete, though, lacking something important—all those people and places we chose not to photograph. For each moment we save, we lose another. For all the viewfinder sees, there is much more it doesn’t.

“To photograph is to frame, and to frame is to exclude,” says philosopher Sontag.|

To understand Sontag’s point, watch a boat—say the Angel Island ferry­—being boarded. The lower-deck windows form a frame. Some travelers we see, many we don’t. We click. We capture. Choose wisely—because the rest are gone forever.