A camera obscura is nothing more than a dark chamber (of any size) with a hole in one side. Outside the chamber, light reflects off an object in all directions. Some light travels through the hole, projecting an upside-down image onto the interior surfaces of the chamber.

All cameras operate on this simple optical principle; whether they be an old-time wooden box with a glass lens and a mirror for making tracings, or a modern 35mm SLR still camera, or a digital video camera.

The invention of the glass lens greatly increased the brightness of the image in the chamber. A glass lens is advantageous, but is not necessary. A small hole will do. It is important to note, when the objects outside of the camera are moving their images move correspondingly on the inside. In this respect, the images created by a camera obscura are more like a movie than a still photograph.

The existence of the camera obscura has been documented in various places and cultures throughout the historic period (since written records). To date, the oldest discovered document to reference a camera obscura was written by Mo Ti in the fifth century BC, describing the image of a pagoda projected inside the enclosed space of an adjacent building.

Enclosed spaces continue to randomly create camera obscuras to this day (though not as often due to the prevalence of the glass window pane, which has too great a surface area to form an image). Contemporary artist Madison Cawein rented studio space in an old factory building where many of the windows were boarded up or painted over. A random small hole in one of those windows turned one room into a camera obscura. When photographer Bruce Cook was a private in the military he had to sit in the back of a dark truck, where to Mr. Cook's amazement, a hole in the side of the truck projected the image of the passing street scene. And there thousands of other individuals who have had similar experiences. Enclosing ourselves from the outside world has led to the formation of camera obscuras for a very long time.