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What makes the pinhole work?

You can demonstrate it for yourself


A  5 X 7 box pinhole camera with its light-proof inner lid and box top removed. The pinhole is located in the center of a brass shim that is painted black and covers the square opening in the camera's front panel.
A 5 X 7 box pinhole camera with its light-proof inner lid and box top removed. The pinhole is located in the center of a brass shim that is painted black and covers the square opening in the camera's front panel.

WHAT MAKES THE PINHOLE CAMERA WORK?

Lightwaves beam off in every direction from the Sun, including towards us on Earth. When lightwaves strike objects on Earth, they scatter in every direction, but they still travel in straight lines. One of these directions is toward our pinhole.

The pinhole allows only the lightwaves traveling directly towards it to enter the camera. They pass through the pinhole, continuing along in straight lines that diverge until they hit the back of the dark box. The brightness of the lightwaves entering the pinhole is proportional to the brightness of the various objects in the scene, therefore a likeness of the outside world - an image - is projected on the wall inside the box.


BUT WHY IS THE IMAGE UPSIDE-DOWN AND REVERSED?

Light travels in straight lines, so a lightwave reflected from the foot of an object must travel upwards in a straight line to reach the pinhole and will continue on in the same upwards direction to be projected high on the rear wall. But, a lightwave reflected from the top of a relatively tall object must travel downward to reach the pinhole opening and will continue down in a straight line after it has passed through the pinhole to be projected at the bottom of the interior wall. Similarly objects from the right side of the exterior scene will appear on the left side of the interior wall. Everything is reversed because all the scene's lightwaves travel in straight lines and must pass through a very tiny opening.

Lightwaves from the bottom of the subject travel in a straight line through the pinhole to the top of the camera's interior. Those at the top travel to the interior bottom, turning the image upside-down.
Lightwaves from the bottom of the subject travel in a straight line through the pinhole to the top of the camera's interior. Those at the top travel to the interior bottom, turning the image upside-down.

Any image taken by a pinhole (or any other) camera, including this self-portrait by Daniel Bouman, appears upside-down and laterally reversed (top).  Picture taken with a hand-made Kodak 126 film-box camera - Exposure time 2 sec at ƒ/128.
Any image taken by a pinhole (or any other) camera, including this self-portrait by Daniel Bouman, appears upside-down and laterally reversed (top). Picture taken with a hand-made Kodak 126 film-box camera - Exposure time 2 sec at ƒ/128.

DEMONSTRATE IT YOURSELF

You can easily demonstrate the principle for yourself.

  • Completely darken a room by blocking its window with a large piece of cardboard that has a six-inch diameter hole cut out of its middle.
  • Stand outside the darkened room at night with a bright flashlight.
  • Kneel down so the flashlight touches the ground and shine the light through the opening onto an interior wall.
  • Its beam will be seen high up on the wall.
  • Now, stand on a high ladder outside and shine the light through the hole into the room.
  • This time, its beam strikes the base of the interior wall.
  • Like the pinhole camera, light from the top of the exterior scene travels through the hole to the bottom of the room, and vice versa.
That's the principle of the pinhole camera.


The tremendous depth-of-field of a pinhole camera is illustrated in this picture of springtime crocuses taken using a Kodak 126 film-box camera. Exposure - 4 sec at ƒ/128.
The tremendous depth-of-field of a pinhole camera is illustrated in this picture of springtime crocuses taken using a Kodak 126 film-box camera. Exposure - 4 sec at ƒ/128.