Spark image

The pinhole camera

The pinhole camera is a very simple piece of apparatus that demonstrates a number of points about the physics of light.
The pinhole camera was first used in about 1000 AD and since then it has been a simple way of producing a correct image of a scene. Artists from the sixteenth century onwards used a pinhole camera to help them get the correct proportions for a painting.

In its basic form it is simply a light-tight box with a pinhole in one end and a translucent screen of, say, tracing paper at the other. You can see a diagram at the top of the page.
Light from the object goes through the pinhole and produces an inverted image in full colour on the screen. This is because only light from one point on the object can reach a given point on the screen.

Changing the distance from the pinhole to the object will change the size of the image as you can see from the two diagrams above. If more than one pinhole is made in the end more than one image will be formed on the screen.

A bigger pinhole will allow more light to get in and so the image will become brighter, but it will also be blurred as the next diagram shows. This is because light from one point on the object can reach more than one point on the screen.

Pinhole cameras need not be focused like ordinary cameras, they are always in focus. However, if you bring a pinhole camera very close to the object you are effectively increasing the size of the pinhole and the image will become blurred. You should notice that the inside of the pinhole camera is painted black to stop unwanted reflections inside. It is also helpful to have a light shield over the back to shade the screen. The fact that the pinhole camera gives a clear image is very good evidence for the fact that light travels in straight lines.

Photographs with pinhole cameras

It is possible to take a photograph with a pinhole camera providing that the object is brightly lit and that you use a long exposure. If the screen is replaced by a piece of photographic paper in the dark and the pinhole covered with a flap, the pinhole camera is ready. The object should be lit by a projector and the pinhole camera put about 1 m from the object. An exposure of 8 to 10 minutes should be something like correct. It is even possible to take "360o" photos by making a hole in each side of the camera box and using a cylindrical roll to mount the film in the centre.

© Keith Gibbs 2020