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A camera obscura in your classroom

A vivid demonstration of the camera obscura, suitable for any age group, can be set up in the classroom. The room must be darkened as much as possible and a small hole a few millimetres across is made in a window blind. You need to set up a screen on the wall opposite the hole. On a sunny day you will see the view outside the room inverted on the screen.

The image may not be very bright but the brightness can be increased by enlarging the hole. Unfortunately, this will reduce the resolution but the image can be improved by placing a convex lens over the opening.

Young children could make a very simple camera obscura and use it to draw a picture. Take a shoebox and make a pinhole in one end and cut the other end off and replace it with a piece of tracing paper. If you point the pinhole towards a window or a bright object such as a light bulb, you will see an inverted image of the object on the tracing paper which can then be traced.

Older children could make a more sophisticated version. Take a larger box and cut a small hole in the top surface and insert a convex lens. Then place a mirror above the lens at 45 degrees to the horizontal so that light from an object in front of the box is reflected and passes through the lens and is focused on the bottom surface of the box. You can use this to draw a portrait if a pupil sits in front of the box and another inserts his head through an opening in the side of the box - draped with a blanket so that no extra light can get in - and traces the image on paper.

* Holbein's Georg Gisze displays some strange distortions. The tablecloth looks normal on the left hand side but the centre seems to be domed. The box of coins appears tilted at a different angle from the table on which it is sitting. David Hockney suggests that these distortions were introduced when the artist had to refocus his lens to paint the various features. Set up a still life of objects on a patterned tablecloth and use a camera obscura to draw a picture. Are the distortions anything like those in the Holbein picture?

This subject gives rise to some interesting discussion topics for the classroom. If the old masters were so good at looking, why did they not spot the distortions which are apparent to us in their final works? Does it matter if painters use optics to make their pictures more realistic?

ResourcesSecret Knowledge, Rediscovering the Lost Techniques of the Old Masters by David Hockney (Thames and Hudson)Vermeer's Camera by Philip Steadman (OUP)The debate for and against the Hockney hypothesis is presented in great detail at hockneyoptics

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