Depth of field for a camera

Class practical

A model camera using lamps, to investigate depth of field and magnification.

Apparatus and Materials

• For each student or group of students
• Lamp, stand, housing (one left- and one right-handed), 2
• Multiple slit
• Holder for slit
• Plano-cylindrical lens, approximately + 7 D
• Piece of pale coloured filter to fit lamp
• Power supply for lamp
• Barriers for creating aperture

Health & Safety and Technical Notes

Warn students that their lamps may become hot if they are left on for a series of experiments of this type.

Read our standard health & safety guidance

The switch-on current for two 12 V 24 W lamps may trip the overload cut-out on some supplies. Try connecting one lamp first and then adding the other.

Procedure

1. Place the lamps side by side, about 1 metre from the lens, and look at the images A' and B'. Place a piece of pale colour filter in front of one lamp. This will make it easier to tell which rays come from which lamp. Hold a sheet of plain paper vertically across the two image points to represent a photographic film.
2. Move one lamp much farther or nearer, to produce an out-of-focus patch on the film made by light from that object-point. Bring in barriers to stop down (reduce) the lens aperture and observe the effect on the focus of the blurred lamp.

Teaching Notes

• Arranging the lamps in this experiment is not easy, and students will need help.
• Two lamps can be used to represent two objects, or the top and bottom of a single object. If the light from the two lamps is of different colours, the path through the camera from different objects will be seen clearly. In the first diagram below, both objects (parts) are at the same distance from the lens.

• In the second diagram, above, one lamp is moved forward, so each object's distance from the lens is different and their images are not in focus in the same plane.
• However, if the aperture is reduced, less blurring of the image of the moved object occurs in this image plane. This is shown in the third diagram, above.
• The overall clearest images of both objects are seen in a plane situated between the two images.
• For a spherical lens, this would be called the circle of least confusion. Cheap, fixed focus cameras have tiny apertures. For average scenes they produce a reasonable photograph if the film is placed at the circle of least confusion.
• With the lamps side by side, students can measure the separation of the lamps (size of the object) and the separation of their images (size of the image), and so calculate the magnification (or in this case, diminution).
• Explain that this object lies across the lens axis; it is not an object lying along the axis of the lens.

This experiment was safety-tested in January 2007