Spark image

The camera

In its simplest form the camera uses a single convex lens to form a real inverted image on a film. The film is coated with a light-sensitive material such as an emulsion of silver bromide on a transparent or paper base. Figure 1 is a diagram of a simple camera.

The important parts of the camera are:
(a) the lens that produces the image,
(b) the shutter that controls the time for which light is incident on the film (this time may vary from a thousandth of a second to a few seconds), and
(c) the diaphragm (Figure 2) which controls the amount of light falling on to the film per second by using more or less of the aperture of the lens. The aperture of the camera lens is usually described by the f-number or relative aperture which is defined as follows:

Relative aperture (f-number) = focal length of lens/diameter of aperture

that is, a camera with an f/8 lens is one with a focal length 8 x the diameter of the lens. You will find that the aperture control on a camera usually has the following f-numbers on at:

f number 2 2.8 4 5.6 8 11 16 22 32
square of f number 4 7.8 16 31.4 64 121 256 484 1024

The reason for this rather curious series of f-numbers is that the square of each is approximately double that of the previous one. Therefore changing from one to the next will double the aperture of the camera (since the area of the lens is proportional to the square of its diameter).

An important property of a camera is its depth of focus, this is the distance that the film can be moved without spoiling the image. This will correspond to a depth of field, which is the range of object distance that will still give a satisfactory image. This is greater for a lens with a short focal length and for larger f-numbers. The depth of focus can be improved by putting a 'stop' over the front of the lens and then reducing the size of the hole in the centre of the stop so that only a small part of in the centre of the lens is used.

Example problem
Calculate the diameter of an image of the Sun formed by a camera with a lens of focal length 50 mm if the angular diameter of the Sun when seen from the Earth is 0.5o.

The image of the Sun must therefore also subtend an angle of 0.5o with the axis of the lens of the camera (as you can see from Figure 3) and therefore:

Diameter of image = 50 x tan 0.5 = 0.44 mm

1. A camera with a shutter speed of 0.01s and a lens of focal length 50 mm is used to photograph a car 100 m away and moving at 50 ms-1 perpendicular to the line of sight. By how much will the image on the film be blurred?

2. The telephoto tens system of a camera consists of two lenses: a convex lens of focal length 12 cm mounted 4 cm in front of a concave lens of focal length 12 cm. How much larger will the image of a distant object be if the telephoto lens system is used compared with the image produced by the convex lens alone.

3. A camera with a focal length is used to take a photograph of a footballer 30m away. If the footballer is 1.85 m high calculate the size of the image produced on the film.

See also: CCD camera

© Keith Gibbs