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.
aperture ring
shutter speed control
lens film
shutter
diaphragm
focusing ring
object
image
Figure 1
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 4 7.8 16 31.4 64 121 256 484 1024
diaphragm
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).
open – dim light closed – bright light
Figure 2
1
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.
f
Example
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
o
when seen from the Earth is 0.5 .
Sun
o o
0.5 0.5
o
The image of the Sun must therefore also subtend an angle of 0.5
with the axis of the lens of the camera (as you can see from Figure 3)
and therefore:
Figure 3
diameter of image = 50 x tan 0.5 = 0.44 mm
50 mm
1. A camera with a shutter speed of 0.01s and a lens of focal length 50 mm is used to photograph a
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car 100 m away and moving at 50 ms 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
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