essential skills: digital photography in available light
Intensity and duration
Intensity
Light intensity is controlled by the aperture in the lens. The aperture is a mechanical copy of the
iris existing in the human eye. The human iris opens up in dim light and closes down in bright light
to control the amount of light reaching the retina. The aperture of the camera lens must also be
opened and closed in different brightness levels to control the amount or intensity of light reaching
the image sensor. The right amount of light is required for correct exposure. Too much light and
the image will be overexposed, not enough light and it will be underexposed.
As the aperture on a lens (as fi tted to an SLR camera) is opened or closed a series of clicks may
be felt. These clicks are called f-stops and are numbered. When the value of the f-stop decreases
by one stop exactly twice as much light reaches the image sensor as the previous number. When
the value of the f-stop increases by one stop half as much light reaches the image sensor as the
previous number. This is called the ‘halving and doubling principle’. The only confusing part is that
maximum aperture is the f-stop with the smallest value and minimum aperture is the f-stop with the
largest value. The larger the f-stop the smaller the aperture. From a small aperture of f22 to a wide
aperture of f2.8 the stops are as follows (not all lenses have the range of stops indicated below):
f22 f16 f11 f8 f5.6 f4 f2.8
Note > When using a command dial on the camera body to adjust the aperture you may
fi nd the aperture is opened or closed in 1/3 stop increments instead of 1 stop increments,
e.g. the difference between f5.6 and f8 is one stop. The camera’s LCD readout may however
indicate values of 6.4 and 7.1 as the aperture is stopped down from f5.6 to f8.
ACTIVITY 1
Carefully remove the lens from a digital SLR camera if available.
Hold the lens in front of a white sheet of paper.
Look through the lens and notice how the size of the aperture changes as you alter the f-stop.
Record and discuss the relationship between the size of the aperture and the corresponding
f-stop number displayed on the barrel of the lens.f16 f8 f4