essential skills: digital photography in available light
Guide numbers
Portable fl ash units designed to be used with professional cameras are allocated a ‘guide number’
(GN) by the manufacturers and this guide number denotes its approximate power output (guide
numbers are often optimistic or overstated). The guide number is an indication of the maximum
distance at which the unit can be used from the subject to obtain an appropriate exposure. The
guide number is usually rated at ISO 100 in metres/feet (ISO 100, m/ft.) using a designated focal
length and an aperture of f1. Because an f1 lens is a collector’s item and not a typical aperture
available to photographers the actual maximum working distance is usually somewhat lower than
the guide number would suggest.
A unit with a guide number of 56/184 (at 105mm) could correctly expose a subject at 46 feet using
an aperture of f4 (184 divided by 4 = 46). If the lens was changed to one with a maximum aperture
of f5.6, the maximum working distance would be reduced to just under 33 feet.
Gaining a rough idea of the maximum working distance of a fl ash unit is useful for the photographer
who does not wish to get caught in a situation where they do not have enough light to illuminate
their subject. Now that the digital photographer has the ability to review images immediately
after capture using the camera’s LCD screen the worry of underexposing or overexposing their
subject whilst using unfamiliar equipment has largely been removed. It is worth mentioning that
the amount of light refl ected back from the subject matter will greatly infl uence the fi nal exposure
of the fl ash unit when using automated settings. Exposure compensation may be required when
working with a subject with dominant light or dark tones.
Note > Bouncing the light from the fl ash or diffusing the fl ash light in an attempt to soften
the harsh light from a portable fl ash unit will further reduce the working distance that the
photographer is able to work from their subject matter (see page 120).
ACTIVITY 3
Test the guide number of a fl ash unit using a fl ash meter if available.
Turn the fl ash unit to manual operation and full power.
Stand a measured distance from the fl ash unit, e.g. 16 feet.
Attach a diffusing dome or ‘invercone’ to the fl ash meter’s cell and set the ISO to 100.
Aim the fl ash meter at the unit and take an ambient reading of the fl ash (use a ‘sync lead’ or
friend to manually trigger the fl ash using the test button).
Multiply the indicated aperture on the fl ash meter by the distance, e.g. if the subject stands sixteen
feet from the fl ash and records a meter reading of f11 the guide number of the unit is 176.
Note > If no fl ash meter is available increase the distance between the fl ash and the
subject at intervals of 3 feet. Choose the best exposure and then multiply the distance by
the aperture used to fi nd the guide number of the fl ash unit.