70 Part II ✦Creating Great Photos with the Creative Lighting System
5. Take a test shot.Take a photo
and review the results on your dig-
ital camera’s LCD (if you are using
a digital camera). If the image
appears under or overexposed, you
can adjust the output of the flash
by adjusting the flash compensa-
tion or by adjusting the aperture
setting on your camera. When
using bounce flash, you lose two
to three stops of light (bouncing
light results in less light illuminat-
ing the subject as opposed to
using normal flash, thus losing
two-three stops, measured by
aperture settings).
To compensate for light loss, you
can increase your cameras expo-
sure compensation to increase the
amount of light your flash emits to
make up for the loss in exposure.
Alternatively, you can change your
cameras exposure mode to manual
and then adjust the aperture set-
ting to stop down your aperture a
few stops until you get the desired
results.
Explaining FlashExposure andSpecifications
Flash exposure can seem mystifying when
you first attempt to use a flash. You need to
know a lot of settings and use different for-
mulas to get the right exposure. After you
know what the numbers mean and where
to plug them, it becomes quite easy.
I start out by explaining the different aspects
that involve obtaining the right flash expo-
sure. Of course, if you are using your
Speedlight in the TTL mode, all of these cal-
culations are done for you, but it’s always
good to know how to achieve the same
results manually. When you know this infor-
mation, you can use any flash and get excel-
lent results.
In the following sections I explain how to
use the guide number, the distance from
the Speedlight to the subject, and the aper-
ture to determine the proper flash exposure.

Guide number

The guide number (GN) is a numeric value
that represents the amount of light emitted
by the flash. You find the GN for your spe-
cific Speedlight in the owner’s manual. The
GN changes with the ISO sensitivity, so that
the GN at ISO 400 is greater than the GN of
the same Speedlight when set to ISO 100.
The GN also differs depending on the zoom
setting of the Speedlight. Tables 3.1 and 3.2
break down the guide numbers according
to the flash output setting and the zoom
range selected on the Speedlight.
If you have access to a flash
meter, you can determine the
GN of your Speedlight at any
setting by placing the meter ten
feet away and firing the flash.
Next, take the aperture reading
from the flash meter and multi-
ply by ten. This is the correct GN
for your flash.
Tip
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