What Is a Guide Number?
The guide number (GN) for a flash unit measures how well it
can light a subject at a specific ISO sensitivity and with a
precise angle of view (wide-angle vs. telephoto lens). To put
it simply, a higher guide number means the flash is more
powerful, all other things being equal.
Be careful when you are deciding on an external flash unit to
use, whether it is a genuine Nikon Speedlight or an
aftermarket unit. Simply comparing the GN is not enough.
You must understand the settings the GN is based upon.
Many flash units have zoom capability and will be able to
light up subjects farther away when zoomed out. However,
imagine buying a flash unit from a manufacturer who
publishes the GN based on a longer zoom position and then
comparing it to a different flash unit based on a shorter
non-zoom position. The GN rating on the flash that is zoomed
out would seem to be higher than the same unit not zoomed
out. However, unless you are comparing flash unit guide
numbers with exactly the same settings, it is truly like
comparing apples and oranges. For instance, to get an exact
comparison of guide numbers, you would have to know the
following:
▪ Distance from flash head to the subject
▪ Aperture f-stop number in use on camera
▪ ISO sensitivity of your camera’s image sensor
▪ Angle-of-view setting on the flash’s zoom head
▪ Actual angle of view your lens provides (must match
flash head)
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