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The guide number was based on using a film of
specific speed, 100 or perhaps 400. Thus, if
you changed the film you were using, you'd have
to change your guide number. And every time
your flash-to-subject distance changed, you'd
have to recalculate your exposure. This is why
we strongly advise photographers to avoid non-automatic
flash units no matter how inexpensive they might
be. Other features to look for include correct
exposure confirmation, flash-ready indicator,
variable power settings, automatic power shutoff,
and battery-check light. You'll find that most
units from major manufacturers will offer all
of these basic features.
Fortunately, most flash equipment on the market
today from any of the major companies is of
high quality. If you are new to flash photography,
a very useful introduction to the subject is
Electronic Flash, one of the Kodak Workshop
series (KW-12E) published by Silver Pixel Press.
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