Flash Basics - Nikon D300 Complete Manual

only aperture and ISO overlap between the two
exposures.
When set to the defaults, the D300 does its best guess at
making everything work "magically." As you'll learn in a bit,
the default settings for the camera are to "balance" flash and
ambient lighting (what Nikon calls TTL BL). That's not
always what you want it to do, and there are things that can
keep the camera from succeeding at that.

Flash Basics

A flash produces a burst of light by pushing an electronic
pulse across xenon gas, which causes it to react
in response to a signal from the camera that it's time to
produce flash. For flashes with variable power ability, such as
those found in the Speedlight models and the internal flash,
the amount of light actually produced is determined by when
the electrical signal to the xenon is shut off:
• When a flash fires at full power, it essentially gives
everything it has: the xenon gas responds as much as it
can and eventually decays to nothing. That takes about
1/1050 of a second on an SB-800.
• When a flash fires at less than full power, this is done by
stifling the xenon flash prematurely by removing the
electrical impulse. On an SB-800, for example, the "flash"
can be shut off in as little as 1/41,600 of a second (1/128
power).
In order to have any variability in flash output something has
to measure the amounts of light produced and make the
decision of when to shut the flash off. Either the D300 or the
flash itself can both measure and control the amount of light.
Yes, this means that the D300 has something inside it that
measures the light produced by the flash (the 1005-pixel CCD
139 I used to use the word "ignite," but technically that's not correct, so I've modified
my wording slightly.
Thom Hogan's Complete Guide to the Nikon D300
V1.02
139
. It does this
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