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Thom Hogan’s Complete Guide to the Nikon D300
Page 595
in the viewfinder is used for this job
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). Like the ambient
exposure, the metering CCD must get its measurements
before
the picture is taken, which is why you’ll see references to
something called the preflash
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. Why before the exposure?
Because the D300 has no way to measure the flash output
during
the exposure (none of the Nikon digital SLRs do).
If you’re starting to think that there may be more “modes” and
settings coming, you’re right. Flash exposure is no different
than regular exposure: you have to set the camera/flash to do
what you want it to. And there are lots of options you need to
know about.
Digital Flash Differences
For 35mm film cameras, Nikon TTL
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flash sensors are
designed to look at reflections off the shutter curtain before
exposure
and
again off the film during exposure. But the
D300 doesn’t have any film, and the sensor doesn’t reflect
light the same way that film does, so this second exposure test
isn’t performed.
Nikon originally decided to modify its flash system slightly for
digital cameras to include a new flash “mode,” called D-TTL,
or Digital TTL. D-TTL is supported by the D1 series, the
D100, and the D2 series with the SB-28DX, SB-50DX, SB-
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Note that one way the D2 series and the D300 differ is that the D2 series has an
additional five-segment sensor in the mirror box, which looks at the shutter curtain.
This has subtle but real implications. The D300, for example, fires a preflash at the
first shutter release press when set to
M
UP
, the D2 series waits until the second, thus
putting the flash calculations closer to the actual picture taking.
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The name preflash is a little misleading. Most newcomers expect to always see a
separate flash from the main flash. But the preflash usually occurs so close to the
actual flash that you usually don’t distinguish it from the main flash. If you don’t
believe me, set your D300 to
M
UP
with the flash popped up and in a TTL mode. Press
the shutter release. The mirror goes up, but before it does, the camera fires the
preflash. Press the shutter release again. The curtain now opens and the flash is fired
at the power calculated by the preflash. At any shutter speed faster than 1/8 with the
camera in a normal frame rate method (
S
,
C
L
,
C
H
), the preflash and flash are close
enough together that many people can’t distinguish them.
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Just a reminder: TTL stands for Through The Lens. Flash measurements are
performed
by the camera
looking through the lens. In theory, this is the most accurate
flash capability.
