On Sunday, February 9, 2003, at 02:47 AM, John A. Lind wrote:
At 22:13 2/8/03, Mike wrote:
> ...and I don't understand how a ND filter over the flash unit would
not be
> 'overridden' or 'ignored' by the OTF metering - or am I missing
something?
>
The flash was for fill only using Super FP mode. My thought was that
the
OTF metering would make up with natural light the light from the flash
that was blocked by the ND filter. i.e. a longer exposure time. This
would give more weight to natural light without the calculations. Say
a
1:3 fill ratio. The situation was bright low angle sun, harsh shadows.
Shutter speeds faster than 1/60 were desirable. Crowded, which meant
getting in close with a wide angle lens for groups of 3-5 persons.
Shots
i took in Super FP mode were way too heavily weighted to the flash. I
thought that a ND filter would have the effect of lowering the curve
of
the Working Distance Guide in the manual meaning less light from
flash.
Hope that makes it more clear.
mike
I don't believe so. As long as the flash does not "max out" it will
simply dump more energy to the xenon tube. The TTL/OTF metering
determines when the flash will quench . . . and that's when the
TTL/OTF metering sensor inside the camera body finally senses it has
enough light for the exposure.
The biggest use I'm aware of for ND "filters" over flash units is to
reduce their power output and reduce their *minimum* working distance
for macro photography when the flash unit is often placed much closer
to the subject material than for general photography.
BTW, all flash units that use a "quench" to shut them down have a
minimum working distance in addition to the maximum one. The basic
limiter on minimum distance is minimum flash duration. You wouldn't
want a duration shorter than about 1/8,000th second anyway. Most
people think of reciprocity failure as only applicable to long
exposures . . . but there is a limit at the short end too . . .
typically about 1/10,000th second . . . and if a flash is providing
the overwhelming amount of light its duration effectively becomes the
shutter speed.
-- John
That is true of conventional TTL flash, but Super FP mode does not work
that way. The camera metering system does not quench it. It is
constant and exposure is controlled by variations in the shutter speed.
Winsor Crosby
Long Beach, California, USA
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