At 03:31 9/9/02, Tim wrote:
>Therefore, for an "FP" flash to work, it must always have
> the same, long burn duration, regardless of shutter speed.
I think you are assuming that the flash remains ON for the entire time it
takes the slit to traverse the focal plane. That may not be true. I
remember reading somewhere that in super sync mode the flash output is not
continuous but a series of pulses. How many pulses would that be.. If we
divide the length of the rectangular focal plane by the width of the slit
- that gives us the number of slits that can be fitted all at once on the
plane. And since we want illumination in 'all of the slits' there will
have to be so many flash pulses. In other words, the flash fires every
time the window moves a distance equal to its width.
[snip]
With the FP flashbulbs, it was one continuous burn. You're right that an
electronic one cannot do that. My description of a "long burn duration"
with the F280 and other, similar flash units is the series of pulses you
describe. They are of very short duration and spaced very close
together. It has the same effect as a continuous burn. Look at the chart
showing effective GN for Super FP mode at shutter speeds from 1/60th to
1/2000th (meters; ISO 100):
1/60 14.7
1/125 10.4
1/250 7.3
1/500 5.2
1/1000 3.7
1/2000 2.6
The sequence for a one-stop increase in shutter speed (half the light) is a
reduction in GN by a factor of approximately the square root of two
(~1.414) and rounded to the nearest decimeter (10cm). This is exactly what
would be expected with F280 light output remaining the same, regardless of
shutter speed selected.
-- John
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