Since the F280 is a continuous light source with a single power
setting, it seems to me that we need to know the illuminance at
a standard distance. I would say the spec we need is:
The distance at which the F280 equals "sunny f/16"
That's all: its strength as a continuous source relative to sunlight.
Shutter speeds, ISOs, apertures, filters,...nothing else matters.
Since it's continuous, we only need to know how strong it is relative to
other sources.
I haven't seen anybody give the number, but from the curves given, it looks
like about 0.75m. That means for a daylight -2 stop fill, the flash should
be at 1.5m. Yuck. That's worse than the T20. The F280 has a higher T mode GN
than the T20, but in F mode it wastes most of its energy storage on shutter
curtains.
Other calculations would be semi-easy, too. If you are indoors, determine
how far ambient light is below sunny f/16, then determine from the main
subject distance the same number for the F280. The difference will tell how
far below the subject a distant background will be.
Paul Farrar
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