t> Basically the calculation is the same, the difference is that the
t> distance is from flash to bounce point to subject. The X factor is how
t> high the ceiling is, so I don't think there's any rule we can give you.
Is that true? The normal calculations works for spread of radiation,
according to the square root rule (right in English?). Spread from
point source. But bounce flash is so nice because it isn't a point
source. Once it hits the ceiling, it radiates like from a plane
source, for which the formula doesn't work (it doesn't diminish with
the square root of distance). That's why bounced flash looks so
natural, too.
I remember seeing some formula, but there would be much more error
than in the simple direct-flash formula. For bounce, flashmeter or
auto/ttl metering is imho best.
Good light,
Frantisek
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