[philip5cf]

just learn it from the canon expo, apeture x distant, = index. the bigger index mean it can travel further. I don't think you put more flash mean you can have a better photo, flash have a limited, mean 5.6/F + 125/1 is enough, you using double flash then you have to minus DOWN. correct me if I wrong. thanks

[oozz]

It is too difficult to calculate with limited info.
Even you know the GN of every flash, still there are many things ssuch as the firing angle of individual flashs preventing you from calaulating precisely the exposure. Better use a Flash meter for multi-lighting. If no meter available at the moment, just try bracketing. For an ideal case with 2 identical flashs firing at the same angle and same distance to subject, temp down one stop for the combo. That is combined GN = 1.414 x GN of each one.
hmmm.. just see the result on your display if you use a digital gear.
hope this helps.

:)

[micros]

都係用 meter 度好。
廠俾出o黎o既 gn 未必準﹐用舊o左又會差o的。
理論上多燈總 gn o既計算係:

2枝燈 分別係 A 同 B
GN = ( A^2 + B^2) O既 開方

如果再多﹐就 A^2、B^2、C^2 ...... 一路加上去﹐之後將總和開方。

粗略計下都可以﹐要準就唔包。