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Can someone tell me whether the GN on strobes in accumulative?

I have a pair of YS90 which are both rated as GN20. I am trying to work out the maximum aperture that I have get away with for macro shots in a dark envoirnment. Is it as simple as saying 2x20 @ 1.5m gives me about f24.

If they are not accumulative then is their a way of working it out other than trial and error?

Thanks

Daniel

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The light from the two strobes will be accumulative, although the amount of accumulation will be dependent on how much the two strobes overlap. If both strobes were on 100% output and both covering the exact same area, you would be getting twice the amount of light as you would with one strobe.

One difficulty here is that guide numbers are used for distance and f/stop calculations both of which are affected by the inverse square rule. What I mean here is that if you were to double the light output of a strobe, the guide number only increases by the square root of two. e.g. if the GN is 20, doubling the light gives you a GN of 28 (20 x 1.414).

So, if you were using both strobes at 100% and they exactly covered the same area, you could assume a GN of 28 and at 1.5m you'd have f/18. In reality it will probably be less than this as they won't overlap exactly. Maybe start at f/16 and see how that goes.