Flashgun Guide Number (GN) and zoom-head setting relation

Started Feb 18, 2013 | Discussions thread
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Cyclic
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Flashgun Guide Number (GN) and zoom-head setting relation
Feb 18, 2013

Flashgun manuals provide information about its guide number. For different zoom settings (at ISO 100) and different flash power level, the manuals provide guide numbers.

Is there any way (formula, or any relation?) to find GN for different zoom settings given that it is available for one zoom setting? For example, if the GN is, say, 70 m (1/1, ISO 100, 105 mm), then how does one calculate GN of the same flashgun at 28 mm zoom setting at full power, ISO 100.

I tried to understand this relation, but could not get a clear description of this anywhere.

Any help will be appreciated. Thanks!

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