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

Started Feb 18, 2013 | Discussions thread
ktownbill Senior Member • Posts: 2,191
Re: Flashgun Guide Number (GN) and zoom-head setting relation

Cyclic wrote:

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.

The Canon Speedlite 580EX II Flash, (compare prices) (review) has all the features of the 430EX II, but adds a significant number of new functions. The 580EX II can act as a master controller in a wireless flash system, it can be powered by an external power pack for faster recycling and more shots than using the internal “AA” batteries. It has a PC socket for wired operation off camera and a stroboscopic flash option. It also has a small built in “bounce card” which reflects a small amount of light forward when the flash head is tilted up for bounce flash operation.

The 580EX II has more power then the 430EX II and the guide number (m @ ISO 100) varies with zoom setting as follows:

  • 14mm GN=15
  • 24mm GN=28
  • 28mm GN=30
  • 50mm GN=42
  • 70mm GN=50
  • 80mm GN=53
  • 105mm GN=58
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