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flash guide number

TTL is probably fine for my purposes but would like to understand more about guide numbers anyway. So, two questions:
- Is using the guide number ever relevant/useful when bouncing flash?
- The GN in my speedlight's manual is only given for 35mm zoom head. If I am using an 85mm lens, how (if at all) does this affect the guide number formula? (I realize that with my speedlight - Nikon SB-5000 - I can manually adjust the zoom head; I'm just wondering about the effect on the formula.)
Thanks much.

Comments

  • Neil has this very nice article on how to use the guide number of your flash. It includes answers to most of your questions: https://neilvn.com/tangents/tutorial-how-to-use-the-guide-number-of-your-flash/
  • Neil vNNeil vN Administrator
    edited December 2017
    With bounce flash, the Guide Number loses much of its relevance. The GN is based on the quantity of light being known, and the distance being known. When you bounce the flash, the distance is unknowable by the flash, and you also lose light when it is reflected by the various surfaces. 

    This is where TTL is most often the faster way to work. 

    The flash zoom head will only marginally affect the flash exposure, even if the zoom head affects the effective power. You can check this for yourself by bouncing manual flash and seeing how the exposure changes when you zoom from 24mm to 85mm (for example.)
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