Help with guide numbers when using a flash manually

Started Apr 14, 2013 | Discussions thread
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Vito_F
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Help with guide numbers when using a flash manually
Apr 14, 2013

Hi,

am trying to use my canon 420 ex flash with a newly purchased Panasonic fz 200  as I cannot afford a compatible flash at this time.

the 420 is a zoom flash, according to the manual with guide numbers from 23 (at  42 mm?) to 42 (105mm). There is a note that it is for ISO 100 meters.

I am totally confused. Can someone break this down for me into feet and tell me how to calculate the f stop for let's say 4 to 25 feet? There is a chart in the manual

http://www.usa.canon.com/app/pdf/lens/Speedlite_420EX_manual.pdf

thanks,

vito

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