Help with guide numbers when using a flash manually

Started Apr 14, 2013 | Discussions thread
DUSTY LENS
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Re: Help with guide numbers when using a flash manually
In reply to Vito_F, Apr 14, 2013

Vito_F wrote:

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

I do not understand the numbers you give ( (( 23 (at 42mm )) ) or the (( 42 (105mm) )) .

Very seldom would a guide number be given for 105mm or for 42mm .

and 42 meters would be way out there about ( 150 feet ) Nor a very probable guide number either .

You could find the real Guide number with test shots .

Try 10 feet and work backwards by changing the aperture on the lens until you have a perfect exposure .

Then you would have the two numbers you keep for finding the correct exposure .

Guide numbers usually are given for a certain ISO and at a certain distance .

ISO 100 at ( what distance )  would equal a guide number .

Read it again .

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