Help with guide numbers when using a flash manually

Started Apr 14, 2013 | Discussions thread
OP Vito_F Regular Member • Posts: 178
Re: Help with guide numbers when using a flash manually


Vito_F wrote:


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



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 .

Hi Dusty,

Thanks for the suggestion, I will try it but I was looking for help with interpreting the GN chart from Canon. I did include the link but will add the chart at the end of this message.

What is confusing me is the statement about ISO in meters, the "Flash coverage in mm" and the how to use the different GN by distance.

The 420 EX is a zoom flash, although I am not using the TTL as it is not compatible with the camera.

Are these different GN in the chart based on the focal length I am using with my lens? Or is it because I am not using TTL I use one GN and stick with it?

Totally confused.



420 EX GN

Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow