Confused regarding flash Guide Numbers

2 months 2 weeks ago #673093 by Liem Stailey
Good morning guys.  I'm shopping for an external flash and these guide numbers have me running around in circles.  When a flash has a guide  number of XXX, what in the world does this number mean?  Im a beginner and at least for me, this seems complicated.  LOL I just wanted  a flash to use while practicing my portraits.

When you are shopping around for a flash. Can I ask what are you looking at aside from price point?

You all are awesome, thank you in advance!


P.S.  if this basic question, just something I don't  understand!!


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2 months 2 weeks ago #673099 by Shadowfixer1
Guide number relates to output. The larger the guide number the more power it has which means you will get more distance to the subject to work with. The more power also means you can shoot it at a lower power when closer to the subject which gives you more control.

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2 months 2 weeks ago #673107 by Ozzie_Traveller
G'day Liem

"yes" the GN as quoted relates to the power of the flash and it is made up from a simple calc of "Aperture x Distance" at a specific ISO, with 100-ISO being the industry standard

So for a GN of 80 say, it suggests that F8 will expose correctly at 10Ft if the GN is quoted as GN-80 (ft). It may also be quoted as GN-24(metres) as F8 x 3metres is the same as F8 x 10Ft

Hope this helps
Phil from the great land Downunder
www.flickr.com/photos/ozzie_traveller/sets/


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2 months 2 weeks ago #673124 by Liem Stailey
OK, thank you both.  I'm better now and have a clearer idea about guide numbers.  Thank you!


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2 months 2 weeks ago #673168 by fmw
Back in the day, flashguns weren't adjustable.  Their power output was full power.  The guide number was used to calculate the correct aperture based on subject distance.  Today, flashguns are adjustable by changing the flash duration so the unit can expose the subject properly itself based on the aperture you choose and the distance.  What that means is that you don't need to worry about what the guide number of the flash unit is at full power.  You can buy the most powerful one you can afford and it will still work like a smaller flash unit when required.  If you your camera has TTL (through the lens) flash be sure to choose a flashgun that is compatible with your camera's TTL system


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