{"id":10192,"date":"2012-03-08T04:49:16","date_gmt":"2012-03-08T08:49:16","guid":{"rendered":"https:\/\/neilvn.com\/tangents\/?p=10192"},"modified":"2020-06-13T19:19:32","modified_gmt":"2020-06-13T23:19:32","slug":"tutorial-how-to-use-the-guide-number-of-your-flash","status":"publish","type":"post","link":"https:\/\/neilvn.com\/tangents\/tutorial-how-to-use-the-guide-number-of-your-flash\/","title":{"rendered":"Tutorial: How to use the guide number of your flash"},"content":{"rendered":"

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Tutorial: How to use the guide number of your flash<\/h1>\n

GN \u00a0= \u00a0distance \u00a0* \u00a0f-stop<\/p>\n

Your flash’s Guide Number (GN) is determined at 100 ISO, when it gives correct exposure at a certain distance, multiplied by the f-stop<\/p>\n

The idea that we can figure out the manual flash exposure by the combination of distance and aperture (for a given ISO setting), was covered in these recent topics:<\/p>\n

–\u00a0getting the most power out of your flash \/ speedlite \/ speedlight<\/a>
\n–\u00a0practical tutorial: controls for manual flash exposure<\/a><\/p>\n

In these articles, we relied on the display on the back of the speedlight to show us the distance we need to hold the flash from our subject. A flashmeter \/ lightmeter would’ve given us a similar answer. (There might be a discrepancy, since the manufacturers tend to be a little bit optimistic about what the flashgun is capable of.)<\/p>\n

Now, the question is, what do we do if our speedlights don’t show the distance \/ aperture relationship on its display, or if we don’t have a lightmeter on hand?<\/p>\n

<\/p>\n

 <\/p>\n

Using the guide number to calculate flash exposure<\/h2>\n

Well, if we want to be more methodical than just looking at the back of our camera display, we can use the Guide Number of our flash.<\/p>\n

Keep in mind that the GN is given for a specific zoom setting on the flash-head. Zooming from wide to tele on our flash, changes the Guide Number!<\/p>\n

 <\/p>\n

The spec for the Nikon SB-910 speedlight<\/a>\u00a0is given as:<\/strong><\/p>\n

Guide number of 34\/111.5 (at ISO 100, m\/ft., 35-mm zoom head position, in FX format, standard illumination pattern) for high flash output volume<\/p><\/blockquote>\n

This means that our GN (in feet), is 111.5
\nSo if we are using our flash at full power, at say 10 ft from our subject, then we have to use an aperture of 111.5 \/ 10 … which gives us an aperture of f\/11<\/strong><\/p>\n

 <\/p>\n

The spec for the Canon 600EX-RT speedlite<\/a>\u00a0is given as:<\/strong><\/p>\n

Guide Number: 197′ (60 m) at ISO 100 … for the flash-head zoomed to 200mm
\nGuide Number: 118′ (36 m) at ISO 100 … for the flash-head zoomed to 35mm<\/p><\/blockquote>\n

The GN of 118 is close enough to the Nikon’s that the explanation is the same for 35mm flash-head zoom. For the flash zoomed to 35mm, the aperture would be 118\/10 = f\/11<\/strong><\/p>\n

(If your Math insists it isn’t f\/11 keep in mind that the next 1\/3rd stop setting down from f\/11 is f\/13)<\/p>\n

Working at 200mm zoom setting for the flash: \u00a0if we are 20 feet away from our subject, then our aperture is 197\/20 = f10<\/p>\n

 <\/p>\n

Simple as that.<\/strong> The math isn’t scary. We all have smart phones with us, and they all include a calculator.\u00a0If you want all of this in a handy form on a website, here is the Flash Power Calculator<\/strong> on Canon Europe’s site.\u00a0And if you want to read up more about Guide Numbers for flash, the Wikipedia entry<\/a> is thorough.<\/p>\n

Now the question remains … what is the Guide Number of your<\/strong> flash?
\nTime to find that instruction manual again! But firing up Google might just be easier.<\/p>\n

 <\/p>\n

Related articles<\/h2>\n