flash guide number

flash photography: applying the Sunny 16 rule and the flash Guide Number

In essence, if you know the GN of your flash, then you could use (bare) off-camera flash to match the sunlight, without even metering!

There is a super-useful shortcut built into those two simple values: Sunny 16, and the Guide Number.

Now, I am pretty sure that when you hear mention of the Guide Number of a flash, you’re most likely switching off already, thinking that it is just an arcane list of numbers – different apertures against different power settings. But hang in there – this is very useful stuff to have a grip on.

And yes, since: GN  =  distance  *  f-stop
that is what the Guide Number tells you – the distance multiplied by the aperture is the GN.

But there is something immediately useful there in the Guide Number, which is hugely important. If you understand this, then you have an important key in your pocket about how to quickly match bright sunlight with your speedlight. It’s really simple:

Learn more inside…

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using the guide number of your flash to determine flash exposure

GN  =  distance  *  f-stop

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

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:

– getting the most power out of your flash / speedlite / speedlight
– practical tutorial: controls for manual flash exposure

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.)

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?

Learn more inside…

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