Home How-To Helpline July/August 2007 HelpLine
Monday, June 25, 2007

July/August 2007 HelpLine

Light Matters




Illuminating Flash Guide Numbers

Q) I'm considering purchasing an external flash for my camera, but I'm not sure what the guide number is. Should I really be concerned about it?

Mark D.
Via the Internet


A) Cameras can be heavy, especially when you add a longer lens to them. An obvious key to keeping images sharp is keeping the camera still. Last month, I gave you a way to practice using a light touch on the shutter release. But if you can't hold the camera still in the first place, a soft touch won't help.

Simply put, the guide number is a measure of the flash's strength. To compare the light output of each unit, the guide number or GN specification is used. If you're concerned about having a strong flash that can throw light great distances, then you need to be aware of the guide number.

The GN specification is given with an ISO setting and a unit-of-distance measurement, as in "this flash has a GN of 60 at ISO 100 (as measured in feet)." This allows you to plug the guide number into an equation to determine correct exposure. The formula is: GN divided by distance = lens aperture. "Distance" in this case is the distance from the flash to the subject, not the distance from the camera to subject.

With the flash example above, if you had a distance of 15 feet, the aperture setting would be ƒ/4 (60/15 = 4). From this formula, you can take the simple idea that the larger the guide number, the more powerful the flash. It's up to you to determine how much power your budget can afford.


Thanks, Readers!

Before I close out this issue's HelpLine, I'd like to thank all of the readers who have sent in questions-keep them coming. If there was enough time in the day, I'd love to answer all of your questions personally. Unfortunately, I can't, so I apologize if your question has yet to be answered. Please note, however, that I also answer a question each week by clicking HelpLine.


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