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The Phoblographer Explains: How to Use the Guide Number of a Flash

Chris Gampat
No Comments
03/23/2015
2 Mins read

Kevin Lee The Phoblographer Strobies Pro-Flash TLi-C Product Images 2

It’s no real big secret that flashes come with varying amounts of potential power output. Large monolights can do one thing and hot shoe flashes can do another thing. But in general, it can be tough to tell how strong a flash is.

To do this, manufacturers use the rating of a guide number. We’re going to use the Canon 600EX Rt flash as an example.

To figure this out, we’re going to need to do some math.

The guide number is found using the following equation:

Aperture (X) x Distance (Y) = Guide Number

According to B&H Photo’s listing, the guide number of the Canon 600EX RT is 197′ (60.05 m) ISO100 at 200 mm position. So where does all this extra info come from?

– The flash has a zoom head that goes all the way out to 200mm.

– To do this test, they’re shooting at ISO 100.

So in order to figure that out, we’re going do a bit of math. We have the Guide number which in this case is 197. But let’s say that we wanted to fire our flash at f5.6. In that case our equation would look like this:

5.6 x Y = 197

In this case, Y is the distance and we’re trying to find out the distance that a subject will have to be for proper illumination. In a case like this, you’re doing simple algebra. If you divide 197 by 5.6, you end up with approximately 35.18 feet. That means that at a little bit further than 35 feet, you can shoot with your flash at f5.6 and get a proper illumination in the right conditions at ISO 100. The problem with this though has to do with the fact that everyone has a different idea of what a proper illumination is for a scene.

But by using this formula, you can figure out an approximation of how powerful your flash actually is. In general, the larger the number is the stronger the flash is.

canon distance flash formula guide number iso
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Chris Gampat

Chris Gampat is the Editor in Chief, Founder, and Publisher of the Phoblographer. He provides oversight to all of the daily tasks, including editorial, administrative, and advertising work. Chris's editorial work includes not only editing and scheduling articles but also writing them himself. He's the author of various product guides, educational pieces, product reviews, and interviews with photographers. He's fascinated by how photographers create, considering the fact that he's legally blind./ HIGHLIGHTS: Chris used to work in Men's lifestyle and tech. He's a veteran technology writer, editor, and reviewer with more than 15 years experience. He's also a Photographer that has had his share of bylines and viral projects like "Secret Order of the Slice." PAST BYLINES: Gear Patrol, PC Mag, Geek.com, Digital Photo Pro, Resource Magazine, Yahoo! News, Yahoo! Finance, IGN, PDN, and others. EXPERIENCE: Chris Gampat began working in tech and art journalism both in 2008. He started at PCMag, Magnum Photos, and Geek.com. He founded the Phoblographer in 2009 after working at places like PDN and Photography Bay. He left his day job as the Social Media Content Developer at B&H Photo in the early 2010s. Since then, he's evolved as a publisher using AI ethically, coming up with ethical ways to bring in affiliate income, and preaching the word of diversity in the photo industry. His background and work has spread to non-profits like American Photographic Arts where he's done work to get photographers various benefits. His skills are in SEO, app development, content planning, ethics management, photography, Wordpress, and other things. EDUCATION: Chris graduated Magna Cum Laude from Adelphi University with a degree in Communications in Journalism in 2009. Since then, he's learned and adapted to various things in the fields of social media, SEO, app development, e-commerce development, HTML, etc. FAVORITE SUBJECT TO PHOTOGRAPH: Chris enjoys creating conceptual work that makes people stare at his photos. But he doesn't get to do much of this because of the high demand of photography content. / BEST PHOTOGRAPHY TIP: Don't do it in post-production when you can do it in-camera.
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