LATEST: Newly expanded, updated Strobist Gear Guide.

Friday, December 21, 2007

Guide Number: Your Free Flash Meter

Pop quiz:

What f/stop will your flash give you through an umbrella at ISO 100 on 1/4 power at ten feet?

Not sure? Read on...

Guide numbers are basic, core, old-school flash photography knowledge. And I can all but hear the old-timers rolling their eyes and saying, "Gee, Dave, whaddya gonna tell us about next, bounce flash?"

But we have gotten so far from this kind of thinking, what with TTL everything, eTTL and CLS, that many people have never had occasion to think about guide numbers. Which is a shame, really, because a working knowledge of guide numbers (GN) can do something very cool: Get your exposures in the ballpark on the very first test shot.

Find out how helpful -- and easy -- using GN is after the jump.
____________________________


And Now, a Word From the Math Department

Okay, let's start with the definition. The guide number of a flash is the product of the f/stop of the exposure at a given distance at ISO 100. Wikipedia has a whole page on it here. But that's all pretty math-centered and kinda inverse-squarish. And you really do not need to know that stuff to use GN to zero in on a first exposure.

Here's how to use it. On just about any hot-shoe strobe capable of manual, there's a guide number calculator built in. The photo up top is from a Vivitar 285HV, which is a 70's-design manual-capable flash. Let's use this as an example.

The four variables are: F/stop, distance, power and ISO. You plug in any three, and the calculator spits out the fourth. For instance, click here to open the 285 GN dial in hi-res in a new window.

This GN calculator is set for full power manual at ISO 200. It is telling you that, at 30-40 feet, you would get about f/4 out of this flash. And if you set the flash to 1/2 power, you'd get f/2.8 out of it at that distance. Here's the cool thing: If you zoom the head -- even on this old-design flash -- it will move the dial and adjust the result.

Here is an example of more modern guide number calculators. Click here to see it big in a separate window. This display (from an SB-800) is typical of modern GN calculators. Every flash is a little different, but they all work pretty much the same way. One thing to remember: The GN calculator display will only pop up if the flash is at it's direct flash setting, i.e., no bounce. This, for instance, is telling me that I will get f/5.6 at ISO 400 on 1/4 power at 26 feet at the 50mm zoom setting.

Play around with your buttons a bit and you will see how yours works. What I like to do is to already know my ISO, my desired shooting aperture and an estimated flash-to-subject distance. Now, by setting up my GN calculator, I just dial in the different manual power settings until my desired f/stop lines up with my flash-to-subject distance.

So, this is pretty cumbersome and slow and why should you even bother, right?

Well, I'll tell you. The first five or six times you have to dial in your GN calculator and learn where you need to set your flash to get f/4 at, say 10 feet, you'll need the calculator to tell you. Then all of a sudden one day, you'll just know.

And if you learn a good anchor point, like say 1/8 power at 50mm zoom, 13-foot distance at ISO 400 = f/8 (exactly what an SB-800 will do at those settings) you'll quickly start interpolating around that to suit your given situation. And then, you really do not even need your GN calculator to get close on the first test.

"But wait," you say. "I'm a hot shot. I use snoots, grids, umbrellas and gels. Won't that screw it up?"

Yeah, maybe. But consistently so. A snoot will be dead-on with the expected aperture in the center of the beam. Your grid spot might knock it down half a stop. Your shoot-through umbrella might knock it down two stops -- but the difference will be the same, every time. Once you learn how much your umbrella knocks your bare flash down, just adjust for that. Ditto for the gels. A typical full color conversion gel knocks about 2/3 of a stop off of your flash.

If you are having trouble getting a handle on your early test pops -- you know, like getting first shots that are about 5 stops off -- this will zero you in real close, real fast. In fact, there is no faster way to save time on your first test pops than to learn to use your GN calculator.

Until you don't need it any more. Then, you'll be faster yet.


__________

Brand new to Strobist, or lighting? Start here.
Or, jump right into our free Lighting 101 course.
Connect: Discussion Threads | Reader Photos | Twitter

75 Comments:

Blogger Larry said...

I know the answer is gonna be "have you tried it yet?" But, it's 1:25am, I'm reading in bed, my camera is at least 15 feet away, and it would take too much effort to set up my light stand right now. So, I'll take the lazy route and ask, hoping to have an answer before I fall asleep. If no answer, I'll try it later. But here goes. Is it likely that bouncing into an umbrella will be pretty close to about 2 stops less than direct flash using the distance to the subject PLUS the distance to the umbrella? Or does the couple of feet difference not factor in much? I know, it'll be consistent, but just asking anyway.

December 21, 2007 2:30 AM  
Blogger Ken said...

Thanks, I'll have to try this out soon, like, immediately tomorrow morning. I always dismissed the aperture and iso setting on the flash unit as something for TTL function and whatnot. Good to know it's there for a (good) reason!

December 21, 2007 3:14 AM  
Anonymous Brian said...

Sweet post Dave! I just purchased an SB26 and have been wondering about the guide number mumbo jumbo. Thanks for answering all of my, unasked, questions.

December 21, 2007 3:48 AM  
Blogger J. Beckley said...

I was wondering how that dial worked on the side of the 285. Thanks for explaining.

December 21, 2007 4:09 AM  
Anonymous Anonymous said...

This is just.. brilliant!

And it works with the other variables plugged-in: I set a power output on the SB800, dial-in my desired f-stop and I get a read-out showing what distance the flash to subject should be [ball-park figure of course].

Thanks again, David, for showing the way.

December 21, 2007 5:02 AM  
Blogger Adler said...

Thanks a lot David, this will help me save time for sure.

December 21, 2007 5:08 AM  
Anonymous Eric said...

David,

Thanks for this informative post. I'm really just scratching the surface with this off-camera flash stuff (thanks to you) and working my way through your 101 series again. I would recommend adding this post to your 101 series due to the fundamental nature of this information. It certainly has helped my understanding and I'm sure will increase my efficiency. How about a follow-up on GN and multiple flashes?

Cheers!

December 21, 2007 7:55 AM  
Blogger Shaun Krisher said...

How does this relate to a light's specific guide number? an sb-26's GN is 36. How does that tie into these equations? I suspect that knowing this will help us photographers with a franken-kit of 285s and sb-26s and 28s and 24s and 25s and god knows what else!

Thanks, Dave!

My blog for new pro photographers

December 21, 2007 8:02 AM  
Blogger Vlad said...

Thanks very much for this tutorial. It is very helpful for someone like me who is only beginning to learn photography. I have an additional question about using multiple flash units: imagine using two flashes - the main one and the fill one. Metering from the main flash may give me the aperture of f/8, and the fill one f/4. What should be my working aperture? f/16?

December 21, 2007 8:12 AM  
Blogger Claude said...

Again and again, I leave this blog enriched with knowledge. I had frankly forgotten about GN. Thank you.

December 21, 2007 9:00 AM  
Anonymous Anonymous said...

Great post. Now if someone will help me with my SB600, I'll be happy as a cheshire cat. Anyone knows how I can activate the GN calculator on the SB600?

December 21, 2007 9:03 AM  
Blogger Frank Philip Photography said...

I wish I had my SB-600 with me right now as I'm at work, but to my knowledge I don't believe it has the options like the Vivitar 285 and SB-800 or SB-26. There's a manual setting but not as customizable. Am I right, or do I not love my flash enough to know?

December 21, 2007 9:27 AM  
Anonymous brock said...

Now that is a tasty little post, my friend. Talk about "news you can use," that really hits the spot. Like so many others, I've always blown off the GN as "old school," what a mistake.

December 21, 2007 9:50 AM  
Blogger Larry Eiss said...

Great post, thanks! Like others here I had been wondering what all that information was there for and how to put it to use. I'm hoping to work on this over the time between Christmas and the New Year as I'll be off from work. I'd love to see more information on using this with bounce, etc.

December 21, 2007 10:05 AM  
Anonymous Matthew McMullen Smith said...

Nice post. I recently dusted out the corner of my brain that contained this information for my trip to Europe. I wanted to travel light and am still on the fence about a good compact digital so I brought my Contax G2 rangefinder and a sb-24 with a cord to fire it off camera. I utilized the guide number settings on the back and a little strobist knowledge to use my flash as fill and sometimes a main light at night. One of these days I'll post the results which were quite pleasing.

December 21, 2007 10:13 AM  
Blogger Alan said...

the distance measured for the guide number is the total distance the light travels. So if you are bouncing the light off a perfectly reflective ceiling, the distance will from the flash to the ceiling and then to the subject. For the previous poster, yes, the distance is from your flash to the umbrella and then back to the subject. Remember that the ceiling or umbrella is not a perfect reflector so you will lose a stop or two...
another trick is just to memorize your exposure at 8 feet. your exposure will be one stop less at 11 feet, two stops less at 16 feet, and conversely one stop greater at 5.6 feet, two stops greater at 4 feet... do you see where I am going?

December 21, 2007 10:58 AM  
Anonymous jpc said...

The strobe will display the same guide numbers even if it's pointed in the opposite direction, if you are metering through the lens.

Not to speak for DH, What i got from this is; there are ole new skool ways to develop consistency.

Deeper understanding of multiple lights, distance reflectivity comes with time and experience (practice)... Guide numbers are just a great base or "constant" from which to build on. Being able to chimp an image and it's histo instantly without reverting to a polaroid back or waiting for processing $$$ and keeping notes. The amount of time saved is also good biz practice for you and your client... not to mention, it can be fun to learn new old stuff...

now how's about a post on flash metering,

Thanks,
j

December 21, 2007 11:45 AM  
Blogger David MacVicar Photography said...

Maybe I am missing something

But with my 580exII off camera, it doensn't show me the guide numbers. The manual states that you'd have to half press the shutter to find this information out on the back of the flash...if my 580ex II is off camera, how do I figure this out?!!

December 21, 2007 11:45 AM  
Blogger Don Sweener said...

Get a light meter, MEASURE it . Tie a string to your light stand and mark the tape. Its all about the MATH . We all need to go back and read ablout the LIGHT ratios too.

December 21, 2007 11:50 AM  
Blogger kim said...

Thanks for this post! I just got my vivitar flash a week or so ago, so nice timing ;) This will help me out a lot.

December 21, 2007 11:58 AM  
Blogger Jeff the Geek said...

Talk about great timing... I am brand spanking new at lighting having done my best with available light until now. I, literally, ordered two of the Starving Student Wireless Kits from MWPX yesterday (sans wireless... I ordered that from GI). The kits have the Vivitar 285HV, and I was wondering how that dial thingy on the side worked. Wicked cool. I can't wait for the kits to arrive so I can drive my wife crazy with learning this stuff (she's my unwilling model).

December 21, 2007 12:03 PM  
Anonymous Anonymous said...

Now what about Foot Candles or Jules? Isn't there an easy way to covert guide numbers to watt seconds using the SWAG method? I love the hisorical significance of the techniques, but come on, most flash meters use maybe one double A and are well worth the price. I'm with the Sweener, meter, string and move on. Sorry to be a sarcastic pain but.....

December 21, 2007 1:19 PM  
Blogger burns529 said...

I just received my 285HV's in the mail yesterday and used this method on my first shot without even knowing that you had posted this. I can say that even with estimating the distance that the dial got me in the ballpark. I was only about one stop too bright on my test fire. From then it was just getting the look I wanted.

December 21, 2007 1:40 PM  
Anonymous Adam said...

thanks for this bunch of info
I really had no idea how to work guide numbers up until now
This site is awesome , and thanks for not forgetting about us new guys who are having a ton of fun learning

December 21, 2007 1:47 PM  
Blogger lbattist said...

Blogger David MacVicar Photography said...

Maybe I am missing something

But with my 580exII off camera, it doensn't show me the guide numbers. The manual states that you'd have to half press the shutter to find this information out on the back of the flash...if my 580ex II is off camera, how do I figure this out?!!


David, set your camera to EXTERNAL MANUAL metering. To do this, you need to enable Custom Function to "3". Hold down the C.Fn function until you can adjust your custom functions, and then change the 5th function to "3". After that, your "E-TTL" mode is going to change to "E". It will display ISO, and f/stop information. The distance calculated is going to be in the bottom in terms of meters.

December 21, 2007 1:52 PM  
Blogger lbattist said...

Also, change the first custom function to display feet.

December 21, 2007 1:58 PM  
Blogger Larry said...

Alan, Thanks for confirming the distances to bounce surface and back to subject are a factor.

For all the SB-600 users, it doesn't have the "calculator" capability. But, It does have all of the information you need to do the calculations in a table for reference. I think I'll be laminating a shrunken copy for myself here soon.

December 21, 2007 2:07 PM  
Anonymous Anonymous said...

vlad said; "Metering from the main flash may give me the aperture of f/8, and the fill one f/4. What should be my working aperture? f/16?"

i am wondering this as well and getting out my 1960s copy of photo math stuff my grandather gave me... he had it all memorized. allll of it...

December 21, 2007 2:54 PM  
Anonymous Ray said...

Heck ya, this has to be added into Lighting 101.

Great post. You have to pour a foundation before you can make a skyscraper!

December 21, 2007 3:06 PM  
Blogger lbattist said...

Here is a guide for 580EX II users:

http://www.usa.canon.com/dlc/controller?act=GetArticleAct&articleID=946

December 21, 2007 3:19 PM  
Anonymous Jens_H said...

well, i'm learning soemthing new each day. can't tell how much i enjoy reading your posts david.

thanks a lot!

jens

December 21, 2007 3:30 PM  
Anonymous Pamela Vasquez said...

I have a 580EX...not the EXII...I just checked the manual and it doesn't seem to have that option that the EXII has. Is this possible? Man...now I see why people complain about canon!

David....you are awesome!
Pam Vasquez

December 21, 2007 4:39 PM  
Anonymous Anonymous said...

Thanks Ibattist for the Canon info!
Debbi

December 21, 2007 4:59 PM  
Anonymous Mickey said...

OK, I have three 285's and plan on getting a forth for pur manual/ivitar auto use as it works no matter how many lights are used. I will get atleast one 580 for ETTL2 sometime next year. So how do you use the guide no of multiple flashes to determine what Fstop to use. I under stand using ratios and the two main ways to adjust light on the subject, a) move light further away to decrease or B0 turn amount of light down. If all lights are equal then how do you figure out fstop? also do a demo of this please using two and three light setups, I think between the setups single, dual, and three strobes everyone should be able to tell the setup for any number of light setups. Also is the distance the distance between the Flash and the subject or the subjet and the camera, I beleive it is the flash and the subject but then doesn't the distance between the subject and the camera play some factor? Is that where the sutterspeed plays in.

December 21, 2007 6:16 PM  
Anonymous Mickey said...

OK, I have three 285's and plan on getting a forth for pur manual/ivitar auto use as it works no matter how many lights are used. I will get atleast one 580 for ETTL2 sometime next year. So how do you use the guide no of multiple flashes to determine what Fstop to use. I under stand using ratios and the two main ways to adjust light on the subject, a) move light further away to decrease or B0 turn amount of light down. If all lights are equal then how do you figure out fstop? also do a demo of this please using two and three light setups, I think between the setups single, dual, and three strobes everyone should be able to tell the setup for any number of light setups. Also is the distance the distance between the Flash and the subject or the subjet and the camera, I beleive it is the flash and the subject but then doesn't the distance between the subject and the camera play some factor? Is that where the sutterspeed plays in.

December 21, 2007 6:17 PM  
Anonymous Jerry said...

The flash guide numbers are based on the assumption that the light is a point source (for the inverse square law to work). Passing the light through an umbrella I think invalidates this. Of course the further the umbrella is the bigger opening you need, but not with an inverse square law relationship.

December 21, 2007 6:18 PM  
Blogger Bob Fowler said...

"The flash guide numbers are based on the assumption that the light is a point source (for the inverse square law to work). Passing the light through an umbrella I think invalidates this. Of course the further the umbrella is the bigger opening you need, but not with an inverse square law relationship."

Nope, the umbrella (or face of the soft box) becomes the light source. The law of inverse squares still applies, you're just dealing with another (usually lower) starting point.

December 21, 2007 8:43 PM  
Anonymous thanos said...

I don't know about the 285 calculator but to my Metz C-2s my impression is that the calculated distance is the MAXIMUM flash distance. For correct exposure the Metz manuals state that the subject should be at the middle third area of the distance between the flash and the subject. So, I guess different manufacturers define the calculated distance differently.

December 22, 2007 5:06 AM  
Blogger Don Sweener said...

WHY DOES ALL OF THIS MATTER!!!!
WE ARE ALL SHOOTING DIGITAL!!!
LAST TIME I CHECKED , WE DIDN'T HAVE TO PAY TO SEND OUT OUR FLASH CARDS AND HAVE PIXELS DEVELOPED.
SHOOT IT , "CHIMP IT " , WRITE IT DOWN FOR FUTURE REFERENCE!!!
DIGITAL IS FREE. LET THE PIXELS FLY WHERE THEY MAY !!!

December 22, 2007 10:55 AM  
Blogger David said...

Don,

This s a way for your first shot to be in the ball park. It will also quickly allow you to be more intuitive about your flash's power settings.

Just because you have lots of bullets doesn't mean it's not a good thing to have a sight on your gun...

-DH

December 22, 2007 11:34 AM  
Anonymous Anonymous said...

Clear and interesting. As usual.

December 22, 2007 12:26 PM  
Anonymous MikeD said...

So after setting my Canon 580ex II to EXTERNAL MANUAL metering, the way I do FEC is to increase or decrease the ISO or the Aperture? That sucks! Thinking of switching to Nikon or just use Nikon flashes.

December 22, 2007 10:26 PM  
Blogger lbattist said...

^^ Yeah... I started with Canon because that's what the people bought at work for me. I should have stuck with Nikon... or anything else for that matter.

December 23, 2007 1:00 AM  
Blogger Don said...

I did a quick Google search to see if some one had mad a manual GN chart for the sb-600 .
I found this one .why reinvent the wheel.
http://swannman.wordpress.com/2006/06/03/howto-sb-600-flash-distance-guide/

December 23, 2007 9:13 AM  
Blogger Q Master said...

I guess this is great to know but if I use my flash in iTTL more with CLS I can specify the output for each flash unit the my D80 will fire!

The SB-600 manual is here
http://www.nikonusa.com/pdf/manuals/Speedlights/SB-600.pdf

December 23, 2007 11:30 PM  
Blogger Ben Roberts said...

Another reason why this all matters when we're shooting digital is -- doesn't it just make you feel damn cool to be able to nail your settings within a stop or half stop the first time around?

I know when I happen to hit it, it makes me feel great. And that's worth something that no number of test pops can buy. :D

Thanks for a great blog, and another useful post.

December 26, 2007 2:40 PM  
Blogger Tim Sohn said...

so it sounds like with a 580EX II the flash will determine the amount of light output as long as you give it the ISO/fstop. I guess you can't learn what power it's going to output at? Bummer.

December 28, 2007 1:38 PM  
OpenID photodomo said...

Thanks for the post. I found another resource on guide numbers that was also good, though some of the animation on the tutorials is hokey.

The site offers an excel file to calculate guide numbers, f-stop and maximum flash range. Pretty cool.

dowload GN calculator

page is here

December 30, 2007 2:49 PM  
Blogger silvicone said...

This is just.. brilliant! Thanks for this informative post. I'm really just scratching the surface with this off-camera flash stuff (thanks to you) and working my way through your 101 series again. Again and again, I leave this blog enriched with knowledge. I had frankly forgotten about GN. Thank you. http://toatechestiile.ro

March 12, 2008 10:04 AM  
Anonymous Anonymous said...

Regarding 580EX II, you can choose your power settings.

You can choose full power 1/1, half power 1/2, quarter 1/4, eighth 1/8, and so on to 1/128. You can also choose 1/3 stops in between.

So looking at 1/2 power, for example, you can choose among:

1/2 power + 2/3 stop
1/2 power + 1/3 stop
1/2 power
1/2 power - 1/3 stop
1/2 power - 2/3 stop

The stops are merely more or less power. You'll note that the major settings are factors of 2 (one full stop). Thus, these 1/3 stops are just intermediate settings between the full stops.

http://www.the-digital-picture.com/Reviews/Canon-580EX-II-Speedlite-Flash-Review.aspx
(excerpt below)

One more flash mode: Manual mode. If your subject distance is consistent (formal portraits for example), a manual flash output setting can eliminate shot-to-shot exposure variations. Using the control dial, it is easy and fast to set manual flash power output. Manual flash output can be controlled in 1/3 stop increments - 1/1, 1/2, 1/4, 1/8th, 1/16th, 1/32nd, 1/64th & 1/128th. Become familiar with this mode.

April 05, 2008 11:48 PM  
Blogger Joe Martinez said...

Hi all . First off to the admin , Great site you have here.

I'm a pro racing photographer and I use a Canon digital along with an Vivitar 285 HV.

I shoot at night with this flash and most of the time I run at 1/4 power and at F/4. Depending on the track lighting I will use 800 ISO or 1600 ISO.. The type of Photography I do takes a fast shutter speed.. This flash will shock you for it's all out power.

Im lighting all the cars and getting good depth of field with this setup.
http://picasaweb.google.com/jmart16164/Week92008
Take a look at some of the photos and see what an old flash can do.

For this type of photography I don't need to use TTL .. In fact TTL is a lost cause with my work LOL. Once you learn the ins and outs of F Stop and Shutter Speed and ISO you can do great things with this flash for a fraction of the cost of the TTL type flash units.

I own 540EZ's and 550 EX flash units and I still like this old 285 better. It can light far better than the speed lights and you can set it up to make great exposures if you learn the ropes so to speak..

Again to the site admins , Really nice place you have here.

Joe
L&J Photography.

PS it has a claimed Guide number of 120 at ISO 100.
I find with my setup it can light 90 feet in 1/4 setting at ISO 1600.

When I shot with film I ran it at ISO 800 and 1/2 power.. It has never let me down and I feel even with it's age it is still the best falsh out there.

May 03, 2008 9:05 AM  
Blogger brett said...

Hey, I know this page is pretty old by now, but hopefully someone still looks at it now and then and can help me out.
I'm still pretty new to flash, and so I feel like I'm missing something completely obvious in this article that everyone else understands. Why don't you need to know the shutter speed you are using when using the flash calculations? For instance, if I figure out that 1/8 power at 50mm zoom, 13-foot distance at ISO 400 = f/8, won't my shutter speed still affect the picture? When I calculate the aperture or flash intensity, am I supposed to be assuming a certain shutter speed? Or is something just going right over my head?
Any help would be much appreciated! Thanks.

June 18, 2008 4:34 PM  
Anonymous Jon-Paul said...

Hey Brett, If I've got this right, Shutter speed only affects the amount of ambient light falling on your sensor. Because the flash burst is of a specific (and very short) duration, your shutter speed can be 10 seconds yet the flash burst has been, gone, and left it's impact on the sensor. Leaving the shutter open any longer only allows extra ambient light in.

June 19, 2008 9:49 PM  
Blogger brett said...

Jon-Paul: Ah! Thanks alot, that explains so much!

June 23, 2008 12:14 PM  
Blogger GeoWulf said...

The time you have to be concerned with shutter speed is when you are dealing with 'sync'... If your shutter curtain to flash pop is 'off' then you are sure to capture the less of the image and more of the camera's shutter curtain.

July 18, 2008 4:17 PM  
Anonymous Anonymous said...

I just bought a 285HV and the guide chart confused me. I tried to enter the ASA,f/s,distance etc, but there are only two dials which allow one to select either the amount of flash (e.g full 1/2 1/4) and another setting on the side between 1 and 400 with many increments inbetween. Lost me: back to the manual because there has to be a hidden dial somewhere else.

July 30, 2008 8:53 AM  
OpenID antony.se said...

I've just started a couple of weeks ago to follow the Strobist groups.

But I have to admin this is the post I've been looking for the most!

I can't stop bugging my girlfriend to help try out this new knowledge.
Everything just fell into place - great post! Thank you!

September 08, 2008 3:49 PM  
Blogger jonthelens said...

Hi,
I have just bought 3x 285s from ebay for £100 inc postage, http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=160286874362
Just hope all is ok with them now.
Does anyone know which are the best radio triggers to use as i know that Elinchrom Skyport are not compatible with the 285s at the moment.

cheers
jon

September 26, 2008 2:47 AM  
Blogger SP said...

Thank you David, like others this is an informative post. I am starting to go through the assignments. I now understand how to use the flash better.

Shiva

November 03, 2009 10:20 PM  
Blogger talkinglens said...

Thanks for the excellent post Dave.
Know I know how to better use and adjust my 580EX and can use my 2 Vivitar 283s. I also learned of a VariPower sensor which can make life a little easier!

November 06, 2009 4:31 PM  
Blogger Tony Mac said...

"Nope, the umbrella (or face of the soft box) becomes the light source. The law of inverse squares still applies, you're just dealing with another (usually lower) starting point."

Once you diffuse that light, the inverse square law no longer applies, it's no longer a point light source.

February 08, 2010 6:24 PM  
Blogger obakesan said...

Hello

I just popped by to mention to you that you have forgotten to mention the effect of the room. The GN system as far as I understood took this into account with some assumptions on a standard room. After all if the GN calculation was independent of room effect then it would mean that a subject in a black room would be exposed the same as the subject in a white room. If subject distance flash and aperture remain constant I'm sure you will not see constant exposures because of the ambient bounce back.

:-)

February 23, 2010 1:24 AM  
Blogger RF Guy said...

very good tutorial. my question is, what if i use flashes? both at the same distance or at different distances from the subject. thanks.

June 15, 2010 11:59 PM  
Blogger Will said...

Good to get a more comprehensive explanation of guide numbers than I have seen so far. Many things still confuse me though.

If you know the guide number and the distance to the subject then you will also know what f/stop to shoot at.

How does this work? The flash doesn't know about the ambient lighting so how does it know what aperture will create the best exposure?

Is there any way to know, for a given guide number and distance, how many equivalent stops it will lighten the scene by?

October 06, 2010 4:49 PM  
Blogger iXpose23 said...

Hi, David.. Question on your post, the idea behind this is to ensure fast, predictable outcome from your flash right? That's why you didn't go deeper with the inverse-square law I presume (takes more time..) But then, the whole deal of strobist.com is to use flash in an indirect manner (bouncing is also permissible) and I ask myself, if the flash is off the hot-shoe, say angled at 45 degrees camera left, won't you need to use trigonometry to ensure that the outcome is predictable?

This is from I book I'm reading lately, that you also "recommend" on this blogsite; "Light, Science and Magic". In general it says that when speaking of GN's, the assumption is that the flash is mounted on or near the camera.

What do you say of this?

November 05, 2010 5:22 AM  
Blogger acerimusdux said...

This is a very helpful post, but it probably should have started with the very basic:

GN/distance = f-stop (at ISO 100)

For example:

"The first five or six times you have to dial in your GN calculator and learn where you need to set your flash to get f/4 at, say 10 feet, you'll need the calculator to tell you. Then all of a sudden one day, you'll just know."

Or, you'll just look at the specs and see it's a guide number 40 flash, and you'll just know the proper exposure (40/10 = f4.0) from shot one without having to mess with the calculator.

Granted, the procedure described is still needed for those many situations when you don't know the guide number, because you are shooting through a gel or umbrella, etc. But it's still helpful then to learn the actual guide number of that setup. For example, if your 40 GN flash loses two stops shooting through an umbrella, the umbrella setup now has a 20 GN.

December 03, 2010 12:51 AM  
Blogger Artista Posse said...

This comment has been removed by the author.

December 29, 2010 8:49 PM  
Blogger Harborneit said...

Guide numbers work in theory, but they have two basic problems.

First problem is that manufacturers just plain lie about the power of their strobes. A real GN of half what's quoted by the maker isn't uncommon.

Second problem is that Guide Numbers rely on the inverse square law applying, and it really doesn't. The inverse square law only works for a plain open light source like an old-fashioned incandescant bulb. Small hotshoe strobes give out a focused light, which means the effective (or virtual) origin of the light is somewhere behind the gun. And it's this virtual origin that should be used to calculate the f-stop from the GN.

All-in-all you'll be lucky if the strobe maker's GN gets you in the right county, let alone the right ball-park. I'm speaking heres an old-timer who grew up with Guide Numbers as the main way of working out flash exposures. Take it from me, unless you really know your strobes Guide Numbers are really not that useful.

February 02, 2011 7:14 PM  
Blogger owen_ogrady said...

Hi,
I just want to know, do i dial the same settings that are in the flash gun into my camera also.
Regards,
Owen.

March 08, 2011 1:37 AM  
Blogger The People of Detroit. said...

Jesus christ, some of you are over thinking this. Its just light, folks. Take a guess, then give it a lil more or a lil less. There's not need to be a flipping mathematician.

March 21, 2011 12:55 AM  
Blogger Efrain BG said...

Hi, all!

I'm (not so much) relatively new to this lighting stuff.

I understand the math about guide numbers but there is one thing that I still don't quite get:

What about shutter speed? this guide number thing is intended as some sort of sunny 16 rule? as in "shoot at 1/ISO speed and you will need this aperture"?

Also, I get the calculator, giving me a ballpark figure for exposure (assuming I have the proper shutter speed). But what do I get from a spec that states "42 m ISO 100 - 85 mm"? I understand this is a maximum flash output power... but with what aperture was this measured?

If the answer is somewhere else in this site I would really appreciate a pointer to the article.

Cheers!

September 01, 2011 2:17 PM  
Blogger Efrain BG said...

I get it now... forget my last comment! (can I erase it?)

September 15, 2011 1:44 PM  
Blogger H. Graham said...

Can we please get this added to Lighting 101? This is one of the few articles that actually explains how light mods can affect guide numbers.

August 16, 2013 9:36 AM  
Blogger etortxu38 said...

These guide number calculations mesmerized me during months. At a time I eventually did the math, so here you can see the result strobistcalculator
Maybe it will be useful for starting people too.

October 10, 2013 6:18 PM  
Blogger john said...

Vlad said...

Thanks very much for this tutorial. It is very helpful for someone like me who is only beginning to learn photography. I have an additional question about using multiple flash units: imagine using two flashes - the main one and the fill one. Metering from the main flash may give me the aperture of f/8, and the fill one f/4. What should be my working aperture? f/16

multiple flashes is quite interesting and simple really 1,2,4,8 flashes at the same power level each change the power at most a full stop.
so 1 at f4 is the same as 2 at 5.6 4 at 8 and 8 at f11 it'll never be more than 1 stop with 2 equal power strobes.

in the case of f8 +f4 the stronger flash wins so f8 is the starting point but it will be over exposed a bit.
the calculation is sqrt(fa2 +fb2 +fx..)
or sqrt(8 squared + 4 squared)
or sqrt(64+16)= sqrt(90) 9squared is 81 and 10 squared is 100 so 9 something fstop (9.48 but i needed a calculator for that) so between f8 and f11 a half stop possibly, but since its never more than a whole stop with equal power you may find f8 is still fine. Maybe tweaking iso might give you a finer control? or dropping the power of the main flash a 1/3 of a stop

November 16, 2013 8:15 AM  

Post a Comment

<< Home