tutorial: how to use the guide number of your flash

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?

using the guide number to calculate flash exposure

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.

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!


The spec for the Nikon SB-910 speedlight is given as:

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

This means that our GN (in feet), is 111.5
So 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


The spec for the Canon 600EX-RT speedlite is given as:

Guide Number: 197′ (60 m) at ISO 100 … for the flash-head zoomed to 200mm
Guide Number: 118′ (36 m) at ISO 100 … for the flash-head zoomed to 35mm

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

(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)

Working at 200mm zoom setting for the flash:  if we are 20 feet away from our subject, then our aperture is 197/20 = f10


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

Now the question remains … what is the Guide Number of your flash?
Time to find that instruction manual again! But firing up Google might just be easier.


video tutorials to help you with flash photography

If you like learning by seeing best, then these video tutorials will help you with understanding flash photography techniques and concepts. While not quite hands-on, this is as close as we can get to personal instruction. Check out these and other video tutorials and online photography workshops.


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22 Comments, Add Your Own

  1. 1Edy says

    Hello Neil,
    really great post …
    Neil I realized that to find the right exposure on my subject I must divide the number judas (34 for SB 910 with power 1/1 and zoom reflector 35mm ISO 100) for the distance
    Example: 34/3 = 11 m aperture
    My question is this:
    how do I get a perfect FILL-IN?
    I have to act on the diaphragm so as to let less light can or should act on the flash output from 1/1 to 1/2 or 1/4 …. 1/16 etc etc??
    (With manual flash my flash is affected by ISO-OPENING-POWER-DISTANCE ………… my flash with TTL flash is affected only by the flash exposure compensation EV -3 ….. +3.)
    But here we are in presence of various manual flash so either the distance or varying the aperture right?
    If I have to change flash power for what value should I set FILL-IN?
    If I change the aperture value which I use to get a perfect FILL-IN???
    To me that your answer is very important!
    Thank you always follow every day
    You are wonderful, thanks for your great work ……
    Edy Trigona

  2. 2 says

    Edy, since the GN is for the flash at full-power, then you simply dial your flash power down to get fill-flash.

    For example:
    1/2 power = 1 stop under
    (similar to -1 FEC, if we disregard whatever TTL algorithms would’ve kicked in)
    1/4 power = 2 stops under
    1/8 power = 3 stops under … which is probably where you want to be.

    But … this is digital. Look, evaluate & adjust.

  3. 4Allen says

    If you click the specifications tab on the B&H link to the Canon Speedlite, above, it does say “… at 200mm position”.

  4. 6Din says

    A dumb question – is there a constraint on the distance between the subject and camera?
    In case of off-camera flash, say I set the distance of the flash from the subject correctly (based on the above calculations), can the camera be at any distance from the subject and still have correct exposure?

  5. 7 says

    Your camera’s distance to the subject has no influence on exposure .. just the distance between your light and your subject. And that is part of the beauty of off-camera flash.

  6. 8John says

    What is the units of measurement with the guide number calculations?

    GN = distance * f-stop

    What is the units?

    How do you use meter and foot with the calculations?

  7. 10John says

    What is the Guide number of:

    1. Nikon SB-600
    2. Nikon SB-700
    3. Nikon SB-900
    4. Nikon SB-910

    Can you add the guide number info to the article?
    Can you add the units of measurement to the article?

  8. 11 says

    This covered in the final part of this article:

    “Now the question remains … what is the Guide Number of your flash?
    Time to find that instruction manual again! But firing up Google might just be easier.”

  9. 12Ann says

    Guide number (GN)=distance (meters) × aperture (f-number)

    1 metre = 3.2808399 feet

    Info from Nikon: Flash Level (Guide Number)

  10. 15John says

    Guide number (GN)=distance (meters) × aperture (f-number)

    The units of measurement of guide number (GN):

    Aperture (f-number): Because f-stop is a ratio, there are no units associated with it. With the cameras we use, everything is measured in millimeters, but it could as easily be measured in feet or cubits. Regardless, ratios are always displayed without units because when you do the division to arrive at the number, the units cancel out.

    f-number = focal length (mm) / aperture (mm)

    Example: A lens with a focal length of 105 mm and an aperture of 25mm in size, has an f-number of 105mm/25mm = 4.2 (the units cancel out).

    Distance (meters: Units of measurement is either meter (m) or Foot (ft).

    Manufacturers provide Guide Number values in either meter or foot values.

  11. 16Dave Graham says

    Guide numbers apply when you are using direct, undiffused flash. Softboxes and wall-bounce depends more on total light output.

    The guide number on the Canon 600EX-RT (60 meters @ iso 100 and 200mm) would indicate less than half the light output than its predecessor 580EX, which is 58 meters at iso 100 and 105mm . (The 105 mm indicates almost four times the area of flash coverage ).

    On the other hand, at the wide end, the new flash seems to beat the old, with 26 meters at 20mm, versus 28 meters at 24mm for 580EX.

    So, the new flash seems to have same or slightly more overall power, and can zoom all the way to 200mm, but is really inefficient when it gets to 200mm, presumably spraying light to the side. I wonder what it’s guide number is at 105mm, perhaps the 580EX beats it.

    David Graham

    • 16.1 says

      The reference table in the 600EX manual gives the following GN info:
      200mm – 60m (196.9 ft)
      135mm – 59m (193.6 ft)
      105mm – 58m (190.3 ft)
      80mm – 53m (173.9 ft)
      70mm – 50m (164 ft)

  12. 18RonR says

    If I have the GN of a flash is rated at one focal length, how would one calculate mathematical the GN at a different focal length, using the differences in angle of coverage. I am looking at a flash that is rated with a GN of 54 at 105mm the zoom head on the flash is 18mm to 180mm. I am in the process of purchasing a flash and I am trying to determine if it adequate for my needs for a 24-105mm zoom lens.

    Thank You

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