The relation that allows us to set the aperture or to choose the distance to take a photo with correct brightness while using a manual flash is this one:

Guide number = f-number × distance, For a certain ISO value.

My question is: why the shutter speed (i.e. time) is not present? It should have influence on that, since longer time = higher brightness and shorter time = lower brightness.

  • 2
    I can't believe we don't have this question already.
    – mattdm
    Nov 5, 2019 at 15:29

4 Answers 4


The simple equation assumes that the flash is effectively the only source of light in your image. This is a reasonable assumption in many cases, because since exposure works on an exponential scale, the amount of light from typical indoor room lighting is a drop in a bucket compared to that provided by the flash. See Do flash guide numbers assume some amount of ambient light built-in? and the chart there for a little deeper exploration of this.

That means that the shutter time doesn't matter at all — the exposure is linked to the light output of the flash, and motion is frozen by its (usually very short) duration.

Of course, if you use a longer shutter speed, the relative amount of ambient light will increase and may become a factor. It will also weigh in significantly if you're some distance away or it's relatively bright in the scene (like, you're outside in the day). If you want that not to happen, see What does it mean to "kill the ambient"?.

See How can I take 'balance flash' pictures with studio lights indoors? or What is lighting ratio? for how to calculate the "fill ratio" — the ratio between ambient light and your flash.


Flash duration is typically much shorter than most cameras' flash sync speed.

If the flash only has a duration of, say, 1/1000 second (or 1 millisecond), it matters not if the shutter is open 1/250 second (4 milliseconds) or 1/25 second (40 milliseconds), the energy from the flash that is captured by the photo will be the same in either case.

What shutter duration does affect is the amount of ambient light recorded. But your formula is strictly for calculating the light energy from a flash and does not include anything about ambient light or the ratio between ambient light and flash.

  • Correct but keep in mind the sync speed of the camera, which is typically around 1/250th of a second. This is actually the shortest time the camera can expose, shutter speeds above this are done by dropping the second shutter while the first is still dropping--which means there's never a time the whole frame is exposed and thus the flash will fail to illuminate part of the frame. (And note it's a lot shorter than 1/1000th of a second, also.) Nov 6, 2019 at 10:56
  • 1/250 is a lot longer than 1/1000. Typical DSLRs and mirrorless cameras with mechanical focal plane shutters take about 2.5-3.5 milliseconds (1/400-1/385 seconds) to transit the sensor. At full power, there are many speedlights with a T.1 duration of up to 1/400 second, though 1/1000 is more typical. They get progressively shorter at lower power. For many studio strobes, a full power dump takes longer than many cameras' sync speed, which is why longer Tv is needed to get the full benefit of the flash.
    – Michael C
    Nov 7, 2019 at 3:52

Both of the other answers are correct, but I think there might be an easier way to understand it:

Consider being in a completely black room with no light... the shutter can be open indefinitely because there is no light available. The only thing that matters is that the shutter is fully open when the flash fires so that all of the sensor can see it. The shutter is fully open at any shutter speed equal to x-sync and slower.

The flash power setting provides the higher/lower brightness, distance determines how spread out (strong) it is when it reaches the subject, and the aperture setting regulates how much gets through.


We use the Guide Number as an aid to gauge the f-number setting.

Accept, as an example, your ISO setting is 200. You consult a table of guide numbers and discover it’s 180. Your principal subject is 10 feet downstream of the flash. What would be the appropriate f-number setting? To compute, we divide guide number by distance -- thus 150 ÷ 10 = 15.5. Now round to the nearest full f-stop = f/16.

In modern times, the shutter speed setting is moot because the source of the flash is an electrical discharge of high voltage applied to a glass tube filled with xenon gas. A super short blitz of light ensues. The blitz is so quick that few, if any shutters have any ability to mitigate. Thus the real problem is synchronization. Now we are talking the timing of the applied charge so that the shutter will be fully open when the blitz occurs. There are exceptions when using specialty high-speed cameras.

The published guide number is just a good guestimate. Several difficult to predict factors come into play. To name a few: Indoor or outdoor scene – height and color of indoor ceiling – reflectivity and contrast of scene - size and shape to flash reflector – plus many more.

To refine the guide number for an actual shoot: On scene, shoot a series at different f-stops. The idea is to discover a setup that delivers a faithful image. Pretend f/11 works. Now multiply the principle subject distance by f-stop used. Say 10 feet @ f/11 – revised guide number is 10 x 11 = 110. Setting for synchro-sunlight: For flash fill in sunlight, likely best is flash subordinate to sunlight by 1 f-stop. We adjust flash-to-subject distance to achieve. Better method is to adjust flash power if possible. If not, set shutter speed to synchronize. Set camera f-number for sunlight exposure. Divide guide number by principal subject distance. Pretend f/11 with guide number of 120.

Now divide 120 by 11 = 11 (rounded ). This is distance in feet flash to principal subject. At this distance, flash intensity mirrors sunlight exposure. Now multiply this distance by 1.4 = 16 (rounded). This is flash to subject distance, one f-stop subordinate.

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