Portable Flash Exposure Basics

What is a Flash?
A flash unit, and an “on camera flash” both work in the same fundamental way. Voltage from a battery is stored in a capacitor, and then as a photo is taken, the voltage is released to the flash tube filled with a gas and produces a flash of light. This burst of light may only last from 1/500th of a second to 1/10,000th of a second depending on your flash model.

There are three types of off camera flashes: manual, automatic, and dedicated. With manual flashes the usually fire their flash with 100% intensity and you have to set your camera accordingly. Automatic flashes will cut off the amount of power in the flash once if feels the subject reaches a proper exposure, but you still have to set your camera. Dedicated flashes will usually communicate with the camera and set both the flash and the camera settings. As each flash becomes more automatic, the price will usually rise, and for the sake of this tutorial we will be discussing certain aspects assuming all you have is a manual flash.

Guide Numbers (GN)
Every flash is rated with a guide number, whether on-camera or a separate flash unit. The guide number is a measure of power and relates to the distance the flash can cover. A guide number may be provided for meters or feet or both, so just stick with one unit for all your measurements and you’ll be fine. Guide numbers are rated at ISO100 which you’ll see makes a difference later. I warn you this section has some math so put your thinking helmets on.

Lets say we have a flash unit and its guide number is 120 (ft) and we have our subject 20 feet away. All you have to do for proper exposure is divide the guide number by the flash to subject distance and the result is the proper f/stop to use. Now, the number you get may not be an f/stop you can select so it’s best to round your number to the closest f/stop available to you.


GN = (f/stop) x (Flash-Subject distance)
f/stop = GN / Flash-Subject distance
120 / 20 = 6

The closest f/stop to the number 6 on most cameras would be f/5.6 so to properly expose our subject we’d use f/5.6. As you increase your ISO, you increase your guide number. Doubling your ISO increases your guide number by 1.4 and quadrupling it increases your guide number by 2. So at ISO200 a guide number of 120 may be 170 or it may be 240 at ISO400. Also at ISO50 it may be 86 (120/1.4).

Shutter Speed and Flash
Notice how I haven’t mentioned much about shutter speed yet? Shutter speed does not control a flash’s exposure because the flash is only firing for a very short amount of time. Instead it controls what we call ambient light. Say we’re shooting at night with a train station behind us. If we shot at 1/60th of a second with a flash, the train station may appear dark, while shooting at 1/15th of a second may make the train station appear properly lit.

You may also have noticed that with a flash you cannot set your camera above a certain shutter speed (usually 1/60th or 1/125th of a second). This is called flash sync. Most SLR cameras have a curtain shutter which is comprised of 2 moving curtains. As one opens and exposes the film/sensor for the proper amount of time, the second releases to cover the film/sensor again. This system allows for very fast shutter speeds but if the flash doesn’t fire at the moment both curtains are wide open (the sync) then parts of the image will be black because the shutter was blocking the frame. Medium Format cameras and some 35mm cameras have leaf shutters which open in a circular configuration. Leaf shutter systems are usually more expensive because the shutter is built into each lens instead of the camera body. The advantage to leaf shutters is flash syncs at all speeds but the disadvantage is a slower overall shutter speed (perhaps 1/500th compared to 1/8000 for curtain shutters). Digital point and shoots can usually sync flash at all speeds because they don’t use shutters, they just sample from the sensor for a certain amount of time, because it’s always on.

Fill Flash Exposure
Let’s say we go outside to shoot and get a meter reading of 1/125th and f/8 for a proper exposure, but our subjects are a bit in the shade so we decide to use fill flash. Now we know our guide number is 120 and at f/8 so the best distance for proper exposure would to have our subjects be 15ft (120 / 8) away from the camera… but we want to get closer. If we increase our f/stop to f/16 we can be 7.5ft (120 / 16) away from our subject, getting a great portrait. Because we closed 2 stops we have to compensate by slowing our shutter speed 2 stops to 1/30th and we’ll have a proper exposure for the scene with fill flash.

Bounce Flash Exposure
Finally, let’s say you have a flash unit that allows you to angle the head to bounce the flash off a wall or ceiling for diffusion. We now have to measure the distance from the flash to the wall, the wall to the subject and the subject to the camera. We get these 3 values and divide them by 2 to get an average distance we can plug into the guide number formula. Lets say the distance from the camera to the wall is 3 ft, the distance from the wall to the subject is 5ft and the distance from the subject to the camera is 8ft. 3+5+8=16/2=8. Now we just take 8 and plug it into our guide number equation to get an f/stop of f/16 (we’re still assuming for example purposes that our guide number is 120). Now because we’re diffusing the light some we’ll end up losing a bit so it’s wise to open up a stop to compensate which means we’d be using f/11 for this shot.

9 thoughts on “Portable Flash Exposure Basics

  1. saqib

    thanks for giving me very use full tips .i never bin found of flash photography but by ur artical i have learn many things . plez keep helping me. Saqib from pakistan .

  2. Roby Davis

    I see you talk about F/Stops, and realize how important it is in knowing them. I wonder have you run across a chart that shows f/stop in Aperture Value, and Shutter Speed. ISO and distant are a lot easier to figure out. Also it would be a big plus to see it in 3rd steps as well as half steps.

  3. alukman

    Hi..I’m usually shooting in low light with on camera flash(580EX) in auto ETTL mode. with iso200 and camera in Manual, set my shutter to 1/125 and aperture at f5.6 will not get a correct exposure (camera meter reading -2)underexpose.bump up the exposure to iso400 and still cant get the correct exposure. I’m using canon 350D with 24-70 L lens and even with shutter at 1/80 still no help…all my photos are underexposed and needs plenty of photoshop to correct it..pls help!!

  4. bing

    hey alukman you have the top of the line flash and an L lens and you dont know how to use your camera? Wow I guess your just another gear fanatic. Go read a book

  5. Mohan

    Wow…Very simple but informative article. I heard the term Guide Number, but never understood the importance. With just a simple formula, I got it, mainly because it is simplified. I always try to use the natural light, may be its time for me to try my flash….

  6. Ken


    In one of the lesson, you mentioned about to figure out an f/stop is by divide the guide number to the flash-subject distance.
    I am kind of slow and confused. I am using a Nikon D80 with flash SB-800.
    Where can I find the guide number on the flash/camera?


  7. Robert


    Not sure if you will check this again and I’m just a beginner to learning about flash photography so bare with me.

    If you look in the manual for the Nikon SB-800 (which I have as well) to the back where the specifications are listed you will see the guide number listed for ISO 100 & 200. The value at 100 is 38m or 125ft and the value at 200 is 53m or 174ft.

    PS. The videos on this site are excellent although I think I’ll have to pick up a more basic book to start out with. Thanks for your articles.

  8. edward

    ive shot strobes for years and want to sync
    a studio pack and a point and shoot by slave, but on slow sync i get the flash to fire from bounce off
    the point and shoots flash with foil but no image?
    thank you,

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