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.