I use flash in my photography on a regular basis. I bring my light with me on every photo shoot. Word photography comes from Greek phōtos - light and graphé - drawing; therefore drawing with light. I like to be in control of light where and how it falls and how it lights my subject.
When I started photographing and using flash, I struggled to make sense of all the numbers, F stops on a lens, F stops on a flash, shutter time on camera, ISO and so on. So I decided to have a good look on how things work.
First of all flash settings. Hypothetically flash gives me a proper exposure of f16 at 1m on full power. With the flash set to its full power I have my lens set at f16, make an exposure, and you get a nice sun tan. Full power is a lot of light coming at you, but you are properly lit and exposed.
Flash power settings divided by one stop. Each time you move one stop up or down you gain or lose half the light emitted by the flash.
Full power to 1/2 is one stop and half the light lost. Full going to 1/4 are two stops and 3/4 of light lost, because you lose half the light at step one and half the avaliable light at step two.
F stops as you can see them on your lens or in camera setting. Smaller the number more light camera lets in. Lens set to f1.2 lets in twice the amount of light then lens set to f1.4 and 4 times more light then lens at f2 and so on.
So after frying my subject’s eyes by shooting at full power from 1 meter distance, I decide I don’t want to shoot at f16, I want to shoot at f4. If I open up a lens by 1 stop, I can lower my flash power also by one stop and only use half the power. Lens set to f11 and flash at 1/2 power. Same exposure, but by allowing more light through the lens I don't need to produce as much light to light the subject. Therefore, if you want to move from f16 to f4 you need to open up the lens by 4 stops, and you dial down the flash by 4 steps to 1/16th of its power. By going from full power to 1/16 power we lose 4 stops of light and by opening lens by 4 stops we gain that light back, Flash at 1/16 power and f4 will expose your subject same as flash at full power and f16.
Here you can see how we got from full power to 1/16 power and f16 to f4.
Please mind all flashes produce different amounts of light and those settings will vary on each flash system. Use of modifiers, soft boxes, umbrellas, beauty dishes or any light shaping tools will change how much light is hitting your subject. Also distance of the flash from the subject will change how much light is available to use. If you change the distance from the flash to your subject from 1 to 2 meters you will have only 1/4 of the light hitting it. We will get to inverse square law of light some other time.