The "flash ratio" is an important way to describe the mix between ambient light and light from your flash. Since the shutter speed doesn't affect the amount of light captured from your flash (but does affect ambient light), you can use this fact to control the flash ratio. For a given amount of ambient light, the mix of flash and ambient light is adjusted using only two camera settings: (i) the length of the exposure and (ii) the flash intensity.
Only Ambient Light
Settings: no flash
1:8 - 1:2
Fill Flash
longest exposure
weakest flash
Balanced Flash
shorter exposure
weaker flash
Strong Flash
shortest exposure
strongest flash
In this tutorial, the flash ratio* is used to describe the ratio between light from the flash and ambient light. At one extreme of this ratio is ordinary ambient light photography (left), and at the other extreme is photography using mostly light from the flash (right). Realistically though, there's always some amount of ambient light, so an infinite flash ratio is just a theoretical limit.
*Technical Note: Sometimes the flash ratio is instead described in terms of the ratio between total light and light from the flash. In that case, a 2:1, 3:1 and 5:1 ratio would be equivalent to a 1:1, 1:2 and 1:4 ratio in the table above, respectively. Unfortunately both conventions are used.
It's important to also note that not all flash ratios are necessarily attainable with a given flash unit or ambient light intensity. If ambient light is extremely intense, or if your flash is far from your subject, it's unlikely that the internal flash of a compact camera could achieve flash ratios approaching 10:1, for example. At the other extreme, using a subtle 1:8 fill flash might be impractical if there's very little ambient light and your lens doesn't have a large maximum aperture (or if you are unable to use a high ISO speed, or capture the photo using a tripod).
Flash ratios of 1:2 or greater are where the topics in the first half of this tutorial become most important, including the flash position and its apparent light area, since the flash can appear quite harsh unless carefully controlled. On the other hand, flash ratios less than 1:2 can often achieve excellent results using a flash that is built into the camera. For this reason, most photographers will likely want to use their flash as a fill flash, if possible, since this is the simplest type of flash photography.
source
