The key light delivers the most light except when using a ratio of 1:1 which is a rather flat and unattractive set up. The idea is to add more light to one side of the face and less light to the other to create a difference in shadow as can be seen in the image to the left. The bigger the difference in the power of the two flashes the darker the fill side will be.
The ratio is worked out by the number of stops between the output of the Key light and the output of the fill light, so:
1:1 Ratio – Same output
2:1 Ratio – 1 stop Difference
4:1 Ratio – 2 stop Difference
8:1 Ratio – 3 stop Difference
|Key Light at f5.6 and Fill light at f5.6 is a 1:1 ratio
Key Light at f5.6 and Fill light at f8 is a 2:1 ratio
Key Light at f4 and Fill light at f8 is a 4:1 ratio
Key Light at f8 and Fill light at f22 is a 8:1 ratio
The ratio is the difference between the meter reading from each light fired on its own.
The basic setup looks like this: