Canon Speedlite 90EX Instruction Manual page 26

The flash ratio range of 8:1 - 1:1 - 1:8 is equivalent to 3:1 - 1:1 - 1:3 in stops (1/2-stop

increments).
The flash ratio under the Z mark is shown in parentheses below the scale.

8:1
24
Set the flash ratio.
2
Take the picture.
3

4:1 2:1
(5.6:1) (2.8:1) (1.4:1) (1:1.4) (1:2.8) (1:5.6)
Set the camera in the same way as with normal
flash shooting.
1:1 1:2 1:4 1:8