Nikon D3100 Reference Manual page 100

A
The Y (M) and E Buttons
Flash compensation can also be set by
rotating the command dial while
pressing the Y (M) and E buttons.
The selected value is shown in the
viewfinder and information display.
A
Optional Flash Units
Flash compensation is also available with optional SB-900, SB-800, SB-600, SB-400, or
SB-R200 flash units. The SB-900, SB-800, and SB-600 also allow flash compensation to be set
using the controls on the flash unit. The flash compensation selected with the optional flash
unit is added to the flash compensation selected with the camera.
t
84
Y (M) button
–0.3 EV
E button Command dial
+1 EV