The default action for the mouse wheel is to adjust the cursor size.
Check to change the mouse wheel action to focusing.
Each moves through the Z-axis by the distance specified in Z distance per wheel click.
Use for frequent adjustments of the Z distance per wheel click.
To navigate one plane at a time, select 0.5 from the State drop-down menu.
To enable both cursor size adjustment and focusing with the mouse wheel:
Also see Using the mouse wheel