Canon EOS 80D Manual (en)

Constant speed mode
Click the [W] button to zoom to wide-angle.
Click the [T] button to zoom to telephoto.
Changing zoom speed levels:
You can change speeds with the speed level switch.
Setting this to [SLOW] provides a slow zoom, suitable for movie 
Setting this to [FAST] provides a faster zoom than [SLOW].
Micro-adjusting zoom speed:
You can finely adjust the speed in one of 15 levels for either 
[FAST] or [SLOW] speeds with the [Speed level] list box.
Speed level switch
If zooming stops, check the message that appears in the [Power 
Zoom Control] window.
Shooting a few test movies is recommended where you will 
perform zooming during movie shooting. Zooming during movie 
shooting may result in recording of changes in exposure or 
mechanical sound of the lens, or may cause images to be out of 
For more cautions for using the power zoom adapter, refer to the 
power zoom adapter instruction manual.
In constant speed mode, if the mouse cursor is in either the [Power 
Zoom Control] window or the [Remote Live View window], press 
<W> on the keyboard for wide angle and <T> for telephoto