'Free' view

[White Owl]

Right click + middle mouse button, then drag up or down. I think that's the default for zoom in external view... at least I can't imagine why I would've deliberately selected that.

[Blackdog_kt]

In a bit more detail, the middle mouse button is only needed to override the mouse cursor and place the mouse in camera control mode. If you have the mouse cursor disabled by F10 you don't need to be holding down the middle button. For example, i use the cursor to click on things and read the tooltips on instruments so all my camera movement controls are preceded by holding the middle button.

Holding the right mouse button down is switching to zoom and strafe controls for the camera, while moving the mouse without holding it down is for the default pan control.