The Zoom tool adjusts the magnification of the current view of a model.
With the Zoom tool, you can change the zoom magnification of a model in the following ways:
key before you click the Zoom tool on a wheel, the current view is zoomed out by a factor of 25 percent.
key before you click the Zoom tool on a wheel, the current view is zoomed in by a factor of 25 percent.
, the Zoom tool uses the center point defined by the Center tool.
+click causes zooming to occur about the current pivot point rather than the current cursor position. Click and
+click perform incremental zooming only if Incremental Zoom-in is enabled on the SteeringWheels panel of the Viewport Configuration dialog. 
When changing the magnification of a model with the Zoom tool, you cannot zoom in any further than the focus point or out past the extents of the model. The direction you can zoom in and out is controlled by the center point set by the Center tool.
To zoom the view with a single click:
To use this function with the Full Navigation wheel, you must enable the Incremental Zoom-in option.
The magnification of the model is increased and you are zoomed in closer to the model. If you hold down the
key while clicking the Zoom wedge, the model is zoomed out or you can hold down the
key to zoom in.
To zoom a view in and out by dragging: