NOTE: Zoom is the default for the left mouse button.
1. From the Viewer, select the image(s) you want to zoom.
2. In the Image tab of the toolbar, click Zoom 
.
3. Hold down the left mouse button and do one of the following:
Drag the mouse up to increase the zoom factor.
Drag the mouse down to decrease the zoom factor.
3. To zoom in on a particular point within the image, press the Alt key, then zoom in.
The image will be zoomed from the point clicked on the image, not from the center of the image.
In all cases, the zoom out option will zoom out from the point clicked on the image.