Zoom to Window: Zooms out to show the full extent of the image.
Zoom to Full Resolution: Zooms in to show the image at 1:1 pixel resolution.
Zoom In: Zooms in one step. Maximum in is at 1:1 pixel resolution.
Zoom Out: Zooms out one step. Maximum out is at full extent of image.
Zoom to Differences: Zooms to show only the area where differences occur.