Select the zoom tool to either one of the mouse buttons. There are 3 zoom tools:

(1)Zoom by moving the mouse horizontally. Click and drag the mouse from left to right.
(2)Crop, select a part of the image to be enlarged. Click the mouse button and choose the area to enlarge.
(3)Magnify glass, magnify the area under the mouse pointer. The image is magnified 4x.

Another option is to use the mouse scroll wheel to zoom in or out. This only works for single images. For multiframe files, the scroll wheel function is linked to the scrollbar.
A panning window is displayed top left, when an image is larger than the image window. Click inside the panning window to move the zoomed image. See Pan tool.

Undo any zooming by choosing 'FIT' in the menu of the image window. This will 'fit' the image inside the window.