The user should have a consistent means of producing zoom-in and zoom-out commands, but the means may differ from product to product. There are several potential zoom interactions depending on product features and form factor; each product should support whatever is appropriate:
Pinch zoom is available in touch products with multiple touch support.
Some applications may support double-tapping zoom. A double tap can toggle between zoomed and normal views, and there may be application-dependent behavior.
A zoom slider can be used to perform zooming events.
Volume keys might be used for zooming in the appropriate applications.