The zoom feature on Mac computers lets you zoom in and out on objects on your screen. You can configure the computer so that zooming is controlled either by the keyboard or the trackpad. In OS X Yosemite, changes to zoom settings are made through the Accessibility section of the Mac's System Preferences panel.
Accessing Zoom Preferences
To configure zoom options, click the "Apple" logo in the top corner of your screen, choose "System Preferences" and then click the "Accessibility" icon. Click "Zoom" on the menu on the left of the screen to load the Zoom preferences. Here you can customize every facet of how zooming will work on your Mac.
Setting Zoom Options -- Basic Functionality
In the Zoom preferences window, place a check mark in the "Use keyboard shortcuts to zoom" check box if you want to zoom in and out on the screen using the keyboard. If you enable this option, use the keyboard shortcuts displayed beneath the check box to control zooming.
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To set up your Mac so that it zooms using trackpad gestures, place a check mark in the "Use scroll gesture with modifier keys to zoom." If you select this method, click the associated drop-down menu and select the key you want to use. If you select "Command," for example, you zoom in and out by holding down the "Command" key and swiping up and down on the trackpad.
Setting Zoom Options -- More Options
A series of other zoom-related options are available for customization in the Zoom preferences window. You can customize image smoothing, zoom focus and zoom style. Zoom Style is particularly helpful. By default, it's set to Fullscreen, which means that when you zoom, everything on your Mac's screen gets bigger. But you can also choose "Picture in picture." When enabled, a small zoom window follows your mouse pointer around wherever it goes. This picture-in-picture method is especially handy for those who have vision issues as it's always active and doesn't require you to continuously zoom in and out on items.
Exploring Additional Zoom Configuration and Using the Window Zoom Button
While you're viewing the Zoom preferences window, click the "More Options" button to further customize your zoom settings. Here you can set maximum and minimum zoom levels and adjust how screen images behave when you zoom in on them.
Apple made an important zoom-related change to its window controls in OS X Yosemite. Most windows on your Mac have three buttons found in their top-left corners: a red one that closes the window, a yellow one that minimizes it and a green one. In OS X Yosemite, when you click the green button the window switches to full-screen mode and zooms in somewhat on the content within it.