“If you have trouble seeing, buy this,” wrote one reviewer.

A change in eyesight is a natural part of aging. Over time, making out the small text on menus or the size 10 font in your paperback can get harder to read. Sure, you could go through life squinting.

The iPhone has a built-in magnifying glass app. Unlike the flashlight app, however, it's an option you need to turn on in settings. CNBC's Todd Haselton shows you how to use the magnifying app on your ...