Your iPhone ships with a built-in magnifying glass. This is intended to be used as an accessibility feature, but it is useful any time you need a close-up look at something. If you are having trouble reading tiny text in a book, for example, you can fire up Magnifier and use your iPhone’s camera to zoom in. You can enable Magnifier by going to Settings > Accessibility > Magnifier.
After enabling this setting, Magnifier will show up as an app on your iPhone. Find it using Spotlight Search or App Library and fire it up; when Magnifier is open, you can use the slider next to the magnifying glass icon to zoom in to real-world objects.