In iOS 11, the native camera app includes functionality that allows you to scan QR codes and access QR code content like URLs without having to download a third-party app.

You can scan a QR code using the camera on your smartphone, tablet, or computer. Scanning QR codes lets you easily access websites, restaurant menus, videos, and more. Most smartphones have QR ...

Now when you click Camera Control, the QR code scanner app will be instantly invoked. You'll also see that there's a Magnifier option that you can assign to the button from this settings menu.