Apple includes an accessibility feature in iOS that's useful if you have a visual impairment, but can even come in handy if your eyes are simply tired or you're struggling to read something like small print, especially in poor light.
It's called the Magnifier, and has several advantages over just opening up the camera app and zooming in to get a better look at something.
Enabling it is easy: Launch the Settings app on your iPhone or iPad, navigate to General -> Accessibility -> Magnifier, and toggle on the Magnifier switch.
After that, all you need to do to use it is triple-click the Side button (or Home button, depending on your device). You can also add it to the Control Center by going to Settings -> Control Center -> Customize Controls, and tapping the green plus button next to Magnifier.
When you launch Magnifier, you'll see a camera-like interface at the bottom of the screen, but with some unique features. The slider controls the magnification of the scene in the lens frame, while the button at the bottom left turns on the flashlight so you can illuminate it. The padlock button next to that locks the focus.
Tapping the big button in the center freezes the image (a frozen image is indicated by a yellow ring around the button), allowing you to move your phone around freely and still look at the image. You can also use the magnification slider to zoom in and out of the frozen image.
Note that when you freeze an image in the Magnifier, it isn't saved to your photo album. But if you want to save the entire image, you can.
Simply tap and hold on the frozen picture and select Save Image from the contextual popup menu. You'll find a Share option in there, too.
Over on the far right of the Magnifier interface is a button made up of three circles that provides access to additional sliders for adjusting brightness and contrast.
If you suffer from color blindness or another visual impairment, you can swipe through several color schemes in this extended menu and even invert the colors to find which combination works for you.
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EDIT: Oh, and perhaps I should also mention, I'm nowhere near 70 and I use Magnifier ALL THE TIME! It is great for reading serial numbers and MAC addresses off hard to reach devices, as I find the auto-focus system to be an improvement for such things over the standard Camera auto-focus. Reading fine print in ads. Etc. Just a handy handy feature.
(a) There are alternative magnification apps available. However the Apple solution seems to do a better job of locking onto focus fast, and then stabilizing the text against jitters. And this seems to have improved in more recent hardware. My iPhone XS seems to be quite a bit more visually stable than my iPhone 7 (OIS? or better processing?). So if this is something you care about and use a lot, it might be worth seeing how well it performs in a newer iPhone compared to what you have, and might make it worth the upgrade.
(b) My experience has been that the triple-click launch method is EXTREMELY dodgy, very likely (30% or so) to lock the phone in some sort of strange state where it's not at all obvious how to get out of magnification mode. Maybe Apple has FINALLY fixed this with iOS 12.1?
For this reason, my preferred launch method is to put a launcher on the Control Center page.