Even at its lowest setting, an iPhone's display can be too bright if you're reading in bed at night or in an otherwise dark room. If you find you're still blinded by the light even after moving the brightness slider all the way to the left in Control Center, there is a way to dim the screen past what the slider allows. In Settings, you can enable a low-light filter and then make it readily accessible from the home button.
1. Go to Settings > General > Accessibility > Zoom.
2. At the top of the Zoom page in settings, tap the toggle switch on for Zoom.
3. Scroll down on the Zoom page in settings and select Full Screen Zoom for Zoom Region and Low Light for Zoom Filter.
Now, with zoom enabled, you can double tap with three fingers to zoom in and out, keeping the low light filter in place either way. That is, if you are zoomed in, you can do a three-finger double tap to zoom out to the normal zoom level and read on an ultra-dim screen.
To make it easier than digging into settings each time you want to enable zoom's low-light filter, head back to the Accessibility page in Settings and scroll all the way to the bottom. Tap on Accessibility Shortcut and choose Zoom. Now, you can triple-click the home button to turn zoom with its low-light filter on and off.