How do I lower the brightness further than the limitations on the iPhone / iPad?
To lower brightness lower than the limit:
Settings app > General > Accessibility > Zoom:
Set Zoom Region > "Full Screen Zoom"
Set Zoom Filter > "Low Light"
Set Maximum Zoom Level all the way to the right (The highest value)
Go back to Accessibility:
Settings app > General > Accessibility > Accessibility Shortcut:
Set "Triple-Click The Home Button For" > Zoom
To Test: Triple click the home button and the screen will go darker.