Turn Off Auto Brightness
The iPhone 8 hides the ability to turn auto brightness off in an effort to help you get better battery life, but if you need to keep your screen at a specific level so that it is easy to read or so your battery lasts longer you can still turn it on.
This will turn auto brightness off completely. From here you will need to use the Display settings or the Control Center to change your iPhone 8 brightness manually.
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