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Turn off auto-brightness; adjust manually
Some believe that the iPhone's "auto-brightness" feature will help conserve battery life by dynamically and automatically increasing or reducing the brightness of the screen, based on how light it is around you. Yet, others argue that this alone can actually lead to the battery draining. It may not be easy to read your iPhone's screen in direct sunlight, and you may not get the most out of your high-resolution display, but this is about conserving your battery rather than anything else.
The best practice seems to point to disabling the auto-brightness. Go to Settings > Wallpapers & Brightness, then reduce the brightness to 10-25 percent, or whatever feels comfortable.