Why does the brightness on my iPhone 5s dim randomly?

I've had my iPhone 5s since November and I've noticed that the screen occasionally dims a bit when it's on full brightness. It's not a huge issue at all, in fact when it dims it's barely noticeable, but it's just been annoying me lately. Does this happen to anyone else with an iPhone 5s? Other than this issue my phone is perfect.

Added (1). And I've also turned auto-brightness off so that's not the issue.

Got to settings
go to wallpaper and brightness
untick auto brightness box

Yes sid fasd

You probably have it on auto brightness. Go into your settings > wallpapers & brightness > turn off you auto brightness. This wil make it stop adjusting the brightness on your phone!

Maybe the auto somehow reactivated on its own.

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