Display screen on galaxy 5s very dim and the brightness is turned all the way up
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When you slide it all the way up is there still a check mark next to the slider for Auto brightness still there?
There could be dust inside your phone near the proximity sensor. That has been an issue with some Samsung phones. Get a can of air and gently blow the air in to the speaker grill of the phone angling the air direction upwards towards the sensors. Make sure not to put the spray too close so that you don't spray the cold frost on to the phone.