Samsung Galaxy Tab A (10.5) - Adjust Brightness Settings
From a Home screen, swipe up or down from the center of the display to access the apps screen.
Navigate: Settings > Display (left pane).
From the Brightness section (in the right pane), slide the blue bar left or right to adjust brightness.
To automatically adjust brightness based on current lighting conditions, tap the Auto brightness switch to turn this setting on o desactivar .