Auto Brightness (Eco) - Panasonic TC-P65ST50 Owner's Manual

Watching TV
For best picture

Auto brightness (ECO)

Contrast Automatic Tracking System (C.A.T.S.) adjusts the screen brightness and
contrast according to the room's ambient light conditions. This ECO feature
automatically dims the brightness of the screen when the room lighting is lowered.
C.A.T.S. features a built-in delay when the lighting conditions change. The adjustment
happens gradually.
If you prefer a consistent brightness, disable C.A.T.S. by:
Menu > Picture
Or
Menu > Picture
* Standard picture mode automatically sets C.A.T.S. to On.
> scroll down to C.A.T.S.
> Picture mode then select viewing mode such as Custom.
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and turn it off.