What went wrong with this shot D5100 with f/1.8 lens

If you are in an auditorium and the lights aren't changing, then use P or A mode to find the right exposure you like and then switch over to manual mode and use those exposure settings.

If you are in P or A mode, everytime you aim your camera at someone with a white or black shirt for instance, your camera will increase or lower exposure based on what it seeing, thus potentially ruining everything else in the scene.

If you are under non-changing lighting conditions, it's best to take your camera off any kinda auto exposure modes because there is no reason to keep changing exposure. Obviously, if you want to go up and down in aperture for creative reasons, then you should adjust shutter speed or ISO accordingly.

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Antonio
http://www.intensitystudios.com