a2 Built-In AF-Assist Illuminator

Nikon D5100 custom setting menu a2 built-in AF-assist illuminatorExplanation: You can choose to deactivate the little lamp called AF-Assist Illuminator here if you want.

Recommendation: If you find the light disturbing because it can distract people just give it a try without. In many situations where the D5100 switches on the lamp it is still able to focus without.