Nikon D5100 menu buttonExplanation: Pressing both buttons with a green dot next to them at the same time will reset important camera settings to their default values. You need to keep them pressed for about two seconds until the monitor appears blank for a moment and the shooting information should be switched on before.

The impacted settings and Nikon’s default values for them are:

setting default value
picture quality JPEG NORM
image size L
white balance automatic (AUTO),
fine tuning neutral
ISO sensitivity AUTO and SCENE: automatic
other modes P, S, A, M: ISO 100
release mode single

exception: continuous for scene modes sport and pet portrait

AF focus tracking automatic AF-A

exception: manual focus in effect mode „night vision“

AF focus point selection automatic
exception: some scene modes use single focus point or dynamic selection
AF (Live-View) AF-S
AF focus point selection
wide in modes P, S, A, M and some scene modes

portrait AF in AUTO and some scene modes

exposure metering matrix
exposure lock off
Active D-Lighting (ADL) off
bracketing off
Picture Control the currently chosen picture control configuration stays active but will be reset to its initial values
HDR off
flash exposure compensation off
exposure compensation off
FV lock off
flash synchronisation fill flash
exception: some scene modes use other setting
multiple exposure off
program shift off

Recommendation: The reset is a useful help because it puts back all settings that are most frequently adjusted.
But if you prefer settings different from Nikon’s defaults you have to restore them after each reset.