Nikon D5100 Reference Manual page 5

Choose On to use the exposure indicator to determine whether the camera is
correctly focused in manual focus mode (_ 65; note that this function is not available
in shooting mode M, when the exposure indicator instead shows whether the subject
is correctly exposed).
Indicator Description Indicator Description
Camera in focus.
Focus point is slightlyin
front of subject.
Focus point is well in
front of subject.
Focus point is slightly
behind subject.
Focus point is well
behind subject.
Camera can not
determine correct
focus.
[] Using the Rangefinder
The rangefinder requires a lens with a maximum aperture of f/5.6 or faster and is not
available in live view. The desired results may not be achieved in situations in which the
camera would be unable to focus using autofocus (_ 40).
Select the increments used when making adjustments to
shutter speed, aperture, exposure and flash compensation, and
bracketing.
At the default setting of Off, exposure only locks when the AE-L/
AF-/button is pressed. If On is selected, exposure will also lock
when the shutter-release button is pressed halfway.
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