Nikon D5500 - Manual (Page 151)

127 P, S, A, and M Modes
Autoexposure Lock
Use autoexposure lock to recompose photographs after using
M (Center-weighted metering) and N (Spot metering) to
meter exposure; note that autoexposure lock is not available in
i or j mode.
1
Meter exposure.
Press the shutter-release
button halfway.
2
Lock exposure.
With the shutter-release button
pressed halfway (q) and the subject
positioned in the focus point, press
the
A (L) button (w) to lock
exposure.
While exposure lock is in effect, an AE-L
indicator will appear in the viewfinder.
3
Recompose the
photograph.
Keeping the
A (L) button
pressed, recompose the
photograph and shoot.
Shutter-release
button
A (L) button