FujiFilm FinePix F550EXR - Manual (Page 51)

33 More on Photography
Focus Lock Focus Lock
To compose photographs with off -center subjects:
1 Focus: Position the subject in the focus frame ( )
and press the shutter button halfway to lock focus
and exposure. Focus and exposure will remain locked
while the shutter button is pressed halfway (AF/AE
lock).
2 Recompose: Keep the button pressed halfway.
3 Shoot: Press the button all the way down.