Nikon D90 User Manual page 78

58
Do not change the distance between the camera and the subject while focus lock is
in effect. If the subject moves, focus again at the new distance.
• Continuous Mode (pg. 65)
Use the AE-VAF-L button to lock focus in continuous mode.
• See Also
Custom Setting f4 (Assign AE-L/AF-L Button; pg. 200) controls the behavior of the AE-lIAF-L
button.