Nikon SB-28 - Manual (Page 81)

When the light is too dim for normal autofocus operation, the
SB-28’s AF assist illuminator LED automatically turns on when set to
illuminate dark subjects when the shutter release button is
pressed halfway.
• Take care not to block the AF assist illuminator LED while shooting.
• The AF assist illuminator LED turns off as soon as the subject is in focus.
• The AF assist illuminator LED will not light up with an F5, F100,
F80-Series/N80-Series or F65-Series/N65-Series camera unless the
camera’s central focus area is selected.
81
Autofocus flash operation in dim light
For autofocus cameras only