Samsung SCC-B2391 Instructions page 12

Lenses with zoom function
Image an object with high resolution(letticed) at a distance of 3 to 5 m
and zoom in the lens as close to TELE as possible. Then adjust the lens
E
focus bar until the object is seen best.
Zoom in the lens as close to WIDE as possible and adjust the BACK
FOCUS adjustment bar until the object is seen best.
Repeat from to above 2 or 3 times until the focus on the ZOOM
TELE side is in line with that on the ZOOM WIDE side.
BACK FOCUS
CONTROL BAR
12