In cephalometric radiography, the distance between the subject and the source of X-ray is
(A) 2 feet (B) 48 inches (C) 4.8 metres (D) 5 feet
(D) By convention, the distance from the X-ray source to the subject's midsagittal plane is 5 feet.
Contemporary Orthodontics, William Proffit, 6th ed, pg-174