The paralleling technique generally requires what distance from the X-ray source to the film?

• A. 8 inches
• B. 12 inches
• C. 16 inches
• D. 20 inches

Answer: (C)

In paralleling technique, the goal is to minimize distortion and blur by keeping the image geometry as true to life as possible. A longer distance from the X-ray source to the film helps achieve this because it reduces magnification and penumbra, yielding a sharper, more accurate image. The standard distance used is 16 inches (40 cm). With the tooth positioned between the source and film, increasing the source-to-film distance lowers magnification (M ≈ SID/(SID − OID)) and improves sharpness, which is why 16 inches is preferred over shorter distances like 8 or 12 inches. Very long distances, such as 20 inches, would lessen beam intensity and complicate exposure, so 16 inches represents a practical balance between image accuracy and exposure practicality.