What is the significance of source-to-image distance (SID) in radiography?

• A. Increasing SID increases magnification and worsens sharpness.
• B. SID has no effect on image quality.
• C. Increasing SID reduces sharpness and requires less exposure.
• D. Increasing SID reduces magnification and improves sharpness but requires higher exposure to maintain receptor exposure.

Answer: (D)

Increasing source-to-image distance reduces the geometric blur that comes from the divergent X-ray beam. With the receptor farther away, the rays fan out less across the object, so magnification goes down and sharpness improves. But the beam’s intensity falls off with distance (inverse square law), so to keep the same receptor exposure you must raise the exposure (increase mA·s). In short, larger SID makes images less magnified and crisper, but requires higher exposure to maintain the same image receptor dose.