Magnification increases when which relationship holds between SOD and OID?

• A. SOD increases and OID increases
• B. SOD decreases and OID decreases
• C. SOD increases and OID decreases
• D. SOD decreases and OID increases

Answer: (C)

Magnification in radiography depends on the geometry of the distances between the x-ray source, the object, and the image receptor. The magnification factor M equals SID divided by SOD, and since SID equals SOD plus OID, this can be written as 1 plus OID over SOD. This shows magnification grows as the ratio OID/SOD increases. So to increase magnification, you want the object to be closer to the source (smaller SOD) or farther from the image receptor (larger OID), or both. The relationship that produces a larger magnification is SOD decreasing and OID increasing. If both SOD and OID increase or decrease together, the ratio stays about the same; if SOD increases while OID decreases, magnification decreases.