To minimize magnification, which combination of distances should you use?

• A. Increase source-to-object distance and decrease object-to-receptor distance
• B. Decrease source-to-object distance and increase object-to-receptor distance
• C. Increase both distances
• D. Decrease both distances

Answer: (A)

Magnification in radiography comes from the geometry of the beam as it travels from the source to the object and then to the receptor. The size of the image relative to the object depends on how far the object sits from the source (SOD) and how far the receptor sits from the object (OID). A useful way to think about it is that magnification grows with larger OID and shrinks with larger SOD. In fact, the magnification factor is essentially 1 plus the ratio of OID to SOD.

So to minimize magnification, you want the object as far from the source as possible and as close to the receptor as possible—that is, increase the distance from the source to the object and decrease the distance from the object to the receptor. This makes the beam more parallel when it hits the object and reduces the extra projection beyond the object that enlarges the image. Increasing SOD while reducing OID works best to keep the image size close to the true object size.