Which object-to-image distance (OID) would cause the greatest amount of penumbra?

• A. 0.5 inch (1.3 cm) OID
• B. 1.0 inch (2.5 cm) OID
• C. 1.2 inch (3.0 cm) OID
• D. 2.0 inch (5.1 cm) OID

Answer: (D)

Penumbra, or edge blur, comes from geometric unsharpness in the imaging setup. When the distance between the object and the image receptor (OID) increases, the rays from the finite-sized focal spot diverge more before reaching the image plane, so the shadow edge of the object becomes broader. This means larger OID leads to more blurring at the edges, assuming the focal spot size and the source-to-image distance stay the same. Among the given options, the largest OID is 2.0 inches, so that distance produces the greatest penumbra. The smaller OIDs produce progressively less blurring. (Using a smaller focal spot or increasing the source-to-image distance would reduce penumbra overall.)