Which object-to-image distance (OID) causes the greatest amount of penumbra: 0.5 inches or 2.0 inches?

Which object-to-image distance (OID) causes the greatest amount of penumbra: 0.5 inches or 2.0 inches?

• A. 0.5 inches
• B. 2.0 inches
• C. Both equally
• D. Neither

Answer: (B)

Penumbra is the blurred edge you see on an image, caused by the finite size of the focal spot and the projection geometry. When the object sits farther from the image receptor, the rays coming from opposite parts of the focal spot diverge more before reaching the image. That larger divergence makes the shadow of the object spread out more on the image plane, increasing the penumbra. With an object-to-image distance of 2.0 inches, compared to 0.5 inches, this blur is much larger, so the image has the greatest penumbra at the larger distance.

Penumbra is the blurred edge you see on an image, caused by the finite size of the focal spot and the projection geometry. When the object sits farther from the image receptor, the rays coming from opposite parts of the focal spot diverge more before reaching the image. That larger divergence makes the shadow of the object spread out more on the image plane, increasing the penumbra. With an object-to-image distance of 2.0 inches, compared to 0.5 inches, this blur is much larger, so the image has the greatest penumbra at the larger distance.