Spatial resolution is improved when

Spatial resolution is improved when

• A. OID decreases, SID increases
• B. SID decreases
• C. OID increases
• D. Focal spot size increases

Answer: (A)

Spatial resolution improves when geometric unsharpness is reduced. The geometric blur, or unsharpness, is roughly proportional to the focal spot size times the object-to-image distance (OID) divided by the source-to-image distance (SID). So, smaller OID and larger SID both shrink this blur, sharpening the image. When OID decreases and SID increases, you minimize the penumbra and sharpness improves because there’s less magnification and less blur from the finite focal spot. If OID were to increase or SID were to decrease, geometric blur would grow and resolution would worsen. Likewise, increasing the focal spot size makes more blur regardless of geometry, reducing sharpness.

Spatial resolution improves when geometric unsharpness is reduced. The geometric blur, or unsharpness, is roughly proportional to the focal spot size times the object-to-image distance (OID) divided by the source-to-image distance (SID). So, smaller OID and larger SID both shrink this blur, sharpening the image. When OID decreases and SID increases, you minimize the penumbra and sharpness improves because there’s less magnification and less blur from the finite focal spot.

If OID were to increase or SID were to decrease, geometric blur would grow and resolution would worsen. Likewise, increasing the focal spot size makes more blur regardless of geometry, reducing sharpness.