Why does a small focal spot increase sharpness?

• A. Less x-ray beam divergence.
• B. Higher energy photons only.
• C. Longer exposure time.
• D. More scattering.

Answer: (A)

Sharpness is limited by geometric unsharpness from the finite size of the X-ray source. The edge of a structure is formed by rays from every point of the focal spot, so a larger focal spot creates a wider penumbra and blur along the edge. A smaller focal spot reduces this penumbral blur, producing a crisper, more defined edge and a sharper image. The other factors—photon energy, exposure time, and scattering—affect penetration, motion blur, and contrast, but they don’t directly reduce the geometric blur caused by the focal spot size.