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Geometric Unsharpness (Penumbra)
Edge blur from the focal spot's size. Small focal spot, short OID, and long SID keep edges sharp.
The rule
Geometric unsharpness, also called penumbra, is the blurred border around the edges of structures in the image. It happens because x-rays come from a focal spot with real width, not from a single point. Each edge is projected from many points across the focal spot, so its shadow fades instead of stopping sharply. It is present even when the patient is perfectly still.