Increasing the object-to-image distance (OID) while the source-to-image distance (SID) remains constant will cause focal spot blur to do what?

Increasing the object-to-image distance (OID) while the source-to-image distance (SID) remains constant will cause focal spot blur to do what?

• A. blur decreases
• B. blur increases
• C. no change
• D. blur becomes unpredictable

Answer: (B)

Focal spot blur, or geometric unsharpness, grows with how far the object sits from the image receptor. The amount of blur is proportional to the focal spot size times the object-to-image distance and inversely proportional to the source-to-image distance. When the source-to-image distance is held constant, increasing the object-to-image distance makes the blur disk projected from the finite focal spot larger on the image, so edges become fuzzier. In short, moving the object farther away increases blur.

Focal spot blur, or geometric unsharpness, grows with how far the object sits from the image receptor. The amount of blur is proportional to the focal spot size times the object-to-image distance and inversely proportional to the source-to-image distance. When the source-to-image distance is held constant, increasing the object-to-image distance makes the blur disk projected from the finite focal spot larger on the image, so edges become fuzzier. In short, moving the object farther away increases blur.