What are the two effects of increasing the object-to-image distance (OID)?

What are the two effects of increasing the object-to-image distance (OID)?

• A. Increased spatial resolution
• B. No change
• C. Decreased spatial resolution and increased penumbra
• D. Decreased penumbra

Answer: (C)

Increasing the object-to-image distance makes the image more magnified, but it also increases geometric unsharpness. This happens because rays from different parts of the finite focal spot spread out more as they travel to the image plane, so edges in the image blur. That blur around edges is called penumbra, and it grows with the distance between object and image. So the image loses sharpness (lower spatial resolution) and the penumbra becomes larger. The other possibilities don’t fit because increasing OID does not enhance sharpness, and it does not leave the image unchanged or reduce the blur.

Increasing the object-to-image distance makes the image more magnified, but it also increases geometric unsharpness. This happens because rays from different parts of the finite focal spot spread out more as they travel to the image plane, so edges in the image blur. That blur around edges is called penumbra, and it grows with the distance between object and image. So the image loses sharpness (lower spatial resolution) and the penumbra becomes larger. The other possibilities don’t fit because increasing OID does not enhance sharpness, and it does not leave the image unchanged or reduce the blur.