Where does the resolution measure the sharpest?

• A. Anode end
• B. Cathode end
• C. Center of the focal spot
• D. Edge of the field

Answer: (A)

The main idea is geometric unsharpness from the finite focal spot and the tube’s anode geometry. Because the anode is angled, the projected size of the focal spot on the image plane isn’t the same across the field. Toward the anode end, the focal spot’s projection is smallest, which reduces penumbra and yields the sharpest detail. As you move toward the cathode end, the focal spot appears larger due to the oblique projection, increasing blur and reducing resolution. The center sits between, and the edges of the field are most affected by off-axis geometry and magnification, making them less sharp than the anode end.