Final answer:
The effective focal spot size in X-ray imaging decreases as the target angle decreases, due to the line focus principle, resulting in a sharper image.
Step-by-step explanation:
Because of the line focus principle, the effective focal spot size decreases with decreasing target angle. The line focus principle is utilized in X-ray tubes to produce a smaller effective focal spot on the target, thereby enhancing the sharpness of the image while maintaining a sufficiently large area on the anode to handle the heat produced during X-ray generation. When the target angle is decreased, the actual length of the focal spot stays the same, but the projected length, which is what affects the sharpness of the X-ray image, becomes smaller, leading to an improved resolution.