Which statement about the line-focus principle is true?

• A. The apparent focal spot size increases when the anode angle decreases
• B. The apparent focal spot size increases when the anode angle increases
• C. The apparent focal spot size remains constant regardless of the anode angle
• D. The line-focus principle describes filtration changes

Answer: (A)

The line-focus principle shows how the apparent size of the focal spot on the image is governed by the angle of the anode. The actual focal spot is fixed on the target, but the way it projects toward the image receptor depends on the anode angle. When the anode angle is smaller (more shallow), this projection magnifies the focal spot, making it appear larger on the radiograph and increasing geometric unsharpness (blur). Conversely, a larger anode angle reduces that projection, so the apparent focal spot size decreases and sharpness improves, within heat-loading limits. Filtration changes are not described by this principle, which is why that statement isn’t correct.