If the focal-film distance is changed, which quantity should be recalculated to maintain proper exposure?

• A. kV
• B. FFD
• C. Exposure time
• D. mAs

Answer: (D)

When the distance from the focal spot to the film changes, the beam spreads differently and the amount of radiation reaching the film shifts according to the inverse square law. Increasing the focal-film distance makes the beam more diffuse and reduces exposure at the film; to restore the same receptor exposure, you adjust how much radiation you’re producing, which is controlled by the mAs (the product of tube current and exposure time). So you recalculate mAs to compensate for the distance change. If you double the distance, you’d typically increase mAs by about fourfold to maintain the same exposure, while keeping kV the same to preserve penetration and contrast. Exposure time is part of mAs, but the metric you adjust to maintain exposure is mAs.