Which formula correctly calculates the new mAs when adjusting the FFD?

• A. Old mAs × (New FFD² / Old FFD²)
• B. Old mAs × (Old FFD² / New FFD²)
• C. Old mAs × (New FFD / Old FFD)
• D. Old mAs × (New FFD² / Old FFD)

Answer: (A)

Exposure at the film follows the inverse-square law: the beam intensity at the receptor varies with 1/d^2, so to keep the same receptor exposure when the distance (FFD) changes, the mAs must scale with the square of the distance ratio. Set the old exposure equal to the new exposure: Old mAs / Old FFD^2 = New mAs / New FFD^2. Solving gives New mAs = Old mAs × (New FFD^2 / Old FFD^2). This is the correct way to adjust mAs when changing FFD.

For example, if the FFD goes from 80 cm to 120 cm and the original mAs is 8, new mAs = 8 × (120^2 / 80^2) = 8 × (14400 / 6400) = 8 × 2.25 = 18 mAs.

The other options don’t follow the inverse-square relationship: reversing the ratio would decrease exposure with a longer distance; using only a linear distance ratio ignores the squared law; and missing the square on the denominator gives an incorrect scale.