CoreFull access
Exposure maintenance formula
When SID goes up, mAs must go up by the square of the distance change to keep the same receptor exposure.
The formula
The exposure maintenance formula, also called the direct square law or the density maintenance formula, gives the mAs that keeps receptor exposure the same when SID changes: new mAs = old mAs × (new SID ÷ old SID)². The new SID goes on top. The answer moves the same way as the distance: farther means more mAs, closer means less.