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Dose Area Product
DAP = air kerma × field area. Tighter collimation cuts it; distance from the tube does not.
The rule
Dose area product, or DAP, is the air kerma in the x-ray beam multiplied by the area of the beam. It is reported in units such as Gy·cm² or mGy·cm². It is usually measured by a flat transmission ionization chamber mounted at the collimator exit, so the reading includes all the radiation leaving the tube housing toward the patient; some systems calculate it from technique and collimator settings instead. You may also see it called kerma area product, or KAP.