The F-factor, in diagnostic radiology, is the conversion factor between rontgens and rads (or, in more modern units, coulombs/kg to grays). In other words, it converts between the amount of ionization in air (rontgens) and the absorbed dose in tissue (rads). The two determinants are of the F-factor are the effective Z of the material and the type of ionizing radiation being considered. Since the effective Z of air and soft tissue is approximately the same, the F-factor is approximately 1 for many x-ray imaging applications. However, bone has an F-factor of up to 4, due to its higher effective Z.