Mean energy imparted in relation to the focus to object distance

The relationship between the energy absorbed by the patient at the exposure of a radiographic image and the focus to object distance is analysed. It is found that the mean energy imparted, or the integral absorbed dose, is the same irrespective of the focus to object distance when the energy fluence and the field size are both normalized at the film plane. When the field size is kept constant within the object whilst the focus to object distance is varied a limited increase in the mean energy imparted occurs when the focus to object distance is decreased; this statement is valid for the focus to object distances which are usually applied in diagnostic radiology. When estimating radiation risks the importance of the focus to object distance is found to be of limited interest.