Source image distance

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Focus film distance (FFD) is the distance between the X-ray source and the film in diagnostic radiography. It has recently been superseded due to the replacement of film with computed radiography or direct digital radiography detectors. There are now various possible alternatives: source image distance (SID); focus image distance (FID); source receptor distance (SRD); and focus receptor distance (FRD). SID seems to be the most common - perhaps as it was already in use as a term in fluoroscopy to describe source intensifier distance.