What is the film focal distance (FFD) in radiography?

• A. Distance of the xray plate from the beam
• B. Distance from the beam to the patient
• C. Image sharpness
• D. Exposure time

Answer: (A)

FFD is the distance from the x-ray tube’s focal spot to the image receptor (the film plate). This is the spacing the beam travels to reach the film, so describing it as the distance of the x-ray plate from the beam captures that exact measure. This distance helps define the image geometry: together with how far the patient is from the plate, it influences magnification and sharpness, and it also affects how strong the exposure at the film will be due to the inverse-square law. The other options refer to exposure time, image sharpness as a property, or to distances involving the patient, not the focal spot to film distance.