Which distance is measured from the focal spot to the object?

• A. Source-to-Image Distance
• B. Object-to-Image Distance
• C. Source-to-Object Distance
• D. Focal Spot Size

Answer: (C)

In radiography, distances are defined by where they start and end. The distance from the focal spot to the image receptor is called the source-to-image distance, while the distance from the object (the patient) to the image receptor is the object-to-image distance. The distance from the focal spot to the object itself is the source-to-object distance. This particular distance is fundamental for geometric setup and exposure calculations because it directly influences beam intensity at the object (via the inverse-square law) and, along with the source-to-image distance, helps determine image magnification.

The term focal spot size refers to the physical size of the X-ray source, not a distance, so it isn’t what’s asked here.