The ratio of image size to object size is called what?

• A. Magnification
• B. Distortion
• C. Penumbra
• D. Mottle

Answer: (A)

Magnification is the ratio of the image size to the object size. In radiography, the x-ray beam fans out from the focal spot, so structures tend to appear larger on the image than they are in real life. How much magnification you get depends on distances: increasing the distance from the source to the object (SID) reduces magnification, while increasing the distance from the object to the image receptor (OID) increases magnification. The standard way to express it is image size divided by object size (often described in radiography as SID divided by SOD).

This concept is different from distortion, which is about changes in the shape or relative dimensions due to alignment or beam angulation; from penumbra, which is the blur at edges caused by the finite size of the focal spot; and from mottle, which is random variation in image brightness due to underexposure.