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Magnification Factor and OID
MF = SID ÷ SOD. Divide the image size by MF to find true object size; keep OID small and SID long.
The rule
Every radiographic image is at least slightly larger than the part itself. The magnification factor, or MF, tells you how much larger. MF equals SID divided by SOD, where SOD, the source-to-object distance, is SID minus OID. An MF of 1.0 would mean no magnification at all, which would require zero OID. Even a part resting on the detector has some depth, so MF is always slightly above 1.