Which statement correctly describes the relationship between object-to-image distance (OID) and magnification?

• A. more magnification is present with long OID
• B. more magnification is present with short OID
• C. magnification is independent of OID
• D. magnification is unaffected by SID

Answer: (A)

Magnification increases as the distance between the object and the image receptor grows, because the image is formed larger than the object when the object lies farther from the detector. The magnification factor is M = SID / SOD, and the object-image distance (OID) equals SID minus SOD. If SID stays the same and OID increases, that means SOD decreases, so the ratio SID/SOD becomes larger and magnification increases. For example, with SID 100 cm and SOD 80 cm, OID is 20 cm and M = 100/80 = 1.25. If OID increases to 30 cm (SOD becomes 70 cm), M ≈ 100/70 ≈ 1.43, a larger magnification. So more magnification is present with a long OID.