When the SID is 40 inches (102 cm), the actual SID should match the displayed SID within a range of:

When the SID is 40 inches (102 cm), the actual SID should match the displayed SID within a range of:

• A. +/- 0.2 inches (0.5 cm)
• B. +/- 0.4 inches (1.0 cm)
• C. +/- 0.8 inches (2.0 cm)
• D. +/- 1.6 inches (4.1 cm)

Answer: (C)

Accurate SID (source-to-image distance) indicators are essential because small deviations change magnification and exposure calculations. The tolerance for SID accuracy is typically a percentage of the indicated distance. For a 40-inch SID, the standard tolerance is ±2%. That means the actual SID should be within 0.8 inches of the displayed value (40 inches × 0.02 = 0.8 inches), which is about 2.0 cm. So the acceptable range is 39.2 to 40.8 inches (roughly 99.5 to 103 cm). This is why the correct range is ±0.8 inches (±2.0 cm).

Accurate SID (source-to-image distance) indicators are essential because small deviations change magnification and exposure calculations. The tolerance for SID accuracy is typically a percentage of the indicated distance. For a 40-inch SID, the standard tolerance is ±2%. That means the actual SID should be within 0.8 inches of the displayed value (40 inches × 0.02 = 0.8 inches), which is about 2.0 cm. So the acceptable range is 39.2 to 40.8 inches (roughly 99.5 to 103 cm). This is why the correct range is ±0.8 inches (±2.0 cm).