What is the acceptable deviation for Light & X-ray Field Alignment relative to source-to-distance?

• A. +/- 2%
• B. +/- 5%
• C. +/- 1%
• D. +/- 10%

Answer: (A)

Light field alignment and the actual x-ray field must stay in close agreement because the light field is used to position and size the exposure, while the radiation field is defined by the beam at the patient and can diverge with distance. Expressing the tolerance as a percentage of the source-to-distance makes the requirement consistent at any SID. A deviation of ±2% keeps the light and radiographic fields matched within a practical, clinically safe margin; for example, at 100 cm SID, this corresponds to about 2 cm. Larger tolerances would risk exposing outside the intended area, while a much tighter tolerance would be unnecessarily difficult to achieve in routine practice. Therefore, ±2% is the acceptable deviation.