Accurate entries for Distance to target and Focal Length are REQUIRED for accurate displacement measurements. Displacement calculations depend on these entries with an error equivalent to the error in the measurements. The closer you are to an object, the more critical the accuracy of the entries becomes because the error has a larger impact. For example, a 1 inch (25 mm) error at 12 inches (305 mm) is a larger percentage than a 1 inch (25 mm) error at 100 inches (2540 mm).