In repeated focal-length measurements, students consistently read distances from the front surface of a thick lens instead of from its principal plane. Which of the following actions would most effectively reduce the systematic error introduced by this practice?
a) Use a smaller object to minimize depth-of-field effects during focusing.
b) Measure both object and image distances from the same principal plane reference point.
c) Average a larger number of independent distance trials.
d) Measure both distances from the lens’s center of curvature instead of its surface.