A measurement instrument and its associated method of use to record a pupil center position of a persons eye and determine rear vertex distance to a lens. The instrument has a chassis and a free swinging assembly that is pivotably attached to the chassis. The swinging assembly has alignment points that remain in a horizontal plane as the swinging assembly swings. The instrument is attached to a set of eyeglass that are worn on that persons head. A prism is suspended adjacent the eye. The pupil is viewed through the prism. The offset of the viewed image is measured and used to calculate the rear vertex distance.
