The measuring magnifier by Bausch & Lomb has a Hastings Triplet lens. This means that the lens is a compound lens specially engineered so that there is no distortion near the outer edge of the lens, and the color of the magnified image is true throughout the visual field. The measuring magnifier is adjustable for individual focus and provides a flat, distortion-free image. The transparent body allows ambient room light to fall on the magnified area. The transparent body fits on to any one of four reticles (sold separately) on which are etched rulers, lines, curves and angles."