Computer-assisted eye examinations IV. "Additive" lens systems in eye refractors.
After a brief introduction to the optical principles and terms, formulas are derived which demonstrate that with conventional methods and designs it is not possible to summate with accuracy the labeled power of three or more ophthalmic lenses in tandem. The effective power of sphero-cylindrical  combinations in all currently available eye refractors, when powers of eight to ten diopters are exceeded, may differ from the summated labeled power by a quarter diopter or more, which can be clinically significant. This error can be directly and readily eliminated only in computer-actuated refractors by including a computer program which calculates the effective power and makes the necessary correction.
D003936:Diagnosis, Computer-Assisted; D006801:Humans; D007909:Lenses; D008722:Methods; D055095:Optics and Photonics; D012029:Refraction, Ocular