§ 29. Mr. Galeasked the Secretary of State for Social Services if he will detail the relevance to an optical prescription which exceeds the power of plus or minus five dioptres of the inclusion of a back vertex distance.
§ Mr. Kenneth ClarkeThe effective power of a lens varies according to its distance from the eye. The positioning of the trial frame or refractor head during a sight test does not always correspond with the positioning of the spectacle frame of a patient's choice. A record of the back vertex distance used during the sight test enables the dispenser to determine by a simple calculation of by reference to a standard set of figures whether any compensating modification of lens power is necessary for a particular frame. We are advised that this information is usually important only for higher power lenses where the degree of modification may be significant. Individual practitioners decide at what point it is necessary to include the back vertex distance in a prescription.