Understanding how simple magnifiers provide image enlargement.

The simple magnifier is a positive power, usually single lens placed between the page and the eye to give an enlarged image. Its 'magnification' is typically expressed as its dioptric power divided by four but this presumes that the reader uses the magnifier in a specific way. Magnifiers rarely meet their rated specification so they often disappoint the expectations of both patients and eye-care practitioners for vision improvement. In this paper, I describe the basic optics of the simple magnifier and a protocol for the provision of a suitable magnifier to meet general vision needs. Advice on the use of a magnifier should be given to patients at the time of prescribing, thus ensuring practical outcomes for vision improvement and success in meeting the patient's vision needs.