Low vision magnifiers--their optical parameters and methods for prescribing.

Information provided by manufacturers of low vision magnifiers is usually insufficient to allow the clinician to predict accurately the resolution improvement that patients may be expected to achieve. We have measured and tabulated the key optical parameters of 92 stand magnifiers and 53 hand-held magnifiers. For the fixed focus stand magnifiers, the image location and the equivalent power of the lens system have been determined and the enlargement ratio has been derived. For each magnifier, 3 different eye-to-lens distances (2.5, 10, and 25 cm) have been considered and, for each of these, the Equivalent Viewing Distance (EVD), the eye-to-image distance, and the theoretically predicted field width have been computed. The EVD is useful in predicting resolution performance because, for a given patient, the visual resolution limit will be directly proportional to the EVD. The eye-to-image distances allow the clinician to consider whether the patient will be in satisfactory focus or whether adjustments need to be made to the power of any reading addition. The stand magnifiers are listed in order of the EVD they give when the eye is a moderate (10 cm) distance from the magnifier. For the hand-held magnifiers, the equivalent powers have been measured. The EVD for a hand-held magnifier will be the same as the equivalent focal length of the magnifier if the lens is used so that the image is at or close to infinity. The tables include supplementary information on the size of the magnifier lenses and whether the magnifiers incorporate battery, electric, incandescent, or halogen lighting systems. Examples are presented to illustrate how the tables might be used in the selection of magnifiers to meet the resolution and other needs of individual patients.