A practical logMAR near reference table for low vision practitioners: Design and applications

Abstract

LogMAR charts are particularly useful for visual acuity (VA) measurement for low vision patients as they simplify conversion of non-standard viewing distance visual acuities to standard values. Also, the process of predicting various quantities involved in the prescrip-tion of optical devices for patients are greatly facilitated. However, the processes involved in the use of this logMAR principle require series of multiplication and division by the logMAR ratio of 1.2589. A table computed and pre-sented in this article provides easy reference for the conversions and predictions needed for the various quantities at near. The table (Table 1) is computed using the logMAR scale of 1.2589 in logMAR, M and N notations. The table con-tains VA values between logMAR 1.4 (10M or 80 point) and logMAR 0.18 (1.3M or10 point) in steps of 1.2589 for reduced (non-standard) viewing distances ranging from 31.8 cm (0.32 m) to 3.2 cm (0.03M). The table contains values which can be used to convert visual acuity values measured at reduced near view-ing distances to standard visual acuity values. The values also can be used to represent any of the quantities involved in the prescription of optical devices for low vision patients. The use of the table for converting reduced distance VA values to standard values and for predict-ing optical powers and visual performance of low vision patients are discussed with relevant examples. Like all principles based on the log-MAR principles, the table is quite versatile in VA conversion and prediction of the various quantities such as print size that can be read by the low vision patient when factors such as viewing distance or power are varied.

Keywords

• Conversion table, logMAR, near visual acuity, low vision, magnifiers, print sizes