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.