closer working distance and two JNDs of higher dioptric power.” “What do you mean about JNDs of working distance and dioptric power?” E. N. asked. Professor McIver pointed to the logMAR chart/ calculator and said, “Every step between lines is a JND, and it doesn’t matter if the JND is an acuity, a working distance, or a dioptric power; a JND is a JND is a JND. When we want to improve acuity to a desired goal from the measured baseline, we have to reduce working distance and increase dioptric power by the same number of JNDs as is the number of steps from measured to goal. In our example above, we know the patient is reading at 40 cm. By finding 40 on the chart, we see two steps smaller for a closer working distance is 25 cm. So, we must move the chart to 25 cm for the patient to see it. However, since most of our low vision patients are presbyopic, we also need to adjust the add power upwards. The dioptric equivalent of the original working distance is 2.50 D. Making that higher by the two JND steps necessary takes us to 4.00 D. If you then put in a 4.00 D add and hold the chart at 25 cm, all other things being equal, the patient will be able to read 1.0 M. No calculations, no formula, no guessing; just using the slide rule-like power of the logMAR chart’s progression unleashes the power of predictive analysis.” “That is so unexpectedly easy,” Remi stated. “All we have to do is measure properly, set a goal, count the steps between measured and goal, then implement based on JND. Are you saying that now I can just use predictive analysis for my low vision patients and not be so nervous?” “Yes, Remi, it is easy, but most people I have encountered over the years try to make it more difficult or think it is some kind of ‘voodoo’,” Professor McIver said. “Just a few simple steps takes you from current VA to goal-meeting without fuss. The key is to do the parts in order, and since there are only four parts, it is simple to remember.” “What about distance vision?” E. N. interjected. “Many patients with partial vision want to see better for travel, television, or other distance tasks like driving. How do we help them when distance is, by definition, zero diopters?” “Great question, E. N.” Professor McIver said, turning toward him. “The same principles apply, except instead of diopters, we use telescopic power or times (x). Follow me on this: when we have fully corrected a patient to their best distance visual acuity, what is the telescopic power in play? Anyone? Trick question, actually, as it involves the multiplicative identity of anything times one is itself. Therefore, our fully corrected patient in our exam chair is using their natural telescope power of ‘one’. We still take best visual acuity with actual letter size and actual test distance, part one of predictive analysis, and if it is not good enough, we set a goal, which is part two, before counting the steps between our measurement acuity and goal acuity, that being our part three. Then, for part four, we implement it, but instead of starting at the working distance or diopters, we start at 1.0, which is the patient’s incoming telescopic power, and count up the chart. Here is an example: a patient sees 20/100 but wants to see 20/40, the common visual acuity needed for obtaining a driver’s license. From the logMAR chart/calculator, we see that it takes four JND steps to get from 20/100 to 20/40. Now, go to 1.0 on your logMAR chart/calculator and count up four steps: 1.0—>1.25—>1.6—>2.0—>2.5, and there it is, we need a 2.5 x telescope to get our example patient to see their goal. Again, no formulae, no calculations, no guessing and no voodoo. By the way, the most common error here is to mistake telescopic power of x for dioptric power or ‘D’; don’t fall into that trap.” “My goodness, Professor McIver, that is a lot to take in,” Hugh slowly said. “No wonder it hasn’t caught on.” “I get your point, Hugh,” Professor McIver said understandingly. “But I believe that trying to memorize reams of formulae that have many assumptions built in is really the harder path. Once you recognize the power and simplicity of logMAR thinking and predictive analysis, and how they free you from formulae, calculating, and guessing, I think you will wonder how you practiced low vision without it. All you have to do is practice with predictive analysis and become familiar with it in order to appreciate its utility and power.” “That’s it for now.” Another Added ‘Bennettefit’ “Science advances by the relentless examination of the commonplace; that some of its greatest discoveries have been made through fascination with what other men have regarded as not worthy of note.” –Jules Henry “Well, if it isn’t Logan Mars,” Professor McIver said, looking up from his desk during office hours. “Please come in. Is something on your mind?” “Good afternoon, Professor McIver,” Logan began. “I am intrigued by the applications of logMAR thinking and predictive analysis and see how they can rationalize my approach to low vision exams, but Optometry & Visual Performance 167 Volume 14 | Issue 2 | June 2026
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