it seems there must be more to it. I mean, I see how to use the logMAR chart/calculator to quickly get the appropriate add to meet patient’s reading goals and also to use it for choosing telescope power for distance, but not everyone uses M-Units or metric distances. What happens then?” “Very good observation, Logan. I have planned that for today’s lecture and workshop. Let’s get to the lecture hall and address your questions with the whole class,” Professor McIver said. “Walk with me.” “Logan brought a good question to me during office hours,” Professor McIver said, opening class time. “He is interested in going beyond the basics of logMAR thinking and predictive analysis, especially regarding non-metric measurements such as reduced Snellen acuity measurements or the ‘n’ (point) system. Let’s jump in. First, there are another few things for you to memorize.” “Wait, Professor McIver, you said we didn’t have to memorize stuff with LogMAR thinking,” protested Karen. “Now you say we need to memorize more. That sounds like a bait and switch.” “Karen, what I said was you don’t have to memorize as much,” countered Professor McIver. “Besides, it is on the logMAR near chart/calculator if you forget something, so memorizing just helps you along but is not essential. Let’s look at what I mean. Everyone, pull out your handy near logMAR chart and note what it says at the top: 16 inches or 40 cm. Now notice that those are numbers on the chart that are four steps apart. Now you can easily transpose inches to cm as they are a constant ratio like the logMAR chart/ calculator! Since the number of cm is always larger than the number of inches for the same distance, you always know which way to go on the logMAR chart. For instance, 10 inches is 25 cm. Therefore, if someone tells you they measured a near acuity at 10 inches, you know the numerator of the reduced Snellen fraction is 0.25 m.” “That is useful, alright,” Karen conceded, “but that only gets us the numerator. What about the denominator in cases where the reported near visual acuity is in reduced Snellen or point?” “Let’s solve for that now!” Professor McIver exclaimed, happy for the lead-in to the next part. “look at your near point logMAR chart and find the 1.0 M size row. Got it? Ok, now look across the row, and you should see 20/50. If someone reports a reduced Snellen acuity of 20/50, you know the letter size was 1.0 M; that is your denominator. Now, hopefully, they also record a working distance. If that is 40 cm, then your actual psychometrically accurate reduced Snellen fraction would be: 0.4 m/1.0 M. If, however, the working distance is reported as 10 inches, you can quickly count up four steps to 25 cm and record the VA as: 0.25 m/1.0 M.” “Now, the point system uses an actual size (1 pt = 1/72 inch), and you have to memorize this: 8 pt = 1.0 M, but once you have that, you can see they are only separated by a single step with a decimal place moved. To transpose from point to M-Units, you just go one step up the chart and move the decimal to the left. Also, you recall from above that since 1.0 M = 20/50 (on a standard logMAR near chart), then 8 pt = 1.0 M = 20/50; another of the few things useful to memorize. “So, if a near acuity is reported as being 20/100 at 10 inches, in the past you might have been flummoxed as to what the near visual acuity actually is, but now, using those minimal memorized factors and the power of the logMAR chart/calculator, you can determine the actual working distance as 25 cm and the actual size as 2.0 M, giving you a psychometrically valid reduced Snellen fraction of 0.25 m/2.0 M.” “Is that all you can do with logMAR thinking and predictive analysis?” came the smart aleck anonymous voice from the back of the classroom. “While sarcasm has not incorrectly been described as an example of intelligence poorly used,” Professor McIver retorted, “there is another useful bit of logMAR magic you might find charming. In my travels, it has come in quite handy. Specifically, one can translate miles to kilometers and back, as they are separated by only two steps on the logMAR chart/calculator; i.e., 100 km = 63 miles or 40 kilometers = 25 miles. This is especially good to know when one has a speed limit to mind in a foreign land.” “Is that it?” came the plaintive voice again. “Is that all that you have for us?” “As a matter of fact, no, there is another added logMAR ‘Bennettefit’ that is something you might find much more useful in clinical practice than just miles to kilometers and back,” Professor McIver again countered. “Reciprocals of diopters and meters are built into our logMAR chart/calculator. Once again, take another look at your near point logMAR chart/ calculator and find the 1.0 M. Got it? Okay, now go one step up to find 1.25 and one step down from 1.0 to find 0.8; those are reciprocals. 8.00 D focuses at 12.5 cm, and 1.25 D focuses at 80 cm. Now, return to 1.0 and count three steps up from there to find 2.0, then three steps down from 1.0 to see 0.5: 5.00 D focuses at 20 cm and 2.00 D focuses at 50 cm. Quickly you can see that reciprocals are symmetric about 1.0! No more Optometry & Visual Performance 168 Volume 14 | Issue 2 | June 2026
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