Almost all of us have been to the eye doctor before and sat behind that machine with all the knobs and dials. It’s got a very distinct, steampunk look to it. It looks like it was designed by someone with instructions to “make something medical-looking and make it look like it’s from the time of the Industrial Revolution.” It’s iconic. And it’s anxiety-provoking.

Why Do We Use A Phoropter?

That machine is called a phoropter. And despite its old-timey look, it is still the most accurate test we have for prescribing glasses. “It’s 2023!” you may say, “Shouldn’t we be able to do that with laser scanning devices or something?” That’s how I feel too! And yet, here we are. Optical biometry, laser interferometry, and all the other wonders of modern science can’t touch what we can achieve with the humble phoropter.

That’s part of what causes the feelings of anxiety when we sit behind the phoropter. This test is the most accurate way possible to prescribe glasses. Yet, your answers in the form of “one” or “two” are the only measurements used. It seems like there’s so much room for error. Especially when “one” and “two” are barely any different, and you can’t tell which one you like more.

How Does A Phoropter Work?

Today, I’d like to reduce that anxiety. I sit on the other side of the phoropter and turn those knobs and dials. But for you to feel better about the exam, you don’t have to know what they all do. You only have to know how your answers decide the way those knobs and dials turn. Your answers aren’t like trying to fire arrows and hit a target. Your answers are like a game of “hot or cold.”

Remember playing that as a kid? Wandering around trying to find something, you’d get clues that were very simple, but exceptionally good at leading you to a spot. There were only two words. A simple “warmer” or “colder” — repeated over and over — was enough to let us find a location that would otherwise require a lot of words to describe.

That’s the game I get to play every day at the phoropter. You are the one giving the clues. When you say “better one” or “better two” as you look through different lenses, what you’re really doing is leading me with answers of “warmer” or “colder.” This includes every now and then when you’ll say, “ugh, I can’t see anything.” That’s the equivalent of the infamous, “ice cold, you’re freezing!”

Are Phoropters Accurate?

But the anxiety you may face usually comes from the opposite issue. It’s when you can barely tell, or can’t tell at all, the difference between “one” or “two.” And that is where I have truly good news! When you struggle to tell the difference between two choices, that is the equivalent of the thrilling, “HOT! You’re on fire!” Because we’ve made it to our destination.

There’s a reason for that. Phoropters determine prescriptions and prescriptions are measured in diopters. The smallest unit of measurement in a prescription is a quarter of a diopter. Therefore, the smallest increment on the phoropter is a quarter of a diopter. For example, if choice one is -2.25 diopters then choice two would be -2.50 diopters. There is no choice available between those two.

Yet, no human eye (except by extreme coincidence) has a prescription that’s exactly -2.25. It might be -2.375 or -2.415 or any other tiny increment between -2.25 and -2.50. So, you can imagine what happens when someone with a prescription at one of those tiny increments in between -2.25 and -2.50 has to answer which is better. They look too similar to tell the difference! That’s our “you’re on fire!” moment. We have found our answer. It happens right at the moment when you’re saying to yourself, “I’m failing this test because I can’t tell which one is better.”

One final note: In case you’re now wondering, “But that means you only got close to the number without finding the exact prescription down to an eighth or tenth of a diopter.” First off, I admire you. You’re a person who values exactitude and so do I. Second, it genuinely doesn’t matter. The cut-off had to be somewhere, whether it was a quarter or an eighth or a hundredth of a diopter. The reason a quarter is chosen is because it is exceptionally rare for a human eye to be able to discern an increase in clarity in units smaller than that. It’s because our eyes vary slightly throughout the day. So when you can’t tell the difference between “better one, better two” it means we’ve found the prescription that will cover you most accurately throughout the day.