Is this better? Or this?
July 16, 2013 9:20 PM   Subscribe

How do automatic optometric machines work? Why don't U.S. optometrists use them (or why do they do a manual followup?)

I got a new pair of glasses made on a recent trip to Japan, and the optometrist measured my prescription using an automatic machine. In the past, I've had similar automatic measurements in both the U.S. and Japan, but you could see the older machines doing an automatic focusing of a high-contrast pattern (a black-and-white plus sign, IIRC), and even when a U.S. optometrist used a machine, they followed up with the manual Frankenglasses: "is this better, or this?"

The most recent Japanese machine was much faster, and I never saw the image come into clear focus (it looked like a blurry multi-color hot air balloon, FWIW). The glasses I got seem to be fine.

I'd like to know:
  • Was the "incomplete focus" measurement intentional?
  • How does a machine adjust for the fact that the patient's eye can do some focusing too?
  • Is there a significant benefit in doing a manual followup after a machine measurement, or is that just a full-employment program for optometrists?
I'm guessing that the focusing image that looked blurry to me, was intentionally chosen to trick my eyes into trying to focus at infinity so that the machine's measurement wouldn't be affected by my eyes focusing closer.
posted by spacewrench to Health & Fitness (12 answers total) 8 users marked this as a favorite
I haven't had a "Better one, better two?" exam in the U.S. for several years now.
posted by The Underpants Monster at 10:11 PM on July 16, 2013

For what it's worth, last month, I had the "hot air balloon" one, followed by a "1 or 2? 2 or 3?" in the same exam, in an office so high tech that they had the new NON-airpuff glaucoma check (with the little handheld laser-thingie that rests on your forehead and bounces beams off your eyeball). (What?! That's what it was!) My doc said the "hot air balloon" exam narrows the reference points to enable her to limit her approach to the "1 or 2?" exam. Take that as you will.
posted by The Wrong Kind of Cheese at 11:07 PM on July 16, 2013 [2 favorites]

Till now I used to get my eyes checked at a little eyewear store in suburban Maryland, and I have no idea what you're talking about.

How does a machine adjust for the fact that the patient's eye can do some focusing too?

The purpose of the pupil-dilating (cycloplegic) drops is to paralyze the ciliary muscle of the eye, which is the mechanism by which the eye can selectively focus on objects at varying distances. The goal isn't really to blind you for three hours. Did you get those drops administered during your visit?
posted by Nomyte at 12:14 AM on July 17, 2013

The 'blurry hot-air balloon' machine my own doc uses is the glacoma check that Wrong Kind of Cheese mentions: nothing to do with how good or bad my eyesight is. His "1 or 2? 2 or 3?" lensflipper thing is used in conjunction with an alphabet chart. (In Virginia, FWIW.)

Maybe it's my Luddite tendancies, but I kinda LIKE having the eye doc manually checking each variation, as opposed to making it totally automatic..... not sure I TRUST machines enough for that.
posted by easily confused at 2:03 AM on July 17, 2013

The automatic eye checkers work kind of like how an autofocus works on a camera. The device projects an image onto the retina, and then the circuitry reads the image back and adjusts the focus until the image is clear. It converts whatever adjustments it made into a lens prescription and off you go.

However, what is perfect to the machine might not be the most comfortable to the patient. That's why doctors still do the manual checks.
posted by gjc at 3:09 AM on July 17, 2013 [2 favorites]

I asked this question of my optometrist at my last appointment--that with all the technology out there, you need to do the "which is better" thing.

Her answer: There is technology that can figure out the exact prescription you need by looking at your eye (I assume the machine you refer to), but sight depends on two things--the status of your eye and how your *brain* interprets the image. So, the optometrist can figure out the prescription for the first part, but has no idea how your brain is interpreting or compensating for what it sees or is used to seeing. So, they need to do the "which is better" thing to compensate for the brain part. Very cool! Brains *are* awesome!

In the end, they often use the direct measurement of the eye for people who can't communicate what looks better: babies, very mentally challenged adults, etc.
posted by chiefthe at 4:48 AM on July 17, 2013 [10 favorites]

My optometrist does both. Actually, it's a butt load of tests.

My Rx is a beast though, so we may narrow it down with the automatic part, but with amblyopia, astigmatism, presbyopia and hyeropia, shit, there's only so much a machine can do.

I've had my head in the hooty-owl a bazillion times and I always have to do the , "is this one better, how about this, what if I do this?"
posted by Ruthless Bunny at 5:53 AM on July 17, 2013

My optometrist used the machine that auto-focused the image I saw. My M.D. used the "better 1, better 2" method. The optometrist told me that M.D.s tend to correct for different aspects of vision than O.D.s do.
posted by infinitewindow at 7:09 AM on July 17, 2013

Best answer: There are certain brands that will give you glasses measured automatically using a Shack-Hartmann Wavefront Sensor—with the added benefit of removing higher-order imperfections in your vision (more than defocus and astigmatism) so that you don't see the halos around bright objects at night. Additionally, these glasses offer far more precision in terms of how much correction is done—the standard increment is 0.25 diopters. However, it's my understanding that, as gjc said, people like the slight blurriness that comes from the 1/2 3/4 test.
posted by Maecenas at 7:29 AM on July 17, 2013 [1 favorite]

Response by poster: Thanks, everyone. My new glasses were done with the "hot air balloon" machine exclusively, no eyedrops and no manual followup. (It was an "eyeglasses superstore" with young clerks, possibly supervised by an optometrist or ophthalmologist. It was clear that their business model depended on not spending a lot of time getting the prescription right -- they were very forthcoming about their "if it's not right, please come back; we'll replace the lenses for free three times!" policy. Perhaps after a replacement or two, they do a more comprehensive exam.)

I'm interested in Nomyte's answer, though:

I know the eyedrops paralyze / dilate your iris, but by (possibly imperfect) analogy to cameras, that's the light-quantity-controlling diaphragm, not the focal plane adjustment. AIUI, adjusting lens distances (and/or lens shape, in the eye) changes the focal distance (i.e., the exact distance from retina to subject where you get the best possible focus). The iris changes the depth of field (i.e., the range of distances around the focal distance where the focus is still "good enough."

I guess the real point of my question is, how do you use one autofocusing system (the machine) to measure another autofocusing system (the eye)?
Doesn't the eye autofocus mess up the machine autofocus?

But perhaps the Shack-Hartman Wavefront Sensor (thanks, Maecenas!) measures fast enough, and with small enough spots, that the eye autofocus can't really get going. I'll have to read more about that.
posted by spacewrench at 7:41 AM on July 17, 2013

Well, in many systems, there's a fixation target that will stabilize both the lateral motion of the eye and the autofocus (aka accommodation). Additionally, there two things in those eyedrops: there are some that paralyze the ciliary muscle (which controls accommodation) like Nomyte said, but a different compound is used to make your pupil get big (aka mydriasis). So with either eyedrops or a fixation target, there should be very little change in focus. However, it has been established that ciliary muscle paralytics tend to actually shift the optics of the eye to make it more nearsighted than normal, so that can affect the end prescription.
posted by Maecenas at 8:46 AM on July 17, 2013 [1 favorite]

In case you’d like some more technical terms to Google for: The “1 or 2? exam” is called subjective refraction and the machine thing is objective refraction with an autorefractor. I’ve never had an optometrist or ophthalmologist not do both, and as people have already mentioned upthread, I have also been told that this is to not only find the exact refractive error of your eye, but find a prescription that is comfortable for your entire visual system.
posted by wachhundfisch at 11:44 AM on July 17, 2013 [1 favorite]

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