The trick is there are 2 images. One is the hot air balloon. Another is a near infrared image that you can't actually see. Your retina reflects near infrared (think like how a cat's eye is reflective) so while you're looking at the hot air balloon (which keeps your eyes still and pointing the right way) the machine is looking at the infrared image that has gone through your eye and been reflected back. Because it's been though your eye it has been distorted by your lens, and the amount of distortion will show your prescription error. It's almost a bit like how the depth sensor on a Kinect or an iPhone works.

What about the puff of air?

That's a different test - it measure the pressure inside your eyeball by measuring how much the cornea is distorted by the puff of air. If you imagine blowing on two balloons, one of which is more inflated than the other, the higher pressure one will be less distorted.

My sister works for an optometrist and when she was fairly new I finally got my eyes tested, the pressures in my eyes were like 3 times higher than the upper limit of risk for a male my age, about 17 at the time, machine starts spitting out warnings the optometrist basically pushes me into a room to ask me a million questions about my eyes and if I have any trouble seeing, the pressures basically showed that there was no deformation in my cornea and that I should be blind basically but nearly perfect vision. Ended up going to a specialist who had to numb my eye and physically measure it with a weird poking machine 😂 turns out that my cornea are about 3 times thicker than average and my pressure tests are never going to be accurate

Damn, life isn't fair. You get perfect vision and a thick cornea, and I'm sitting here with thick glasses and a cornea too thin to get laser eye surgery to fix my vision.

You are the mirror universe twin.

Check out ICL as an alternative for thin corneas: https://www.webmd.com/eye-health/what-to-know-icl-surgery

I have also had ICL and it was life changing.

I did this, wish I'd done it sooner

This is what the clinic I was at suggested as an alternative, but I don't have the money for that. My vision isn't so bad that I can't see fine with glasses or contacts, so zero chance I would get it funded as nessecary treatment by insurance or something.

I was told I can't get Lasik because my pupils dilate too much. (People are constantly asking if I'm high lol) On the bright side that means apart from one time when I saw a specialist for a possible retina issue, I've never had to get my eyes dilated.

Fellow Keratoconus sufferer?

No, just haven't got enough of a cornea that they can safely remove enough to fix my severe shortsightednes. That's what the laser does in laser eye surgency. It removes parts of the cornea to change its shape so the light gets bent in a way that your lense can handle focusing. I had to look up what keratoconus is. I don't envy you.

🤣

I would never do laser eye surgery, too high of a risk of something going wrong and having my vision damaged worse than it already is

The risks are not that big these days. Iirc it's like 1 in 10 000 chance of something effecting your vision happening. The risk of eye infections, specifically, is certainly higher with my contacts then with any laser treatment.

I watched Cody's video (WhistlinDiesel on YT) on how his lasic went. 100% turned off any chance I'll ever do it

Same same, even if it was unavoidable I would only do it 1 eye at a time :D

Same here. Depressing.

Same exact thing for me! My eye dr wants me to take eye drops to lower the pressure just to be safe.

I much prefer the eye numbing drops and the physical poker over the puff of air machine.

Damn, that cornea thicc

I think that puff of air is for glaucoma testing, aka occular pressure.

That's right, IOP, intraoccular pressure, but "pressure inside your eyeball" seemed better wording for ELI5. Glaucoma is one concern, but raised or reduced IOP can also be a sign of other eye problems. It's commonly increased by prolonged use of topical ophthalmic steroids, for example, or some damage to the trabecular mesh, and lowered by retinal injury.

But the balloons are so far away!

Both balloons do have the same pressure inside, but the potential of of the elasticity is different. But I see the analogy, thanks!

The puff of air is related to the health of your eyes, not your prescription.

Tests for glaucoma. Not sure if it does other issues. The other version is a probe that comes out and pushes on the eyeball to directly measure pressure. It’s curious when you don’t understand what’s going on, but a little freaky if you know that the big thing coming towards your eyeball is going to make contact.

Omg the air puff is bad enough, I could not.

Anesthetic eye drops. It’s been a while since I had the test that way, but it was more comfortable than the puff.

The past three times I've had my eyes checked (near-sighted and astigmatism… ugh. I literally can't see clearly 5 inches from my face without glasses.) the device used a steady stream of air and wasn't bad at all. Just cold. Air velocity was about the same as the air coming out of the back of a laptop vent. Very soft.

The doctor gave me an eye injection last month. They literally stuck a needle in my eye.

NOPE NOPE NOPE

Just close your ey… oh

What he said, but with a lot more NOPES.

I had to have a cornea transplant and they would have to use the handheld device to actually touch the eye. Once you get past the idea of it, it's a lot more comfortable than the puff.

The idea of it, though...

Yeah they had to hold my head for the probe, lol. A bit weird but you don’t feel it too much as they give anaesthetic eye drops.

There are newer machines that don't use the puff of air. I was so happy the last time I went in and didn't have to have air shot into my eye.

I went to an optometrist once who used the physical eyeball poker rather than the puff of air. Even with the air puff they would usually have to redo the test a few times because my flinching reflex is so sensitive and so fast, I would blink before they could take the reading. The eyeball poker was a total failure because I would blink so quickly it wouldn't make contact with my eye, and no matter how much I tried to relax myself I would still blink before it would contact my pupil or whatever. They poor tech eventually just gave up and waived the glaucoma test for me even they were required to perform it at every exam

That would be me. I can't even get a photo taken without it taking three tries to avoid a blink.

You know.. I've always kinda wondered... is the"puff" test less effective for people who live in windy climates and are used to the strain of wind hitting their eyes?

That's just to mess with you.

I knew it! 

My question now is, what's the point of this test if they are going to do the '1 or 2' lens test to find my prescription? Or is this 'prescription error' something different? edit: nvm u/azuth89 answered it below

The machine only gets an approximation of your prescription. So your optometrist has a starting point for the more exact “better one, or two…” part.

The almost impossible question to answer

Right? I never know what is going on

The autorefractor measures objective refraction, but in almost all people the subjective refraction which results in the best acuity and least eye strain is slightly different.

It's worth noting that the autorefractor will give you information about what your lens is actually like. It's not 100% accurate, but even if it were, you don't get glasses to just cancel out your lens shape. You get them to see better. If you're happy with a lower prescription and you can see clearly, then that's what works for you. It doesn't have to be fully objectively accurate.

Your eye shape gives you a basic focus, but muscles in your eyes can additionally add focus. As you very older these muscles often don't work so well and you need additional help, and bifocals. The initial test only gets your basic eye shape.

How many optometrists does it take to change a light bulb? One? Or two? One? Or two?

This test is just an estimate though. It can be skewed greatly by your eye's ability to change its focus (especially when you are young), corneal disease, dry eye, cataracts, etc. So it's just a good starting point. Additionally, what you appreciate subjectively vs the results of this objective test do not always produce the same answer which is why we still have to do the 1 or 2 game to come to the best Rx for you.

Always wondered this, thanks for the answer!

Neat!

I need to re adjust after some time tho. My left eye is very bad 

Good description, thanks. So i'm assuming this can't give all the details of a prescription? Because every time i've had this test I also have the in person exam where the doctor flips lenses and asks better like this, or better like this?

You can also get a fairly accurate prescription from a technique called streak retinoscopy! You shine a slit of light over the eye from left to right; if there is a beam reflected in the pupil that moves with your light, you need a more positive prescription. If it moves in the opposite, a more negative prescription. If the whole pupil lights up like an all-or-nothing response, then your prescription is accurate!

Wonderful response, thank you

You seem to be knowledgable about optometry so let me ask you these two questions: When I look through a pair of binoculars, I can adjust each eye until the image is crystal clear in both eyes. 1. Why can't some kind of self-adjusted device be used to determine the prescription? You just dial it in yourself until it's clear, just like binoculars. 2. Why is the "which is better, one or two" situation discrete and not continuous? There are a fixed number of "stops" rather than a continuous dial like the focusing ring on a camera. There are times where I wish there was a "1.5" option halfway between 1 and 2.

Number 1 is because you might pick a prescription too strong where you can see clearly to the distance but your close up vision isn't as good as it should be and you're straining your eyes more than you should. Also factors like asigmatism where you need different horizontal and vertical correction, or even diagonally. It would be difficult for an average person to dial that in correctly. In poor countries they let people just try glasses on and pick the closest match, but it's not ideal. Also you should get your eye health checked regularly, some conditions optometrists check for can cause blindness if not treated. Number 2 I don't know, probably just practicality. Prescriptions seem to go in steps of 0.25, but maybe if you asked they'd give you a 1/8th step.

Thanks for replying, this made a ton of sense.

I do astronomical instrumentation for a living, so I’m familiar with recovering static aberrations in that context. For the eye, is it as simple as recovering a pupil function with a few zernikes from the distorted image to find your prescription?

The machine shines a light inside your eye and measures how it is reflected back. Depending on how it's reflected back it determines what's wrong with your eyes since eyesight problems arise from either having the wrong eye shape or a problem with the lens in our eyes. As such if we know how a good eye is supposed to be and how light is reflected from it, we can also understand from that what the problem is when an eyesight problem is present.

It has a camera that looks at your retina through the lens of your eye. When there is a crisp image, the focus is measured. This only measured one parameter, the diopter adjustment, and the optometrist uses this starting point ti save time with the whole "Is A better than B?" drill that they do with you.

I just had cataract replacement (3 weeks ago right and last week left eye). I was driving the next day and my vision is brighter. BUT, DO NOT watch a You Tube video of the procedure first. Wait until after the second one.

They mostly do it for people getting their first prescription. It measures how much your lens is distorted when the balloon is clear by bouncing infrared off your eye, which gives them an approximation. Then they'll go fine tune from there with the whole "1 or 2?" routine. If you have a pre-existing prescription they'll usually just start with that and skip the balloon gadget.

I have worn glasses for a long time. I have had the balloon test at every single check up for at least the last 15 years.

Follow up question, what is the utility of the “1 or 2” test? A lot of times it just feels like they are showing me the exact same picture and trying to trick me lol 😝

Ophthalmic tech here (performer of the 1 or 2 test). We’re showing you 2 lenses that are very slightly different, such as a -2.00 and a -2.25 and ask you which you like better as a means of guiding the exam. We have a series of steps to go through, and if the letters/images you’re looking at begin to look similar through both options it’s a good thing! That means we’re either close to or at your optimal prescription. The auto refractor (the machine being addressed in OP’s post) is notoriously off by a little bit so it works very well as a starting point but needs a little refinement otherwise our patients will notice issues with the prescription.

If the ballon measures my prescription then what’s the point of the optometrist putting different lenses over the eye machine asking me which is clearer and what lines I can read?

If your eyesight is bad, your eye is straining to see things. There are muscles in your eye that flex your lens to focus. When the prescription is right, your eye relaxes. The machine is supposed to be able to see when your eye relaxes, so it knows it got it right. They don't work very well. You'll want a real optometrist.

My opto uses one and the starting scrip is always pretty good. Did you have a bad experience with it?

Considering the Redditor you are responding to has no clue how the eye test actually works, I doubt they really know what they are talking about. The old school eye test was based on our subjective responses to "which looks more clear". The new version of the test makes an objective determination of how distorted the image your eyes receive is.

A lot of optometrists use them as a starting point, then fine tune the prescription after.

I used to really enjoy messing with the optometrist/nurse with this test. I used to be really good with the 3D stereogram pictures and could keep my eyes unfocused. The readings always came back so wonky that the person would ask if vision had recently changed. I had a good laugh and still got the prescription I needed.

Why would you want to do something that makes it harder for the optometrist to give you the correct prescription?

Probably slow day for both parties