Please can you explain my glasses prescription?
January 28, 2016 7:08 AM   Subscribe

I am really puzzled by my glasses prescription. The optician said my eyes had got worse, which makes sense since everything has been blurry lately. But when I looked at websites explaining glasses prescriptions, it actually looks like my eyes have improved. Also the optician said my new glasses will let me see 20/20 whereas my old glasses didn't. When I asked why the last optician I saw couldn't make me up glasses for 20/20 vision, she didn't know. Opticians of MeFi, please can you help explain this to me?

Some background: I've been wearing glasses for distance vision since I was 11. I have myopia and astigmatism. I wear my glasses all the time. My eyes got worse year on year until I was in my mid-20s then plateaued. I am now 28.

In July 2014 I had some disturbing eye pain and weird faded out vision, and I was referred by my GP to an ophthalmologist who diagnosed me with acute optic neuritis. I was told this would clear up on it's own and there was no treatment. I saw the ophthalmologist for a follow-up in October 2014 and she said my eyes were back to normal, but to get an eye test from my optician.

I went to an optician in November 2014 and was issued the following prescription:

Sph -3.50
Cyl +3.25
Axis 18

Sph -2.50
Cyl +2.75
Axis 178

I had those glasses made up and everything was fine. I had my final check up with the ophthalmologist in June 2015 and in my discharge letter she wrote that my vision was "6/9 N8" in both eyes.

For the last few weeks my distance vision has been growing noticeably worse and today I had my eyes tested again, this time at a different optician. I only went to a different one because I have moved house and this one was closer, not because I felt there was anything wrong with the last one.

My prescription this time was:

Sph -1.00
Cyl -3.75
Axis 95

Sph -0.75
Cyl -3.00
Axis 75

The optician said that my eyes were much worse than they had been before, but that the new prescription would let me see 20/20 vision, whereas my old prescription had not been as good.

I was curious about all of this so I looked at some websites that explain glasses prescriptions. According to those, the higher the minus sph number, the worse your myopia. However my minus sph numbers have got lower e.g. -1.00 compared to -3.50. The websites make it sound like that means my eyes are seeing better than before, which doesn't really make sense given what the optician said and also my own perception when I take my glasses off and everything is so much more blurry.

And what about the cyl numbers and axis numbers, why have those swapped from being positive to negative and changed so much? I know the cyl is to do with the astigmatism but the optician never mentioned that, she seemed to focus on the myopia when talking about how my eyes had changed.

Finally, what's the deal with her saying that my new glasses will give me 20/20 vision, but my old glasses didn't? Why could they not previously give me glasses to allow 20/20 vision?

Basically, I'm really confused, and only have a rudimentary understanding of what all these numbers mean, and I would be really, really grateful if someone who knows about this could explain my specific prescriptions to me. I just spend £300 on new glasses and I'm half convinced I've wasted my money and somehow mucked up the eye test or something! Thank you.
posted by maybeandroid to Health & Fitness (7 answers total) 5 users marked this as a favorite
sorry, but are you sure you've not got the prescriptions swapped?!
posted by andrewcooke at 7:20 AM on January 28, 2016

This happened to my son a few years ago. The technician (I don't know the proper term) checked his eyes and calculated his prescription for a wildly different result than what we were expecting (about the same difference between your old and new prescriptions). When the doctor came in, I asked about it. She took a look at it and said something like, "No, this is totally wrong. [The technician] is new."

I ended up finding a new eye doctor for him to go to and we haven't had that situation happen again.
posted by Lucinda at 7:20 AM on January 28, 2016

I am not an eye doctor, but just a lifelong glasses wearer. So take what I say with with a grain of salt.

I believe the key here is the change in sign of the Cyl measurement. The eye doctors I've been to seem to prefer to write it as negative, but I think the prescription can be written with Cyl as positive by adjusting the other two numbers accordingly. For example, I think your old right eye prescription could be written equivalently as:

Sph -0.25
Cyl -3.25
Axis 108

Written this way, it's clearer that the new prescription is stronger than your old prescription.

But I could be totally wrong here though, and someone who really knows should come in here and clarify, and I'll be glad to be corrected.
posted by DarkForest at 7:39 AM on January 28, 2016 [4 favorites]

oh, interesting. what dark forest says is described here. in that case, i agree these look ok (but am also no expert).
posted by andrewcooke at 7:46 AM on January 28, 2016

I think perhaps the reference to 20/20 vision is that your old prescription is not giving you 20/20 vision NOW whereas the new one will.

That is to say, the old one gave you 20/20 vision back in the day when your eyes and the prescription were matched up. But your eyes have since changed and the old prescription no longer gives you 20/20 vision. The new one will.

I'm guessing that is all that was meant with that comment.
posted by flug at 8:01 AM on January 28, 2016 [1 favorite]

Yeah; I guess there are two ways to write one of these prescriptions that are equivalent (and have to do with the fact that a positive Cyl value is the same as a negative Cyl value with the Axis offset by ~90).

See here:
posted by Betelgeuse at 8:02 AM on January 28, 2016 [2 favorites]

But I could be totally wrong here though, and someone who really knows should come in here and clarify, and I'll be glad to be corrected.

You got it. You rotate the axis 90 degrees, flip the sign of the cylinder, then add that new cylinder value to the old spherical. (Remember: Adding a negative to a negative means it's more negative!)

Older people issuing prescriptions often write cylinder with positive diopter, because that used to be the only way to make the lenses, we couldn't cut a negative diopter in a cylinder. Now we can, and it results in thinner lenses most of the time.
posted by eriko at 9:09 AM on January 28, 2016 [3 favorites]

« Older Damp surveyors in Edinburgh   |   Should I be worried about rock salt being used... Newer »
This thread is closed to new comments.