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#1
Old 02-18-2013, 09:13 AM
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Location: Las Vegas, NV
Posts: 26,954
Need help with positive/negative eyeglass prescription notation

In the past I've gone to an optometrist to get eyeglass prescription updates, who use negative notaton prescriptions. Recently I went to an opthamologist who uses positive notation. I want to make sure I understand the difference before I submit the prescription for buying glasses online.

My previous (negative) prescription was this:

OD (right) SPH -2.50, CYL -1.75, AXIS 5
OS (left) SPH -2.75 CYL -1.00, AXIS 175

Now I'm told my updated prescription is a minor change. It reads as follows:

OD Sph -3.75, Cyl +1.25 Axis 100
OS -3.75 Cyl +0.50 Axis 90

Now I see that the astigmatic cylinder has been rotated 90 degrees (actually 95 and 85) and the that the cylinder notation is now a positive number. Okay, so that's part of the difference between positive and negative notation. There's still a .75 gap between left ey and right eye, but both numbers are half a diopeter less in absolute value.

However, the spherical correction is now -3.75, still a negative number. Does this mean that the switch from negative to positive only affects astigmatism correction, or does the spherical number mean something else when listed in this method?

What confuses me is that they said my prescription was a minor change, yet the spherical number changed by a whole diopeter or 1.25 depending on eye, which does not seem like a minor change to me, which is making me think I don't understand something.

Anyway, I order my glasses from zennioptical online. There's no checkbox for giving your prescription in positive or negative notation. Does that mean I could just enter my prescription as-is, and they'd understand what was written and give me the right prescription, or do I have to do some sort of conversion to make it consistent with my previous negative-notation prescriptions?

Edit: I just realized that perhaps the increased spherical corresponds to the decreased absolute value of the cylinder corrections. So I get more spherical correction and less cylinder correction and maybe the numbers work out to be similar? Is that part of the difference between the notation, or is the doctor changing the relationship between my spherical and astigmatic correction?

Last edited by SenorBeef; 02-18-2013 at 09:14 AM.
#2
Old 02-18-2013, 10:44 AM
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Location: Suburbs of Chicagoland
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I'll convert your new prescription back to minus cylinder and see if that helps you understand.

Plus cylinder -
OD Sph -3.75, Cyl +1.25 Axis 100
OS -3.75 Cyl +0.50 Axis 90

Minus cylinder -
OD -2.50 -1.25 x10
OS -3.25 -0.50 x180

To convert from plus cylinder to minus, you subtract (well, technically you add it but pay attention to the different signs - you're removing power) the sphere from the cylinder to get the new sphere, keep the old cylinder power but change it to minus, and switch the axis by 90 degrees. So what has essentially happened is your left eye got a little worse in terms of your distance vision (myopia) but your astigmatism doesn't look quite as bad, while your right eye barely changed at all. And yes, the sphere strength changes to account for the different kind of lens power used to deal with the astigmatism.

Last edited by Ferret Herder; 02-18-2013 at 10:48 AM.
#3
Old 02-18-2013, 10:53 AM
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I just realized my axis comment was confusing - we only deal with 180 degrees, not 360. If the original axis is 90 degrees or less, add 90; otherwise, subtract 90.

And yes, traditionally most ophthalmologists work in positive cylinder, while most optometrists work in negative cylinder.
#4
Old 02-18-2013, 10:56 AM
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Aha, strange. The whole reason I went in was that my right eye had changed significantly in acuity while my left eye seemed fine, but the actual prescriptions have a minor change to the right eye and bigger change to the left. Hmm...

In any case, should I enter the converted prescription into zenni's ordering mechanism? If I entered the positive one, would they just convert it on their own anyway, so it wouldn't make a difference? In other words, is there any difference in the actual lenses if I list the prescription one way or the other?
#5
Old 02-18-2013, 11:06 AM
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No difference at all. I'd enter the one that the ophthalmologist gave you in case I somehow had a brain-fart and messed up.

How many letters you can read is just a part of how well you can see or perceive that you see. I've helped study patients with uveitis (an inflammatory disease of the eye) which causes blurring, light sensitivity, hazy vision, floaters, etc., and with treatment, some of them read better than 20/20 ("normal") on the charts but still complain of certain perceived vision problems/deficiencies.
#6
Old 02-18-2013, 11:11 AM
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Yeah, I think that may be the case with me. I'm not sure my right eye has absolutely reduced acuity (I can still see fine details sometimes) but the focus seems weird - it'll kind of fade in and out, making the eye a chore to look through. I've switched from being right eye dominant to left eye dominant because of it. I went to the opthamologist in the hopes that he'd spot whatever the problem was, but I really got half-assed bad service but an asshole with no actual answers. He basically just told me "your right eye needs a new prescription, come back when you get that and we'll go from there".

Unfortunately, with no insurance, it's difficult to pay for another full exam at another opthamologist.

Anyway, thanks, I'll order new glasses, hopefully that'll help some. But with the relatively minor prescription change I'm not hopeful.

Last edited by SenorBeef; 02-18-2013 at 11:12 AM.
#7
Old 02-18-2013, 11:31 AM
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Quick, possibly dumb, but related question: is an astigmatism correction of 0/180 useful? Wouldn't that mean it's aligned with the spherical correction? An astigmatism correction without an axis correction seems pointless to me. But I don't know what I'm talking about so it's probably a dumb question.
#8
Old 02-18-2013, 11:49 AM
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No, it's not "aligned" on the 0/180 axis with anything. Think of the sphere as having even correction throughout the lens, and a cylinder adding a bar of some more correction along an axis. The axis just shows what angle it goes across your eye at. Correctable astigmatism is caused by your cornea or lens not being evenly smooth/spherical across its surface, so the power and axis positioning of the cylinder lens seeks to counteract that.
#9
Old 11-28-2013, 08:31 AM
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I had this exact same issue. My new prescription from a new doctor was given to me in positive cylinder values. I did not check my old prescription but I went ahead and ordered my glasses online from zenni. They emailed me and asked me if the prescription was correct. I didn't call my doctor but I re-checked my entry and confirmed the prescription. When the glasses came, I struggled with them for 3 weeks and gave up. I ended up giving them away and going back to my old glasses.

I had to wait 3 more months to see the doctor again. She still insisted she gave me the correct prescription. But this time she changed the prescription a little bit. It wasn't until today that I figured out why my new doctor's numbers were so different. I found a website that will convert the numbers for you. It is accurate because it shows my left eye has not changed.

I don't know what happened with zenni but I will be sending them the converted prescription, just in case. It seems they should be able to recognize and convert the prescription but apparently not.
#10
Old 11-28-2013, 08:38 AM
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I just wanted to add that even though the original post was several months ago, just in case someone else like me runs across this issue, I read this on another site:

Most optometrist consider plus cyl lens as archaic as they are difficult to find as stock lenses, although they can certainly be custom fabricated.

So you should definitely convert your prescription yourself or end up like me, wasting needed money on glasses that may not work.
#11
Old 11-28-2013, 08:47 AM
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I used to work in a lensmaking lab. We just converted one notation to the other so we could use the blank stock we had, since the optical end result is the same.

I can only assume that optometrists used a refracting device that could provide either notation, and if he didn't work in a lab, he wouldn't know that we only used one system.
#12
Old 02-20-2014, 09:15 AM
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Quote:
Originally Posted by Musicat View Post
I used to work in a lensmaking lab. We just converted one notation to the other so we could use the blank stock we had, since the optical end result is the same.

I can only assume that optometrists used a refracting device that could provide either notation, and if he didn't work in a lab, he wouldn't know that we only used one system.
So a trained optometrist should know when they see the prescription, that it should be converted ? Because I am having a difficult time trying to make zenni understand there is no way I would know to convert my prescription.

The doctor gave me the prescription and I entered it on their site. I don't have any specialized training to know that it should be converted.
#13
Old 02-20-2014, 10:06 AM
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Quote:
Originally Posted by frodosdojo View Post
So a trained optometrist should know when they see the prescription, that it should be converted ? Because I am having a difficult time trying to make zenni understand there is no way I would know to convert my prescription.

The doctor gave me the prescription and I entered it on their site. I don't have any specialized training to know that it should be converted.
Absolutely; an optometrist, optician or lab should know how to convert it without a calculator, although I had a boss once who cursed when he had to do it. But he had trouble with 2 + 2, too, and I'm not sure my paycheck was accurate.

I'm sure refracting instruments are different now than 40 years ago, but our B&L gadget could give a reading either of two ways for compound prescriptions.
#14
Old 02-20-2014, 10:30 AM
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Quote:
Originally Posted by frodosdojo View Post
So a trained optometrist should know when they see the prescription, that it should be converted ? Because I am having a difficult time trying to make zenni understand there is no way I would know to convert my prescription.

The doctor gave me the prescription and I entered it on their site. I don't have any specialized training to know that it should be converted.
That's just one of many reasons buying prescription lenses online is a bad, bad, bad idea. Any qualified optician would know how to convert plus cylinder to minus cylinder and be able to fill a prescription written either way.

And technically, there is a real but rarely important difference between plus cylinder and minus cylinder lenses. In minus cylinder the cylinder is ground on the back of the lens, in plus cyl it's ground on the front. The difference is subtle, about like the differenc e looking in or out through a curved windshield.

Nearly all lenses are made in minus cylinder (cyl on the back), regardless of how the Rx is written. I can only recallne time during my 15 years as an optician that an ophthalmologist prescribed actual plus cylinder for a specific medical reason. The lenses cost more and took longer to make because it required special equipment that's rarely used.
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