I recently came back from the ophthalmologist with new prescriptions for both glasses and contacts. I wear my contacts all the time, and had noted that my vision in my left eye had become less clear. I only wear my glasses for the half-hour or so before I go to bed at night, so I haven’t updated the prescription on them in a long time, so I’m long overdue in replacing them.
When I took a look at the two prescriptions, though, I noted that the two prescriptions were different. I’m not talking about the curvature info they add to the contact lens prescription; the strengths were different.
Is this common? Can anyone tell me why?
Also, do opthalmologists frequently under-correct — that is, give you a prescription that results in less than 20/20 visual acuity?
I can give only anectodal evidence, but my glasses and contacts are also different. I was told this is because contacts are more effective at correcting vision.
I have the same thing, and my optometrist told me the difference is because the contact lens is sitting right on your eye, while your eyeglass lenses are held an inch or so off. He told me for light prescriptions the difference is negligible so the numbers may be the same, but the difference becomes more pronounced for stronger prescriptions.
That’s what I was wondering ---- if the fact that one is sitting on your eye, vs. the other one being a cm or so away, would make a difference. And I do have a fairly strong prescription, so the difference is notable.
How about the undercorrection issue? I got a complete copy of the exam to take along when I go see my retina specialist next month, and it appears to me that the prescription strength he’s prescribing will not take me to 20/20. Seeing as how I went in complaining that my vision was poor, and I had difficulty seeing to drive at night, I’m wondering why he would choose not to go to a 20/20 correction.
Some eyesight is so bad it can’t be corrected to 20/20, even with contact lenses. Eyesight that bad is usually at or close to “legally blind” territory, though, so it probably doesn’t apply to your situation. Also, eyeglasses and contacts vary in how close they can get you to 20/20; contacts can usually do better than eyeglasses, except for soft lenses on astigmatic eyeballs (soft lenses can’t correct astigmatism). Again, may not be relevant to your situation. I would be interested to hear of any other reasons for not correcting to 20/20.
Mine is something like -11.25/-12.50 for glasses and -10.00/-11.00 for contacts, and I see better with the contacts. Freaked me out as well the first time I compared the prescriptions…
For my lenses, they only come in “steps” of .5 so the eye doctor errs on the conservative side and underprescribes by maybe .25. So technically I may not get to 20/20 because of the restrictions of the lens possibilities.
Part of the difference comes from the different lens spacing in contacs vs glasses. With glasses, the light cone has about an inch to expand or contract before it hits the lens of your eye. With contacts, that distance is reduced to a mm or less.
At one past visit, I was told I couldn’t get 20/20 vision in both eyes. I can get perfect vision in one eye and the other, but not both at the same time.
You might want to update your information there a little bit, bub. I have worn soft contact lenses to acorrect astigmatism, they are called TORIC lenses, and they work quite well.
I believe I am way into legally blind territory (with a contact lens prescription of -8.50 in both eyes), but my vision is correctly to slightly better than 20/20 with the lenses.
Well, what does legally blind mean anyway? Does it mean that you can’t function normally without corrective lenses, or that even with the best correction you can get you’re still unable to drive and so on? Because if it’s the former, then an awful lot of us are ‘legally blind’ and don’t know it, whereas the latter condition is relatively uncommon.
I have a correction of -14.25 (in glasses), and cannot see 20/20, but I do OK. No doctor or DMV person has ever told me that I am ‘legally blind.’ I have always assumed that the term applied to people who cannot see well even with the best corrective measures.
When your eyes accomidate (focus) a large part of the change is due to reshaping the curvature of the interface between the air and your cornea. Soft contacts deform along with your eyeball, AND the lens moves in relation to the retna, and thus allows accomidation with much less effort that when you have glasses of fixed power.
This really suprised me. I put off getting contacts for years, because I liked to take off my glasses when working close, and I thought this would be inconvienient with contacts. Nope, no need to pull them out when doing close work, and no strain either.
There is another reason why contact lenses frequently have different prescriptions than glasses for the same person, which has nothing to do with how close the lenses are to the eye.
That reason is mild astigmatism.
Unlike near-sightedness or far-sightedness corrections, the correction for astigmatism has to be “aligned” along whichever axis in your eye has the deformation problem. This axis can be anywhere from an angle of 0 degrees to 179 degrees (although usually they only bother to specify this in increments of 10 degrees or 30 degrees).
In glasses, correcting for astigmatism isn’t a big problem – you just grind the lens to have the right astigmatism correction power (called the “cylinder” or “spherocylindrical” correction) along the correct axis of the lens, set it in the frame, and you’re done. Contact lenses, though, are another beast entirely. For a contact lens to correct astigmatism, it has to be built at the factory so that it’s weighted at the right point so that that point will rotate to the bottom of your cornea when you’re wearing it. Since there are so many different axes, manufacturing every possible combination of near/far sightedness correction power plus astigmatism correction power plus astigmatism axis is economically impractical.
So, for people with mild astigmatism (less than 1 diopter or so), optometrists will typically forego prescribing contact lenses with astigmatism correction, but will make up for it by adjusting the near or far sightedness correction power of the lens a little bit. Apparently, a little bit of extra nearsightedness correction can actually overcome a little bit of mild astigmatism.
For example, my glasses prescription in my right eye calls for -3.50 nearsightedness correction and -0.50 astigmatism correction. My contact lens prescription for the same eye, however, calls for -3.75 nearsightedness correction and no astigmatism correction at all.
This may account for the differences in your glasses and contact lenses prescriptions as well.
It’s nearly nine years on and I’ve yet to see this adequately explained anywhere online (I blame Big Glasses), so I thought I’d have a go.
Basically, what it boils down to is that, for spherical lenses (i.e. no correction for astigmatism), the difference in prescriptions is the difference necessary to keep the lens focus at the same distance.
A diopter is defined as the inverse of the focal length. A +1 diopter lens has a focal length of +1 meter (one meter “behind” the lens). A +5 lens has a focal length of half a meter, etc.
With negative diopters, the focus is “in front” of the lens (they “spread” the light).
For the sake of simplicity, let’s assume a difference in distance of a centimeter for a contact lens versus a spectacle lens and a (pretty strong) -10 diopter/-0.1m contact lens prescription. To keep the focus at 10cm, the eyeglass lens needs a focus a centimeter closer.
Which is reasonably close to reality for a (very) rough estimate…
That centimeter-or-so difference becomes less significant as the focal length increases. For a -1 diopter contact lens, you end up with something like a -1.01 eyeglass prescription. Practically identical.
At least I think that’s right. I’m kinda making it up as I go along.
One consequence of all this is that, for strong prescriptions, the fit of your glasses can have a big effect on how well you can see. On the plus side, if you’re extremely myopic, you can put off bifocals for a long time by sliding the glasses down your nose.
Ophthalmologists don’t always elect for “perfect” correction, but for a compromise between that technical point of perfect correction and some degree of flexibility and comfort for the wearer. They often soften and back off full correction to give the wearer a slightly more natural-feeling view. This is particularly true with high-correction glasses.
There is less need to do so, and more to be gained from very close correction, with contacts.
IANAO, but I wore heavy correction, glasses and contacts, for 30 years, and still have mild corrective issues that need some subtle choices between optimum points. I’ve had this explained many times by more than one provider.
A lot of that depends on how old you are. When you’re young, and can focus really well (and practically effortlessly), those compromises can be difficult to find.
I saw the world at arm’s length for about thirty years because I was consistently overprescribed. It wasn’t until things started to stiffen up that I realized what had happened.
When a lot of correction is required, I think the goal is largely functionality with a minimum of discomfort. So long I’m still allowed to drive, I figure, “Eh, good enough.”
Which makes it all the more annoying that the damn things only come in .5 dpt increments for us Magoos. I find that a quarter-diopter off gets more noticeable every year…