# Contact Lenses/Eye Glasses. Strength conversions.

Discussion in 'Et Cetera, Et Cetera' started by D_Thoraxis_Biggulp, May 21, 2008.

1. ### D_Thoraxis_Biggulp New Member

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I've noticed glasses are generally expressed as 20/X, while contact lenses are typically a single number, such as 0.50 or -2.75. Does anybody here know how to convert from one to the other? I'd Google it, but I honestly have no idea what to search for.

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2. ### D_Gunther Snotpole Account Disabled

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The 20/X has to do with your vision itself.
Glasses, like contacts, are described in terms of diopters ... 0.50 or -2.25.

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3. ### D_Thoraxis_Biggulp New Member

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Oh, that's what they're called. I usually mean to ask my optometrist whenever I see him, but I always forget.

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4. ### dong20 Gold Member

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Prescription of <0 is short-sighted, >0 is long-sighted. 0 - 5 is low, 5 - 10 is high. Measured in dioptres.

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5. ### DC_DEEP Gold Member

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20/x (or 20/20, or 20/400) is called "visual acuity" and gives a descriptive measurement of your vision. It basically means that at 20 feet, the smallest letter you can see on the chart could be seen by a person with "normal" vision at a distance of (the second number) feet. Therefore, 20/20 is considered "normal." 20/200 would mean that your vision is the same at 20 feet as a "normal" person's vision is at 200 feet.

The diopter is a measure of the strength of a lens. It is the reciprocal of the focal length of the lens (d = 1/f).

A plain lens is a diopter of 0. Positive diopters are for magnifying lenses, and correct farsightedness. Negative diopters are for reducing lenses, and correct nearsightedness.

In addition to diopter, glasses may also have several other corrections not included with most contact lenses - cylinder, angle, and add-power. The cylinder and angle correct astigmatism, and add-power (bifocal, multi-focal) corrects for presbyopia, the thing us old folks get...

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6. ### D_Thoraxis_Biggulp New Member

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So 20/100 would be -5.00 in diopters, then? That's all I was after was a conversion calculation.

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7. ### DC_DEEP Gold Member

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Wrong conversion. I don't know if there's a converter available (geek that I am, I've looked before.) 20/100 I'm guessing would be about a -1.00 to -1.5 diopter, but I would not guarantee that.

Mine used to be 20/200, with a -1.75; now it's closer to 20/400, with a -2.75. I think.

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8. ### D_Thoraxis_Biggulp New Member

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Hm... When I first started wearing glasses 15 years ago, I had 20/50. When I switched to contacts, 4 years later and with a new prescription, I didn't bother to find out what the corresponding focal measurement was.

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9. ### prepstudinsc Gold Member

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I'm -9.5 in both eyes. I don't know what it converts to in the 20/xxx formula.

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10. ### D_Thoraxis_Biggulp New Member

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Man, and I thought my -5.5 was bad. Whenever I get my prescription filled at Wal-Mart, they have to send out an order for them since the strongest they keep in stock (last I checked) is -4.0.

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11. ### prepstudinsc Gold Member

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My eye doctor has to special order mine, too, because they don't keep a lot of the stong prescriptions in stock--because there isn't much call for them. The receptionist told me that they usually only keep up to -6 or so.

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12. ### D_Gunther Snotpole Account Disabled

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With glasses, I take a -6.50 or something like that.
But, besides being short-sighted, I also have conical corneas ... which cause a literal cone-like deformation of the corneal surface.
To correct these, I need hard ... not soft ... contact lenses.
(A soft lens would just conform to the deformed surface of the corneas.)
And for some reason, the contacts have to be much less strong than the glasses to get equivalent correction. So they're something like 4.00.
Glasses don't work well for conical corneas. I get two images in the right eye, and a lot of flare in the left eye.
Yet, with contacts, you would never know my eyes were other than fully functional.

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13. ### Mickactual Gold Member

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Eyeglasses - Ugh! I can never seem to get eye prescriptions that work well for me.
I'm nearsighted in my left eye, and farsighted in my right eye.
My distance glasses work fairly well: they're clear glass in the right eye & prescription in the left.
It's the reading glasses that kill me: with clear glass in the left eye and prescription in the right eye, everything is magnified through my right eye, causing an imbalance that makes it impossible for me to see properly. With prescription in both, my left eye (which normally sees fine up close) then becomes blurry. I need something to make the right eye shaprer (not larger), but there doesn't appear to be such an animal.
Also the closer something gets, the blurrier it is with my right eye - thus necessitating different glasses for computer vs. reading (which is held closer).
PS I have astigmatism, but can't handle angle correction in eyeglass prescriptions cos it causes me horrible neusea.
PPS - I can't wear contacts cos I have severe dry eyes (uses to wear 'em - but had to stop).
So...I guess the verdict is we should just trade me in for a newer model, eh?

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