View Full Version : Menaing of vision prescriptions?
12-19-2002, 12:22 PM
What do the numbers mean on prescriptions for glasses and contact lenses? I know my vision requires something between -3.75 and -4.00, but I have no idea what those numbers mean - I presume its not just a code that optometrists have developed, but some kind of real measure. Is there anything other than that number that typically matters for picking glasses/contacts?
Also, when I looked around for an answer to this on the board, I saw a lot of mention of low-cost glasses at Walgreens and other places. Is this an option for nearsighted glasses, and is there anything to watch out for? (I would like to pick up a couple of cheap pairs of glasses to stick around various places as spares).
Duck Duck Goose
12-19-2002, 12:38 PM
Found this tech explanation. Read at your own risk. :D
Dunno exactly what the tech stuff means, but the numbers tell the person who is grinding the lenses exactly how many millimeters he needs to go in which direction to make the curve of the lens go the way it needs to go in order to bend the light rays coming into your eye the way the light rays were bent coming into your eye at the optometrist's office, when he did the thing with the multiple lenses, "better? worse?"
The optometrist eventually arrived at a measurement of a lens that would bend the light coming into your eye so you could see normally, and that measurement is expressed as those combinations of numbers. That tell the lens-grinding person exactly how to grind the lens.
Those Walgreens "reading glasses" are just basically plain old magnifying glasses of different strengths. They don't correct for astigmatism, which is where for perfect vision you need the light coming in bent sideways a little.
If the degree of astigmatism is slight and no other problems of refraction, such as nearsightedness or farsightedness, are present, corrective lenses may not be needed. If the degree of astigmatism is great enough to cause eyestrain, headache, or distortion of vision, prescription lenses will be needed for clear and comfortable vision.
The corrective lenses needed when astigmatism is present are called Toric lenses and have an additional power element called a cylinder. They have greater light-bending power in one axis than in others. If the Walgreens reading glasses give you a headache, you need "real" glasses.
12-19-2002, 12:38 PM
Second question first; NO. Over-the-counter reading glasses may do for the farsighted, but there are no over-the-counter glasses for myopia (nearsightedness). On the other hand, if you mean that your local Wal-Mart has an optician, then sure -- I get mine at Costco. If you have a complicated prescription, or demand particular lens material, design, or treatments, you may find that discount opticians don't give you what you're looking for.
The number is the refractive strength of the lens in diopters, a unit of 1/meters. The strength in diopters is the inverse of the focal length in meters. In your case, the lenses have a focal length of about 25 cm, or 10 inches. The negative sign indicates that the lens is concave, that it causes light to diverge, that its focal point is actually behind it, and that you're myopic.
Finally, yes, the other measurements for contact lenses include base curve and diameter; the other measurements for glasses (and some contacts) includes cylinderstrength and angle (correction for astigmatism).
12-19-2002, 12:46 PM
The numbers are in units of diopters (http://www.geocities.com/ocular_times/diopter.html) and the higher the number the more correction you are getting.
The drugstore glasses are essentially magnifying glasses for reading and close work for people with presbyopia (http://www.allaboutvision.com/over40/).
See also myopia (http://www.allaboutvision.com/conditions/myopia.htm), or nearsightedness.
12-19-2002, 03:20 PM
I seem to recall once upon a time seeing a formula for converting diopters to the old 20/20 format that I grew up on. Anybody know that formula? Or is my feeble mind now creating a past that never happened (distinctly possible)?
12-19-2002, 03:24 PM
When chosing OTC readers, you can use your age as a general guideline, provided you do not already have some need for correction other than for presbyopia (which is caused by the gradual loss of flexibility of the eye's lens to change its shape so as to focus images onto the retina).
Most people start to develop presbyopia between the ages of 40 and 45. As a good guideline, you can start off with readers of about +1.00 strength, and add +0.25 diopters every 2 years, eventually getting up to somewhere between +2.50 and +3.00, which is about the maximum the mature lens will give you.
Never take it for granted that all you need is OTC readers, even if they seem to solve your reading problems. There are lots of other things that an optometrist or opthalmologist will test for in older patients, especially increased intra-ocular pressure, which can lead to blindness, plus things like macular degeneration which may indicate some systemic diseases, or clouding of the lens leading to a cataract.
When wearing readers, the most comfortable position for the material you're reading should be about 14-18 inches away from you. If you have to hold it closer, the lenses are too strong. If you find it blurred and need to hold it further away, they're too weak.
12-19-2002, 05:02 PM
The prescription may look something like this:
O.D. -3.50 -0.25 135
O.S. -3.25 -0.25 85
Which means you need a correction in your right eye of -3.50 diopters spherical plus 0.25 cylindrical with the axis tilted 135 degrees. OS is the same for the left eye and then you have the dintance between the center of the lenses of 65 mm.
markup in eyeglasses is outrageous and mostly because the frames are considered a fashion item. They try to convince you that there's something high tech about eyeglasses but the fact is they are the simplest of things. The lenses cost close to nothing and are cut to fit the frame. Really simple stuff. I get my eyeglasses in China for 1/4 of what I would pay in Europe or America.
12-19-2002, 05:28 PM
Originally posted by ShibbOleth
I seem to recall once upon a time seeing a formula for converting diopters to the old 20/20 format that I grew up on. Anybody know that formula? Or is my feeble mind now creating a past that never happened (distinctly possible)? You really cannot, AFAIK. -3 diopters is roughly 20/200, which is the limit of blindness (if both eyes cannot be corrected better than that, you are legally blind). markup in eyeglasses is outrageous and mostly because the frames are considered a fashion item. They try to convince you that there's something high tech about eyeglasses but the fact is they are the simplest of things. The lenses cost close to nothing and are cut to fit the frame. This is not really true. There are all kinds of qualities in both the frames and lenses. You want lenses that are UV protected 100% and are scratch resistant. Sure, you can get some glass cut to your prescription, but that's not what you want. Frames come in different materials and all qualities, also. Better frames will have nose pieces which are replaceable, and must be replaced (free of charge by your optometrist) occasionally. If you find your glasses slipping, you need to replace the nose pads. Tennis balls have struck my glasses without damaging them at all. Once, the opponent on the othe side was only a few feet away from me and slammed the ball into my glasses. Neither the lens nor the frame broke. That's what sturdy glasses with good qualities can do.
12-20-2002, 01:03 PM
Thanks for the answers, that clears up my question. As far as cheap glasses go, I wasn't thinking about 'cheaper optometrists' but about the $20 thingies I heard about, which I now know are only the reading glasses. If they had those for nearsightedness then I'd pick up a couple of pairs to stash around in case I lost a contact, but I'm not going to worry too much.
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