-7.50 = 20/? How to translate

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-7.50 = 20/? How to translate

Postby ksoldan » Wed Apr 18, 2007 12:45 am

THis seems like a silly question, but I keep seeing things posted in regard to seeing 20/15, 20/20 etc. How do you translate your prescription into these terms? My prescription for contacts is -6.75 and -7.50. Just curious.

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Postby LasikExpert » Wed Apr 18, 2007 5:07 am

Some have created grids, but there is no consistent correlation between the Snellen 20/whatever measurement of visual acuity and the measurement of refractive error in diopters. Someone can be emmetropic (no refractive error) and only able to see 20/40 or worse. Someone can be slightly myopic (nearsighted, shortsighted) or hyperopic (farsighted, longsighted) and able to see 20/15. There is much more to vision than that which is measured by the "Big E" chart or "which is better, one or two?".
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Postby maria22 » Sun Apr 29, 2007 6:29 pm

The 20/whatever system refers to the percentage you see, right? In other words, 20/20 = 100%.
I found out the hard way that no refractive error does not necessarily equal perfect vision....
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Postby LasikExpert » Thu May 03, 2007 1:56 pm

A normal sighted person sees an object at 20 feet away with a measurable amount of clarity. An item 20 feet away with the clarity of an item 20 feet away would be 20/20.

If a person sees an object 20 feet away with the clarity that a normal sighted person would see the object at 40 feet away, vision would be 20/40.

If a person sees an object 20 feet away with the clarity that a normal sighted person would see the object at 15 feet away, vision would be 20/15.

Snellen 20/whatever does not relate to a percentage, but is a comparison against what a normal sighted person would see.
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