If I Have a Contact Lens Prescription of -8.00, How Do I Calculate How Well I See Without My Contacts?
April 22nd, 2010 | See the Top Answers to this Question Below
I know with the contacts I have 20/20 vision, but how do I calculate what I see before the contacts are in?
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Here’s a way to look at it.
How GOOD you see is how well you can determine whether two lines with a space between them are really two lines and not one fuzzy one. That ANGLE is what is measured by those little letters. The way they used to be able to measure vision in the ‘old’ days was to look at the second star in the handle of the big dipper. If you can see that star (with your correction, not without it…that’s just silly), and you can see that it’s NOT one star but two, that’s 20/20.
The numbers of the lens is in diopters. The Power is equal to 1/d where d is in meters.
As a myope you have very strong eyes. Too strong. If you were already focused at a meter in front of you instead of way far away, then you’d be +1.00 too powerful. They’d give your a -1.00 lens to neutralize that power.
If you were focused at a half meter in front of you, your eyes would be +2.00 too powerful and they’d give you a -2.00 to do the job of moving the focal point out to infinity so you can do things like see the board at school, drive, see a movie, see those cute guys over there…
If you were +3.00, that’s a third of a meter in front of you, you’d get a -3.00 to do the work.
If you were a fourth of a meter too powerful a -4.00
fifth of a meter, -5.00
sixth of a meter -6.00
you are a -8.00 which is an eighth of a meter (one eighth of 39 inches) which is a fairly strong lens system.
If you use contacts, because the lenses are ON the surface of the eye, the Rx will be different than your spectacles to make up the difference in distance to the back of the eye.
Take those contacts out and you see clearly about 4-5 inches in front of you and closer, but everything further out is a blur (as you well know).
You can’t accurately calculate your unaided acuity from your prescription; there are other variables. You will need to have your unaided acuity measured.
Having said that, your unaided acuity is likely between 20/800 and 20/2000. Most eye doctors do not have eye charts with letters that big so they will simply tell you that your unaided acuity is worse than 20/400.