Equivalent spherical lens for someone with astigmatism?
I would like to order a brand of contact lens that does not have a toric counterpart, and I am trying to find my "equivalent" prescription. There is another catch, though: the lenses do not come in quarter increments of power, they only go up by .5. For instance, they offer -5.00, 5.50, 6.00, 6.50, etc.
Please tell me what contact lenses I should order if my prescription for astigmatism is as follows:
Right: SPH -6.25 CYL +2.00 AXIS 130
Left: SPH -6.50 CYL +2.75 AXIS 045
And please, no lectures on how I should just suck it up and get torics that correct my astigmatism. I already have some, but now I want these other lenses, thanks.
- FootprintzLv 71 decade agoFavorite Answer
A spherical equivalent only works up to a certain point for contact powers.. You are way past that point. Using a spherical equivalent only works when the Cyl number is no greater than 1.00
That is a glasses prescription to begin with.
It has to be transposed first to a - Cyl format also to be workable...your prescription there becomes
Right : - 4.25 - 2.00 X 40
left : -3.75 - 2.75 X 135
Spherical equivialent is just adding half the Cyl # to the sphere, but only up to a maximum of .50...it never works higher than that.
Taking half the Cyl for the right eye for instance would become - 5.25 sphere...which would be way too strong for you. So taking -0.50 and adding it makes your right eye a - 4.75 Sph , but then vertex distance becomes a problem...so - 4.50 would be best...but you wouldn't have very good vision because all that astigmatism isn't being corrected.
The left eye is even worse...a - 4.50 SPH would be the best vision possible...and it would still be not good at all.
You have too much astigmatism to ignore it , and it's very doubtful you would get acceptable vision with a spherical lens.
My professional advice ??...Forget it , you won't like the poor vision you would get with them.Source(s): Optician
- Flying DragonLv 71 decade ago
I hate to lecture, but I honestly don't think you will be happy with the vision you will get from contacts that ignore your astigmatism. If you had just a small amount of astigmatism, you could get away with this.
For your right eye, you are -6.25 in one axis and 90 degrees from that, you are -4.25. Normally, they would split the difference, this would give you a - 5.25, but this would leave you over-corrected by a whole diopter in one axis and under-corrected by a whole diopter 90 degrees from that. You are really going to notice this. The situation with your left eye will be even worse. One axis is - 6.50 and 90 degrees from this you are -3.75 the "average"being -5.125. If the prescription you gave is for glasses, usually the strength for contacts will be slightly reduced vs that for glasses when it is greater than about -4.00 to account for the fact that the contacts sit right on your eyes, not spaced away like eyeglass lenses, so maybe you could get a -4.50 or -5.00 for each eye.
Again, I am warning you, I don't think you will be happy with them and you are probably wasting your money. Also, most of these on-line places won't even sell you contacts unless you have a valid prescription for them.
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- Anonymous1 decade ago
You should base it on your eyeglass prescription.
We do not know whether you gave your eyeglass prescription or a contact lens prescription or something else.
If you are fitting yourself you might want to try:
The equivalent sphere for the Rx you gave is:
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