# Astigmatism eye perscrip.

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why is one number a + and the other a -? Farsighted and nearsighted??

My RX is sph +0.50 cyl -0.75 (both eyes)

R axis 165 L axis 5

In:

It’s a measurement for lenses in general called ‘Diopters’ or ‘D’

The basic jist of it for convex lenses (farsightedness uses these) is that if you have a light straight on the lens, then there is a place called the focal point all that light converges. You measure the distance of the focal point in meters (f), and the Diopters of that lens is 1/f. The shorter the focal point, the stronger the lens, and the more ‘Diopters’ it has.

For concave lenses (for nearsightedness), it works a bit differently–they don’t focus light, they diffuse it. So, if you traced the lines of how widely that lens diffuses the light, it would trace back to a point behind the lens. That is your ‘virtual focal point’ and like the other lens, you take 1/f, and that’s your ‘Diopters’ to measure the strength of the lens. As you’d expect, this would be negative, to show the ‘focal point’ is behind the lens.

A planar lens, that is, one with out any concavity or convexity, is just a plane of glass. It does no focusing or diffusion, and so those are measured as lenses with ‘zero diopters’ of power.

So–positive means a focal point, negative means a ‘virtual vocal point behind the lens’, and zero D lenses are strictly cosmetic.

[https://images.app.goo.gl/foFuajM8dt6B9hvFA](https://images.app.goo.gl/foFuajM8dt6B9hvFA) is a visual aid to explain it.

As for the disparity in those measurements what you have is astigmatism–that’s where your horizontal focal point and your vertical focal point differs, so to do that, they need lenses that have different focal points based on crosswise vs up and down. So you get two more measurements:

Cylindrical power, which shows how much different your lens must be in a specific direction in order to counteract that astigmatism’s disparity.

Axis is the direction that Cylindrical power is not needed–where your astigmatism matches the spherical power of the lens. 90 means a vertical angle, and 0 and 180 reflect horizontal angles. R Axis is your right eye, and L Axis is your left eye. Some prescriptions put the right eye as OD, and the left eye as OS, out of the latin.

Add is additional strength for bifocal lenses on the bottom of the lens.

Prism is when your eye is misaligned, say your lens shoots to the left, or to the right, instead of the center of your retina.

So, looking at your prescription, your glasses aren’t that strong. It wouldn’t be correct to say you are near OR far-sighted. What it means is that you have astigmatism, and you’re farsighted horizontally-ish-almost (spherical prescription is +0.25), and you’re nearsighted vertically (cylindrical is -0.75, so that’s enough to put you below 0)

What usually causes the really extreme variances is having mixed astigmatism, where one meridian is nearsighted and the other is farsighted due to the astigmatism. Meridians are the east/west and north/south lines of the eye.