Just Exactly What is Astigmatism?
If your prescription (Rx) has a correction in the Cylinder (CYL) and AXIS sections, this means you have an astigmatism.
If you have an astigmatism correction on your Rx – and the vast majority of eyeglasses wearers do – you may be experiencing one or more of these symptoms when you are not wearing corrective lenses:
- Blurry vision
- Double vision
- Having to squint to see near and far
- Difficulty focusing on printed words
- Eye strain
- Tired eyes
What causes an astigmatism? It’s predominantly genetic, and it’s usually present to some degree at birth. It may increase or decrease with age. Someone may also develop an astigmatism following an injury to the eye, eye disease, or eye surgery.
An astigmatism results when one or both of the two parts of the eye that bring images into focus – the cornea and the lens – are aspheric. This means that they are not perfectly round and smooth spheres, like a ping pong ball.
An aspheric cornea or lens is more like the oblong shape of a football.
When the cornea and lens are round and smooth, they focus light directly onto the middle of the retina, at the back of the eye, making what you see look crisp and clear. In this case, your cornea and lens are perfectly spherical.
You don’t have an astigmatism if it says “SPH,” meaning “spherical,” on your Rx in the CYL section. Writing “SPH” in the CYL section is one of the ways an eye dr. indicates that the cornea and lens are perfectly round – spherical – and that no astigmatism is present. It’s not uncommon to have an astigmatism in one eye but not in the other.
Another abbreviation for “no astigmatism present” is DS, short for the Latin phrase diopter simplex, which means “unaffected,” indicating that this eye is unaffected by an astigmatism.
But let’s say you have an astigmatism correction on your Rx. If so, you will see a minus or plus number in the CYL section of your Rx.
If the CYL number is preceded by a minus, this means that the aspheric cornea or lens causes light to come to a focus point in front of the retina. It stops at a place that’s too near – not far enough to reach the retina – for you to see well. If the CYL number is preceded by a plus, this means that the aspheric cornea or lens causes light to come to a focus point behind the retina. It stops at a place that’s too far – not near enough to reach the retina – for you to see well.
Each type of astigmatism is corrected by the number in the CYL, which is literally a cylinder placed on your lens, which would be a convex (curved outward) cylinder if the Rx calls for a plus CYL, or a concave (curved inward) cylinder if the Rx calls for a minus CYL.
You will also see a whole number, somewhere between 1 and 180, in the AXIS section. If you have a plus or minus number for the CYL but nothing written in the AXIS field, call your eye dr.’s office. There must be an AXIS if there is a CYL.
Here’s why: the AXIS is literally the degree of angle at which the cylinder is placed on the lens.
For example: if the AXIS number is 180, the astigmatism is corrected by placing the cylinder horizontally on the lens. If the AXIS number is 90, the astigmatism is corrected by placing the cylinder vertically on the lens.
Sometimes the eye dr. will write the AXIS number with one or more zeros before the actual AXIS number, such as 005 or 090. Those preceding zeros are just placeholder digits and the actual AXIS number is 5 or 90. The preceding zeros won’t show up on the AXIS drop-down window on the Your Prescription page where you enter your Rx numbers. You can ignore those zeros.
Every Rx lens Zenni Optical offers can accommodate an astigmatism correction. A strong astigmatism correction would be one that has a number in the CYL category between 2.50 and 6.00, plus or minus on the Rx. The highest CYL number we can correct is plus or minus 6.00, and most of the lenses we offer can accommodate this CYL correction.
Because correcting a strong astigmatism is more complicated than correcting a mild astigmatism, an extra-strength charge is assessed on orders of glasses with single-vision lenses when the Rx indicates a strong astigmatism. Bifocal or progressive lenses with high CYL numbers that call for strong astigmatism corrections come with no extra-strength charge.