What is "axis" in optometry?

Question: I ordered glasses for my daughter online, and they worked perfectly, but I didn't have an "axis" number. She lost them. Now I'm trying to order a less expensive pair from another company, but they won't take the order without the number. Is there a sort of standard number that generally works well, or do I have to get it measured? And what is it a measure of? In simple language, please.

Answers:
The axis number is a measure of astigmatism. If she doesn't HAVE astigmatism, the optometrist might have left that section blank. However, if there is a second correction factor for each eye (cylinder), the second one refers to the astigmatism. In order to correctly make the lenses, the lab needs to know the axis (which is the angle of the correction). Difficult to explain, hope this helped.

You generally can just call the optometrist who examined her and get a duplicate Rx, if it hasn't been too long.

Other Answers:
The three basic components of an ophthalmic prescription are sphere, cylinder, and axis. The sphere component of the prescription is described by lens power. Lenses with plus sphere power, correct for far-sightedness or hyperopia, while lenses with minus sphere power correct for near-sightedness or myopia. The spherical portion of a lens can be visualized as a slice from the edge of a sphere; the smaller the sphere, the steeper the curve, and thus more dramatic the power.

Often prescriptions also have a cylinder component to correct for astigmatism. Just as the spherical component can be visualized as a slice from the edge of a sphere, the cylinder component can be visualized as a slice from the edge of a cylinder, parallel to the axis of the cylinder. Light rays passing through a cylinder component will converge or diverge as with the spherical component, however the focus will be along a line instead of a point. This is why cylinder and axis must go hand-in-hand. As with sphere power, cylinder power is expressed in diopters (D) and is calculated as the inverse of the distance in meters between the focal line or meridian and the lens. Axis is expressed in degrees between 0 and 180. Axis measurements between 180 and 360, merely duplicate those between 0 and 180. In other words, an axis measurement of 270 degrees is the same as a measurement of 90 degrees, likewise a measurement of 190 degrees is the same as a measurement of 10 degrees. So, for simplicity, axis is always expressed between 0 and 180 degrees.

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