Axis
Spherical lenses have a single power in all meridians of the lens, such as +1.00 D, or −2.50 D.
Astigmatism, however, causes a directional blur. Below are two examples of the kind of blur you get from astigmatism. The letters are smeared out directionally, as if an artist had rubbed his or her thumb across a charcoal drawing.
A cylindrical lens of the right power and orientation can correct this kind of blur. The second example is a little bit more blurred, and needs a stronger cylindrical lens.
But notice that in addition to being smeared more, the second example is smeared out in a different direction.
A spherical lens is the same in all directions; you can turn it around, and it doesn't change the way it magnifies, or the way it blurs:
A cylindrical lens has refractive power in one direction, like a bar reading magnifier. The rotational orientation of that power is indicated in a prescription with an axis notation.
The axis in a prescription describes orientation of the axis of the cylindrical lens. The direction of the axis is in degrees measured anti-clockwise from the horizontal line through the centers of the pupils when viewed from front side of the glasses (i.e., when viewed from the point of view the person making the measurement). It varies from 1 to 180 degrees.
In the illustration below, viewed from the point of view of the person making the measurement, the axis is 20° if written in plus notation or 110° if written in minus notation.
The total power of a cylindrical lens varies from zero in the axis meridian to its maximal value in the power meridian, 90° away. in the example above the axis meridian is located in the 20th meridian, and the power meridian is located in the 110th meridian.
The total power of a lens with a spherical and cylindrical correction changes accordingly: in the meridian specified by axis in the prescription, the power is equal to the value listed under "sphere". As you move around the clock face, the power in a given meridian will get steadily closer to the sum of the values given for sphere and cylinder until you reach the meridian 90° from the meridian specified by the axis, where the power is equal to the sum of sphere and cylinder.
Read more about this topic: Eyeglass Prescription
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