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Explain Axis in Eyeglasses

When you get a prescription for eyeglasses, you will see a chart where the optometrist will write numbers for your prescription. These numbers represent nearsightedness or farsightedness, cylinder and axis. Cylinder and axis are the numbers that refer to astigmatism and how it affects the focal points in a person's field of vision.

  1. The Shape of the Eye

    • The human eye is a sphere. Light enters it and refracts evenly. This even refraction translates through your optic nerve to images in your brain. You see things clearly, completely in focus, as they really are. Deviation from this standard of perfect vision is common throughout life. Some people are born with vision that is not perfect and require corrective lenses early in life. Others lose perfect vision through the natural aging process and begin wearing glasses later. In all eyeglass prescriptions, you will see a series of numbers.

    Positive and Negative

    • Nearsightedness and farsightedness are represented in eyeglass prescriptions through positive and negative numbers in the first part of the prescription. If there is a positive number, you are farsighted. If there is a negative number, you are nearsighted. The larger the number, the more nearsighted or farsighted you are. This refers simply to how you focus on particular objects close to you and far away from you. You can have these numbers on your prescription and still require a cylinder and axis number before your eyeglass prescription will help you see.

    Cylinder

    • If you have astigmatism, your eye is not spherical; it is shaped more like a football. With astigmatism, two other numbers, cylinder and axis, are important to get your prescription correct. The cylinder number refers to how much astigmatism you have. The greater the number, the more astigmatism you have in your vision. Imagine a plus ( + ) sign but keep the vertical and horizontal lines separate. With astigmatism, you will have two focal points on these different axes, not just one for the entire plus sign.

    Axis

    • The final number is the axis of your astigmatism. To get your eyeglass prescription correct, the doctor needs to not only determine how much astigmatism you have, but just what axis your eye has. Astigmatism means your eye is not spherical. Axis determines at what angle your eye rests. Imagine holding a football on its point, then leaning it to one side or another at a 90-degree angle. The axis in your prescription determines this angle in your eye.

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