What is Axis on an Eye Test? Common Eye Prescription Details Explained
The axis number on your eye prescription will tell your optician in which direction they must position any cylindrical power in your lenses – which is typically required for people who have astigmatism.
The axis number shows the orientation or angle of the eye in degrees ranging from 1 to 180. An axis eye test of 90 will mean that it is a vertical position, whereas a 180 result will mean it is horizontal.
When an eyeglass prescription has cylinder power (CYL value), which is the strength of astigmatism correction needed, the axis value directs where to place the power in the lenses of your glasses – helping those with astigmatism to see better. If the axis eye test value is 180 degrees, you may see it noted as x180.
A higher number from your axis eye test does not mean that your prescription is stronger, it simply determines the position of your eye’s astigmatism.
What is a ‘Normal’ Axis Eye Prescription?
There’s no such thing as ‘normal’ really, everyone and every eye is different! However, people without astigmatism would typically have clearer vision as the light will enter through the pupil and fall in the correct place on the retina. The eyes closest to ‘normal’ with astigmatism would tend to have approximately 0.50 dioptres of negative cylinder power with a 180-axis number.
Eye Test Results Explained: SPH, CYL, and Axis
Spectacle prescriptions are written in a universally standardised format with common notations. Some of these include:
The term “sphere” means that the correction for shortsightedness or longsightedness is spherical, meaning that it is equal in all meridians of the eye. This also indicates the amount of lens power, which is measured in dioptres (D), needing to be prescribed to correct the eye’s near or farsightedness.
If the number appearing under the (SPH) heading has a minus sign (-), you are shortsighted. If the number has a plus sign (+) then you are longsighted.
This indicates the amount of lens power required to correct astigmatism – representing the difference in the greatest and weakest powers of the eye. This is typically separated by 90 degrees. If nothing appears in the CYL column, either you have no astigmatism or your astigmatism is so slight that it is not necessary to correct with lenses.
Cylinder power always follows sphere power in an eye prescription.
If your glasses prescription includes cylinder power, it must also include an axis value which follows the CYL power. The axis value indicates the angle in degrees between the two meridians of an astigmatic eye.
The axis is defined with a number ranging from 1 to 180, with 90 corresponding to the vertical meridian of the eye and 180 corresponding to the horizontal meridian.
Further Eye Test Definitions
You may notice other features on your eye prescription that you don’t have much information about.
PD: Meaning Pupillary Distance, which describes the distance between the centres of your pupils.
ADD: Standing for Addition, this refers to any added correction that may be needed for reading or intermediate use.
BVD: Meaning Back Vertex Distance, BVD represents the distance from the cornea to the back of the lens of your glasses.
Near, Inter, Dist: Each of these refers to the prescription required for various working distances, i.e. reading, intermediate (computer) and distance.
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The axis eye test enables us to determine the axis angle of your eyes, which can determine if you require corrective lenses for long or short-sightedness.
Protecting your vision and ensuring you can see clearly for as long as possible is extremely important and, with a fully comprehensive eye examination at Taylor-West & Co, we can detect even the most minute of changes to your prescription. We can also detect signs of potential eye conditions up to five years before you start noticing issues with your vision.
Protect your visual acuity and prevent the onset of conditions like glaucoma, macular degeneration, and more today.
Sophie Taylor-West is the founder and Director of Taylor-West & Co Optometrists. She is one of our Hove-based registered Optometrists. Sophie specialises in contact lens fittings for both healthy and also “awkward” or more complex prescriptions, helping people who have difficulties seeing anything at all by transforming their sight. She also specialises in myopia management for children and teenagers with short-sightedness. Click here to read more...