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My prescription is more than numbers: what do they mean?

After an eye exam, the optometrist writes a prescription containing the information needed to correct your vision. By reading this article, you will notice that it contains many numbers and abbreviations. Are you curious to know what they mean? Here is some information to help you better understand your prescription.

First and foremost, your prescription is divided in two parts: one for each eye. On the first line you will find the letters “O.D.”, this is an abbreviation from the Latin words Oculus Dexter, which means, “right eye”. On the second line, we will find the letters “O.S.”, which stems from the Latin words Oculus Sinister, referring to the left eye. 

Now, let's take a look at what is in each section of your prescription.

The Sphere
The first part of the prescription is called the sphere. It indicates if you are short-sighted or hypermetropic. If there is a minus sign (-) at the beginning of the first number, it means that you are shortsighted. In this specific case, it is vision at a far distance that is affected. The higher the number after the minus sign is, the greater the myopia. For example, if you have a sphere of -0.25, your vision is minimally affected. On the other hand, if your prescription starts at -5.00, you most likely have difficulty seeing at a distance without glasses. If the sphere indicates "Pl" or "Plano" for both eyes, it means that you do not suffer from myopia or hyperopia.

A positive sphere indicates hyperopia. People affected by farsightedness have either an eyeball that is too short, or a retina curvature of that is not pronounced enough. This prevents the image on the retina from being properly focused, causing blurred vision at close range and sometimes even blurry vision when looking at a distance.

The cylinder
The second column on the prescription refers to the cylinder, which indicates whether your eyes suffer from astigmatism. This condition is causes blurred vision at all distances and is caused by the deformation of the cornea. The cylinder indicates the strength of lenses you need in order to correct this condition. The third column on your prescription represents the axis, which indicates the degree of astigmatism.

The Addition
The fourth column usually indicates if you have presbyopia, a natural aging-related condition that affects near vision. The numbers following the “+” sign represent the strength to add to the sphere in the lower part of progressive and degressive lenses. This column is identified by “Add”, which simply means “addition”. In some cases, an addition may be prescribed to relax the eyes while reading.

The prism
The prism indicates the shift of the image necessary to correct a misalignment of the two eyes. It compensates for the muscular imbalance between the eyes. This number is expressed in prismatic diopters and the letters indicate the orientation of the prism. The indication "BU" means that the base is pointing upwards, "BD" is pointing downwards, "BI" is pointing inwards, and "BO" is pointing outwards.

The acuteness
Visual acuity represents the ability to clearly see an object at a distance, with or without glasses. It is indicated by the abbreviation "AV". Acuity can either be measured on a base of 20 or on 6. A result of 6/6 or 20/20 indicates excellent visual acuity. On the other hand, a patient who scores less than 6/21 or 20/70 is considered to be visually impaired. This means that they can only see an object properly if it is at a distance of 6 meters (or 20 feet). A person with good visual acuity can usually see correctly as far as 21 meters away (or 70 feet).

The vertex
The vertex, identified by the letters "VTX", indicates the distance between the eye and the lens, or between the eye and the contact lens. This distance is zero in the case of contact lenses: this is one of the reasons why eyeglass prescriptions differ from contact lenses prescription.

Before you buy glasses, it's important to make sure your prescription is up-to-date, so your new lenses correct your vision in the best possible way. Make an appointment now with your IRIS eye care professional!

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