The Differences Between Contact Lenses and Glasses Prescriptions
One of the most frequently asked questions is whether a glasses prescription is the same as a contact lens prescription. They are not, and the main reason for this is that glasses are positioned approximately 12 millimeters from your eyes whereas contact lenses sit directly on your eyes. You can compare this to holding a magnifying glass at different distances. If you hold the lens at an arm’s length and focus on an object, what you see is very different to when you hold it right up to your eye.
During an eye exam for glasses, the optician will determine your glasses prescription with lenses that sit at a specific distance from your eyes and under the assumption that your glasses lenses will also sit at the same distance. A range of information is assessed, including your lens power (known as ‘SP’), which is required to correct an eye refractive error. This is the cause of nearsightedness (myopia) and farsightedness (hyperopia). The amount of correction required to correct a refractive error is measured in diopters (often abbreviated to ‘D’).
For specific eye conditions such as astigmatism, the AXIS is required – this is a measurement of the cylindricality of the lenses, which will compensate for a non-spherical cornea - the cause of astigmatism. For specific types of lenses, such as bifocal lenses, your glasses prescription will also include an ‘ADDITION’ and ‘DOMINANT’ (also known as ADD) measurement which refers to the magnifying power needed to correct presbyopia. Presbyopia this is a very common condition which develops as you get older and it affects the ability of your eyes to adjust in order to see objects clearly at different distances. Some glasses prescriptions also include a ‘PRISM’ measurement which is required to correct eye alignment problems.
Contact lens prescriptions
With an eye exam for contact lenses, all of this information is still needed, but a contact lens prescription will contain additional information such as the base curve (the curvature of the lenses in millimeters), the diameter of the lenses in millimeters, the lens brand and the expiration date. When your glasses are not for visual correction (i.e. for aesthetic purposes), you don’t necessarily need a glasses prescription. This is not the case for contact lenses. You must have a contact lens prescription even if you are buying lenses which offer no visual correction, such as colored contacts for special effects. Contact lenses are a medical device and they are not suitable for everyone, so be sure to speak with a specialist to assess which option - and prescription - will be better suited for you.
It is worth noting that the lens power for your glasses and contact lens prescription may differ, but this is more likely to be the case when you have a prescription of under -4.00D, which means it does not apply to everyone. Following an eye exam, your optician is legally required to provide a copy of your glasses or contact lens prescription. If you are aged between 18 and 60, we would recommend a thorough eye exam every two years and once a year if you are over 60.