For many people who wear both glasses and contact lenses, the actual eye prescription values are different. This is not unusual because of the difference in the way that contacts lenses and glasses correct vision. Contact lenses are much closer to the eye and do not have a small space between the eye and the glasses lenses.
The tear film can also fill in behind the contact lens and the cornea to help correct or mask astigmatism which needs to be corrected in glasses. Additionally, not all contact lenses come in every possible prescription power – sometimes the lenses will have to be slightly different than the precise glasses prescription.
With the many reasons that a contact lens prescription value may be different than the glasses prescription, along with the need to evaluate the fit of the lenses on the eye, each contact lens prescription should be validated by your eye doctor and never attempted to order using the glasses prescription.
With the space between the eye and the back of the lens in glasses, there is space for light to travel and change the effective power of the glasses.
However, as there is no space between the contact lens and the eye, there is no change to the light and the power of the contact lens is not altered in this way.
If wearing a nearsighted prescription or minus-powered lens, the contact lens power may be less than the glasses' power.
Alternatively, if wearing a farsighted prescription or plus-powered lens, the contact lens power may be higher than the glasses' power.
Also, contact lenses can sometimes mask a small amount of astigmatism due to the tear film filling in behind the contact lens. Usually, contact lenses are not made with less than 0.75 diopters of astigmatism power because the lens will mask this amount on its own.
In glasses, even 0.25 diopters of astigmatism correction can be put into the lenses to help correct the vision.
Contact lenses are manufactured with the goal of reasonably fitting many patients with prescription powers that meet most of the population’s needs.
Often, these contacts will come only in certain powers – for instance, every 0.50 diopter power instead of every 0.25.
A very common change from the glasses prescription to the contact lens prescription is the axis of the astigmatism correction. In glasses, the axis of the astigmatism correction can be specified down to a single degree. In contrast, in contact lenses, the axis is usually only available in ten-degree increments starting at zero and going through 180.
At a contact lens fitting or exam, the doctor will not only ensure that the correct prescription is used in the contact lenses but the fitting and health of the contact lenses will be evaluated.
Even when there are small or no changes to the prescription, eye health needs to be evaluated and insured to be healthy even with contact lens wear.