You may assume that a prescription for vision correction is the same for contact lens as it is for glasses, but this is a common misconception. Learning the reasons why they're different and discovering circumstances when you may need both, are important considerations before you visit your eye doctor.
Why are the Prescriptions Different?
Eyeglass and contact lens prescriptions are necessary to determine the lens power needed to correct your refractive error. This is true whether you are nearsighted, farsighted, or have astigmatism.
The reason why these two prescriptions may vary is that the distance from the eye to the lens is different. Typically, the distance from the eyeball to an eyeglass lens is about 12 millimeters, or approximately half an inch. With contact lenses, the lens rests directly on the surface of your eye.
What Additional Specifications are Needed for Contacts?
The ophthalmologist will do a comprehensive contact lens examination and a fitting to determine what prescription and type of lens will work for you. Not everyone can wear contact lens, particularly if they have eyes that tend to be dry, sensitive, or a condition called blepharitis. Things added to the contact prescription that aren't necessary in an eyeglass prescription include:
The Base Curve: This measurement determines how much curvature is necessary on the back surface -- the part that touches your eye -- of the contact lens. This curve measurement depends on the shape of your cornea and prevents the contact from being too tight or too loose on the cornea which can make the lenses uncomfortable.
The Lens Diameter: This measurement determines the actual size of the lens and is also important to how well the contacts fit and feel. With a perfect fit you shouldn't even be able to tell the lens is on your eye, although clearer vision helps you realize that it's properly in place on your eyeball.
The Lens Material: Different materials make up various brands of contacts. They each have a certain degree of oxygen permeability. For instance, the lenses with the most 'breathability' are perfect for extended wear contact lenses or for people who have a bad habit of falling asleep before removing the lenses.
The Expiration Date: Contacts lens prescriptions are only good for 12 months and this is included on the prescription. After that, you must obtain another contact prescription from your eye doctor. This ensures that if your vision has changed that the prescription changes also. The ophthalmologist will also be able to determine if your eyes are healthy and if you are still a good candidate for contact lenses. In many states, eyeglass prescriptions expire after 24 months. This ensures that your eyesight is as clear as possible and that no changes in vision have occurred.
What If I Want Contacts Just for Fun?
If you don't need vision correction and you want contacts simply to change the color of your eyes, or for a special effect to wear to a Halloween party or some other event, you must still get a prescription from an eye doctor after having an eye exam. Contacts can't be legally sold to an individual without a prescription since they are classified as a medical device.
Is it Necessary to Get a Prescription for Both Glasses & Contacts?
The cost of obtaining a contact lens prescription vs a glasses prescription is just slightly higher than for an eyeglasses prescription alone. Generally, your ophthalmologist will provide you with both prescriptions as long as you scheduled the appointment for a contact lens examination. This allows the doctor to have plenty of time to perform the extra tests necessary to determine if your eyes are suitable for contacts.
Most people need to have a pair of prescription eyeglasses handy even if he or she desires to wear contact lenses almost exclusively. There are always times when it's not appropriate or safe to wear contacts, such as when you are swimming, have an eye infection or eye injury. Many people find contacts uncomfortable or dry-feeling when they have spent an especially exhausting day at the office in front of a computer or when they have to spend an unusual amount of time reading while preparing for an exam at school or a lengthy report at work.
There will also probably be times when you lose a contact and don't have time to replace it, therefore the need for attractive eyeglasses to help you drive or make it through a day at work.
Filling the Prescription
Since it's common to want to shop around for the best prices -- and the most attractive pair of frames, if you want both contacts and a pair of glasses -- it's imperative that you ask for both your prescriptions before leaving the doctor's office. The facility is legally bound to provide you with the prescriptions once they have examined your eyes and you have paid for the visit.
Whether you fill your contact prescription vs glasses prescription at the eye doctor's office, by shopping online, or by visiting an eyewear store, the prescription is necessary to obtain optimum vision correction. This will enhance your outlook on life by providing you with healthy eyes and excellent vision!