Many people assume that a contact lens prescription is the same as their glasses prescription; however, they are different because of the distance factor.
Eyeglasses are normally 12 millimetres from the patient’s eye, whereas a contact lens is positioned directly on the tear film of the eye, which is why the prescriptions cannot be the same. Both eyeglass and contact lens prescriptions aim to correct your refractive error through the power of a lens and while the goal of both prescriptions is the same, factors like the degree of your refractive error and the type of contact lenses prescribed will make each prescription significantly different. The powers that your contact lens prescription provides will likely always be different than your glasses prescription to help you achieve the best vision possible.
The differences are not always obvious and there are a lot of factors that are involved, which is why a professional eye care practitioner must help with your prescriptions and assist you in obtaining your contact lens prescriptions. In many cases, this will occur after the diagnostic fitting and it can take multiple visits before a successful fit has been achieved so that a prescription can be provided. This is especially true for GP lenses and specialty designs like bifocal or post-surgical cases.
The prescription you receive must be accurate and provide you with details, including the name of the lens material, its design, power, base curve and diameter. Contact lens prescriptions often contain even more information, including an optical zone diameter, peripheral design, center thickness and the peripheral curve radius. These additional specifications are not included on a glasses prescription and can only be determined after a comprehensive contact lens exam and fitting.
Every patient can request a copy of their glasses prescription at the end of the eye exam; however, a contact lens prescription cannot be written by your eye doctor until they perform a contact lens fitting or evaluate the fit of your current lenses based on their access to your previous prescription. It is also important to realize that not everyone who needs eyeglasses can wear contact lenses because certain conditions like dry eyes can make contact lenses uncomfortable or unsafe for some patients. For this reason, not everyone can get a contact lens prescription.