Often times your eyeglass and contact lens prescriptions match exactly. Sometimes they don't. Why? Well, there are several reasons.
1. Your glasses may correct for astigmatism. If you have a small amount of astigmatism (<0.75D usually), you often do not need contact lenses to correct that (you see just as well without it!). Also, contact lenses correcting for astigmatism start with at least 0.75D. Smaller amounts are not available.
2. If you have a high prescription (over +/-4.00D), vertex distance (the distance you glasses sit from your eye) must be accounted for. Since a contact lens sits directly on the eye, a little optics calculations must be done to determine the prescription for contacts based off of the glasses prescription (eg. -4.00 in glasses translates to a -3.75 in contact lenses).
3. Contact lens prescriptions come in set increments whereas glasses prescriptions can be made precisely. For example, a glasses prescription of -7.25 would correspond to either a -7.00 or -7.50 in contact lenses since -7.25 is not available.
Still have questions? Contact your eye care provider today!