What is the difference between a contact lens prescription and glasses prescription?

[Killbuck]

I was asked to have an eye exam for contacts,but I just had one for my glasses.The doctor said they need the prescription for contacts.What is the difference between them?

[Killdeer]

This depends on how strong your power is. A contact lens prescription can be the same as a prescription for glasses when: - the prescription is a lower power, ie: -4.00 to +4.00 and - there is absolutely no astigmatism present. If you have any astigmatism or require a lens power greater than +/-4.00, the parameters of the contact lens can change. - if you have astigmatism under 0.50, a spherical lens should be prescribed. - astigmatism of 0.50 to 0.75 can be effectively corrected with an aspheric lens. - astigmatism greater than 1.00 should be managed with a toric lens. If your prescription power for glasses is greater than +/-4.00, you should see a difference in the power of your contact lens prescription. Generally, every time your glasses prescription increases or decreases by a power of 2, your contact lens prescription will increase by an additional power of +0.25. For every power change of 2 in a glasses prescription, the difference of +0.25 power is needed to compensate for the location of the contact lens. Your glasses are much further from the front surface of the cornea than contact lenses.

[Killduff]

Glasses prescription is usually stronger than that of contact lenses. Glasses are placed a little far away from our eyes while contact lenses are directly placed on our eyeballs. So they are much different.

[Killeen]

The prescription on your contact lenses are greater than that on eyeglasses. Since eyeglasses are usually about 12 millimeters from the eye, the prescriptive power of a contact lens - which is positioned directly on the tear film of the eye - will be slightly less nearsighted.