An [[optometrist]] will test your vision and determine the amount of refractive error in your eyes, and write a prescription as close as possible to that. A standard prescription should correct your vision so that there is no [[blur]] at 20 feet. If you can not see clearly on your
prescription lenses at 20 feet, see your optometrist and make sure you know about and understand any underlying conditions you have that affect your eyesight.
A prescription may look like this:
You will note that most prescriptions will round to the nearest quarter [[diopter]]. This is the level of precision available in standard pre-ground lenses, which make glasses cheaper and faster to make than custom ground lenses. You may have a more precise prescription if it is written for specific lens grinding systems, or a less precise prescription if it is written for contacts.
==Contacts Vs Glasses==
''See also [[Lens#Glasses or Contacts]]''
Your prescription is specifically written for either contacts or glasses. [[Vertex Distance]] plays a role in effective lens strength, so the contacts which sit closer to your eye may be written
as a weaker prescription than glasses. Vertex Distance is hard to measure, and contacts may act differently as the eye adapts to them, so if you have a strong lens your optometrist may need to do additional refraction tests with the contacts in and adjust your prescription slightly.