BO is Base out, BI is Base in, BU is Base up and BD is Base down.
= Rounding =
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 it is written for contacts.
= Contacts Vs Glasses =
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.
= See also =