by 
The Curious Bag
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When you leave your eye care professional's office with a written prescription in hand, you'll see several numbers preceded by either a positive (+) or a negative (-) sign.
Types
An eyeglass prescription will have numbers for SPH, CYL, AXIS, PRISM and ADD. A contact lens prescription will list PWR, BC, DIA, CYL, AXIS and ADD.
Similarities
SPH (sphere) in eyeglasses and PWR (power) in contacts indicates the amount of correction needed for 20/20 vision. Both forms of lenses use the terms CYL (cylinder) and AXIS to correct for astigmatism (abnormally curved cornea). ADD is a number that tells the lens maker how to create a bifocal lens by adding a second area of focus.
Unique Numbers
Eyeglass prescriptions use the term PRISM and an associated number to reflect the need to move an image up or down to correct for double vision. (See Reference 2) Contact lens prescriptions use numbers for BC (base curve) that reflect the back curvature of the eye, as well as for DIA (diameter), the distance from one edge of a contact lens to the opposite side.
Features
A positive number in the PWR section indicates farsightedness, while a negative one signals nearsightedness.
Format
A vision prescription has two columns labeled OS (left eye) and OD (right eye), with different numbers in each column.
Source:
All About Vision: Understanding Your Contact Lens Prescription
The Ohio State University Medical Center: Eyeglasses and Contacts
More Information:
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