Contact Lens & Glasses Prescriptions: What's the Difference?

 Is there a difference between a glasses prescription and a contact lens Prescription (Rx)? 

A person’s glasses prescription is measured either using a laser-assisted machine called an auto-refractor or the doctor can use a manual phoropter. Either will do the job. The glasses Rx consist of a spherical portion, an astigmatic portion and the axis. It looks something like this: -2.00 – 1.25 x 90. The sphere (-2.00) and the astigmatic portion (-1.25) are measured in .25 increments. The axis goes from 1 to 180 degrees. There is a different Rx for each eye and they are labeled OD for the right eye and OS for the left eye.
For persons over 40 there may also be a near Rx called the add.  This is added to the distance Rx on the bottom portion of the glass lens and can be invisible or have a line. The invisible bifocals are normally progressives where the near Rx gradually or progressively gets stronger toward the bottom. The lined bifocals are one strength throughout.
The final Rx depends upon the distance between the eye and the actual glasses lens. For mild Rxs this distance is usually not enough to make a difference but for moderate and certainly strong Rxs it is. Depending upon the distance from the front plane of your eye and the back surface of the lens, the glasses Rx will change.
 The contact lens Rx is based on the spectacle Rx but is not necessarily the same. The amount of astigmatism and the strength both affect the final Rx of the contact lens. Not only is the contact lens in contact with the surface of the eye making the distance between the eye and the lens non-existent, the interaction of the fit of the lens on the eye can also affect the final Rx.
The final contact lens Rx also depends upon the fit of the lens, the steepness and diameter. Lenses that fit too tight impair oxygen to the cornea and lenses that are too loose move too much and feel irritating.
Here is an example of a contact lens Rx:
Eye      sphere   base     diameter
OD        -2.00     8.4         14.2
OS        -3.25     8.4         14.2
Here is an example of an astigmatic contact lens Rx:
Eye      sphere cylinder           axis      base     diameter
OD      -4.50    -0.75                180      8.3       14.4
OS       -3.75    -1.50                165      8.3       14.4
The contact lens Rx would also indicate the brand of the lens, the material and/or the wearing schedule (daily, weekly, monthly, etc).
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