Spectacle vs. Contact Lens RX and Prescription Terminology
Glasses sit about 12 millimeters from the eye, while contacts sit directly on the eye surface. These 12 millimeters make a world of difference and can dramatically change the prescriptions between the two. Also, contact lens prescriptions require more specifications than glasses.
What is the difference between contact rx and glasses rx?
Your Two Different Prescriptions
However, contacts and glasses prescriptions are actually slightly different because of their distance from your eyes. Eyeglass lenses are placed approximately 12 millimeters away from your eyes, while contact lenses sit directly on your corneas.
How do I write a prescription for contact lenses?
A written prescription usually includes the following specifications: base curve, diameter, power (or sphere), and then additional figures for Cylinder and Axis if you have astigmatism, and Addition and Dominant figures for presbyopia correction.
What are the different types of eyeglass lenses?
- Concave lenses. These are thinnest in the center. ...
- Convex lenses. These lenses are thickest in the center, like a magnifying glass. ...
- Cylindrical lenses. These curve more in one direction than in the other.
What does +1 mean on contact lenses?
A positive number, for example, PWR/SPH +2.00, indicates hyperopia (long-sightedness). The figure in front of 'Addition' (ADD) determines the correction you need for presbyopia to obtain near-clear vision.
Glasses Prescription explained
A prescription is normally written out by an optometrist like this:
R. -1.00/ -1.25 * 175
The first figure that you will see on an optometrists prescription is the SPHERE (abbreviated Sph). This is the main power of the prescription and represents myopia (shortsighted) when prefixed with a minus sign i.e -1.00, and hypermetropia (longsighted) when prefixed with a plus sign i.e +1.00. Often the Sphere power has a DS after it like this: +3.00DS. The "DS" signifies Dioptres Sphere, which is the unit of measurement for spectacle power.
The next figure you see is the CYLINDER power (abbreviated cyl). This represents the astigmatism in your prescription. Often this figure is shown like this -1.25cyl. Astigmatism is always along a certain angle (axis) from 0 to 180 degrees. i.e -1.00 * 175. Finally the sphere and cylinder power are usually separated by a slash
ie sphere/cylinder * axis The R. signifies the right eye. (L. for the left)
A prescription for glasses forms the basis for a contact lens fitting. If you look at your prescription it will be in a format similar to one of these;
 | R. -3.00DS |
| R. +3.00DS |
R -2.00/-1.00 * 35 | |
R. +2.50/ -1.00 * 180 | |
| R +2.00/ -1.50 * 170
Interchangeable? A prescription for glasses is different than a prescription for contact lenses although they can be similar. A contact lens comes in direct contact with the cornea, which is the clear covering over the front surface of the eye. For this reason, there are additional measurements needed for fitting a contact lens and these need to be included in the prescription. Base Curve (BC) The Same Only Different A contact lens prescription can be the same as a prescription for glasses when:  The Eyeglass Rule requires that optometrists and ophthalmologists provide patients a copy of their prescription after the completion of an eye examination without extra cost. Reference:
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