Believe it or not there is a difference between a spectacle and contact lens prescription. When a lens is placed on the eye it will most likely need to have its power adjusted. This is more evident in moderate to higher prescriptions. In addition the actual fitting parameters need to be documented. Lets take a look at a sample prescription:
O.D. -4.00 8.3 14.4 Brand X
O.S. -3.50 8.3 14.4 Brand X
Letters and Numbers
The first thing we encounter are the initials O.D. and O.S. these are the Latin initials for right eye (Ocular Dexter) and left eye (Ocular Sinister). You may also likely see R right or RE for right eye and L for left and LE for left eye.
Next we see -4.00 and -3.50. These tell us the power of a lens. Some people with slightly higher prescriptions may notice that there is a power variance between their spectacle prescription and contact prescription. This is to compensate for something called vertex distance. The closer or further a lens is moved the more the power will change. In the case of an individual with a plus or magnifying lens the closer the lens is moved toward the eye weaker it will become. This individual may require a little more power for their contact lens prescription. In the case of a person who wears a minus of minifying lens the closer the lens is to the eye the stronger it will become so here the wearer would need a little less power.
A contact lens wearer with an astigmatism may also notice that their prescription power may differ from their spectacle prescription. Often in the case of small amounts of astigmatism the doctor prescribes a spherical equivalent lens. This is where one-half of the astigmatism correction is added to the spherical power. Example:
-1.00 -.50 x 180 becomes -1.25
In the case of a rigid lens spherical equivalent may not be used. This is due to the lenses ability to retain tears under the lens that will correct for the astigmatism. Soft disposable lenses do not have this feature. They tend to drape over the cornea conforming to its shape. This brings us to the next set of numbers.
After the lenses power we come to the base curve. This number is the measurement of the radius of curvature in millimeters of the inside curve. The doctor does his best to try and match the lens curvature to the corneas. As mentioned earlier a soft lens tends to drape the cornea. Because of their flexible nature soft lenses tend to fit more corneas with the a smaller number of base curves. Often a lens manufacturer will only make a soft lens in a few curves. Rigid lenses on the other hand need to have a more accurate fit. Because of their rigid nature a lens that is too steep or too flat may cause discomfort.
After the base curve we come to the lenses diameter in millimeters. Like the base curves soft lenses come in a limited amount of diameters. This is due to their rather forgiving nature when it comes to fitting. Again like base curves the doctor will be more precise in specifying the diameter of a rigid lens. If looked at from a side view we see that the cornea projects further than the rest of the eye. The area were the cornea meets the sclera (the white part) is called the limbus. As the eye blinks and rotates the contact lens will move. The soft lens well cover the limbus. The lenses movement generally will not cause irritation. With a rigid lens it should be small enough to allow lens movement with out running into the limbus causing irritation.
Contact Lens Brands
And last we have the brand. The doctor chooses which lens is best for you based on several factors. Some of these being tear quality and quantity, intended wearing schedule, work and hobby conditions and requirements. The lens that has been chosen by your doctor meets your specific requirements. There fore its strongly advisable that you consult your prescribing doctor before attempting to switch brands.