Eye Care Fun

Why is the Contact Lens Prescription Different than the Glasses Prescription? Aren't they the Same?

If you look at your prescription, you may notice that both prescriptions do not have the same information. Basically, the prescriptions differ because contact lenses sit on the eyes, whereas the glasses usually sit about 12mm away from the eyes. Thus the effective power is different and the doctor needs to adjust that power according to the distance from the eye. In addition, the doctor needs to measure the curvature of the cornea for contact lenses to determine the curvature and the size of the lens to dispense to patient. It must fit comfortably to the eye. You will find that a contact lens examination entails more work because the doctor needs to measure the cornea, put the contact lens on, check the vision with the contact lenses, and check the fit of the lens. It would be difficult for a doctor to come up with a contact lens prescription without doing all of the above.

In addition contact lenses, unlike glasses, are classified by the FDA as a medical device and are regulated differently. As contact lenses touch the surface of the eye there are risks of ulcers, infections and other complications.

Glasses Prescriptions

If you look at your glasses prescription it will look similar to this:

R and L on the left refer to the right and left eye.

Sph is the power you need in order to see clearly. A '+' is for far sighted and magnifies and a '-' is for near sighted and will reduce an object. This is the main correction.

Cyl shows the power you need to correct astigmatism. This is the fine tuning of the lens.

Axis tells the manufacturer of the lens where the power to correct the astigmatism should be placed. It refers to the eye on a 360 degree axis.

Add is the power of the reading correction. This is additional information to the Sph box as this power needs to be added to the Sph to get the actual power.

Prism/Base. This box is only used for people who are seeing double and have eye teaming problems.

This covers the actual prescription for eyeglass correction and is used the world over. The lower tick boxes are for additional and superficial instructions when having the eyeglasses made. The patient usually opts for these at an additional cost.

Anti-reflective Coating helps cut off glare from the sun and bright lights.

Polycarbonate is a very strong substance which is also ultra-light. Glasses are made of this for those who are concerned about the weight of the glasses, particularly if the lenses are thick and heavy.

High Index Plastic can be used to create thinner lens which again are lighter than traditional materials.

Transitions/Sunglasses means the lens will react to sunlight and darken in response. The lens interacts with direct UV interaction and is useful for those living in sunny areas who move in and outdoors frequently.

Single Vision is one vision.

Bifocal refers to a lens which is split; usually the lower half moon is for reading and the upper part of the lens is for long distance viewing.

No-Line Progressive relates to a bifocal lens. The two different parts of the lens may be split along a definite line or may be graduated i.e. no-line progressive.

Readers/Computer refers to glasses which may not be worn permanently or outdoors. They need to be sturdier for being frequently taken on and off, but are not likely to be worn outdoors or for sport.

Contact Lens Prescriptions

The information on a contact lens prescription is very different to an eyeglass prescription. With contact lenses the doctor has to specify the curve and the overall size of the lens. A contact lens Prescription looks like this:

Once again R and L refers to each eye separately.

Lens Name refers to a brand name of the contact lenses.

BC refers to the curvature of the lens. It can be loose or tight.

Dia refers to the size or diameter of the contact lens.

Sph is the power you need in order to see clearly. A '+' is for far sighted and a '-' for near sighted. This is the main correction.

Cyl shows the power you need to correct astigmatism. This is the fine tuning of the lens.

Axis tells the manufacturer of the lens where the power to correct the astigmatism should be placed. It refers to the eye on a 360 degree axis.

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. On a contact lens prescription you will see measurements for the curvature of the cornea and the diameter of the cornea.