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Contact Lens Rx Basics

Base Curves

Base CurveThis tells how much curve is on the back suface of the lens.

For soft lenses, those numbers are in millimeters of radius (8.4mm or 8.7mm) and they can be divided into tenths. With millimeters of radius, the smaller the number, the more curved or steeper the lens will be. So, an 8.4mm lens will be more curved or steeper than an 8.7mm lens.

For rigid gas permeable lenses, the base curve may be in millimeters of radius or diopters (43.00D or 45.50D). These can be divided into eighths or a diopter. With diopters, the larger the number, themore cuved or steeper the back surface of the lens. 45.50D is steeper, or more curved, than 43.00D.

Lens Power

This shows how much and what type of correction is found in the lens. If there is only one number for power, it indicates the sphere power of the lens. These will be minus numbers for those patients who are nearsighted or myopic and plus numbers for those patients who are farsighted or hyperopic.

Toric contact lenses correct astigmatism and they will have a cylinder power listed as well as an axis. This means that there are two major meridians in the lens. The power will gradually change from one power to the other every 90 degrees.

For patients wearing bifocal or multifocal contact lenses, they may have a reading correction listed as well as a distance correction. This value will always be a plus number.


This value indicates the width of the overall lens. Soft lenses are much larger than gas permeable lenses since they fit over the entire cornea and rest on the sclera or the white part of the eye. Diameters for soft lenses may range from 13.5mm to 15.0mm. Gas permeable lenses sit on top of the cornea and will be smaller than soft lenses, often 8.5mm to 10.0mm.


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Oculista Stamford CT

Oculista Danbury CT


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