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Contact vs Glasses Rx?

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Q:  I had an eye exam recently, and got a copy of my prescription for new glasses.   I thought I would try contacts again, but it seems my glasses Rx is not acceptable for contacts.  What’s the difference?

A:  We are asked about this almost everyday, and the differences are easy to explain.  Let’s start at the beginning.  Most offices use an instrument called an “autorefractor” to objectively measure your vision prescription.  This is a very good starting point, but rarely is the final Rx we use for a patient’s vision correction.  With other considerations like your age, your current vision Rx, your occupation and vision needs for other aspects of your lifestyle, your eye doctor will determine your sharpest and most comfortable vision correction.  This is called your “refraction”.

Our vision testing instrument sits about an inch in front of your eyes ( the part where we ask you “which is better, number one or two?”) to decide which lens powers are ideal for you.  The refraction is used to fabricate your eyewear lenses, with other anatomical dimensions for best clarity and comfort.   When a patient wants us to also prescribe contact lenses, we convert those glasses powers by altering the Rx numbers to account for the distance from your glasses lenses to the actual front surface of your eyes!  This is approximately an inch, and the conversion depends upon your vision correction…..meaning, the stronger your Rx the greater the conversion effect.

Generally, nearsighted people will have a milder vision Rx for contacts than glasses, while farsighted people need a stronger Rx in contacts than glasses.

There is one more key element for contact lenses…..the shape of the lenses.   Because glasses sit on your nose, the eye shape has little if any bearing.  But a contact lens sits right on the surface of your eye: your cornea.   So we, like many offices, use corneal topography to actually measure and map the shape of each eye.   Like a “topo” map of a mountain, but more detailed and smaller, this guides us to prescribe the inside curvature of each contact lens.

Lenses that best match the contour of your eye feel the best, and give the healthiest corneas.   Lenses that are too “tight” feel fine at first, but over time cause corneal swelling and other damage.   Lenses that too “loose” are less comfortable and can have fluctuating vision.

Ask you eye doctor about your options in contacts, have the additional tests done, and try contacts again.  Today’s lenses are successful for many people who did not do well just a few years ago.   New technologies have made contacts a great choice for most vision problems today!