What is the normal BC for contact lenses?
Contact lenses do their work by bending light in just the right way to compensate for people's vision defects. This requires the shape of the lenses to be carefully crafted to get the desired effect, like feng shui for your eyes. This is the base curve (BC). So, what is BC in contact lenses? It's a shorthand for the measurement of the interior curvature of the lenses. For obvious reasons, this makes a difference in the overall fit and function of the contacts, so it definitely needs looking into.
What is the normal base curve for contact lenses?
The base curve describes the geometry of contact lenses on the interior, or eyeball-facing, part of the lens. This is the surface that's in contact with the sclera, which is the delicate surface of the eye. BC in contact lenses matters because it affects how the lenses fit the wearer, and if you think ill-fitting shoes pinch, try it in contact lenses.
What BC means in contact lenses is usually how much the lens curves, measured in millimeters. Standard soft contact lenses run between 8-9 mm, while rigid gas-permeable lenses have a somewhat broader range of 7-9 mm. As a rule, a BC of 8.4-8.6 mm generally fits nearly everyone, up to 98% of contact lens wearers. The larger and smaller base curves are more of a niche product for people with pronounced irregularities in the shape of their eyes, which are not coincidentally associated with more severe vision problems.
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Factors influencing base curve selection
Eye care professionals decide on a BC after taking measurements of the eye. They're looking at a person's eye shape, their corneal curvature, and whether they have any special aberrations that could affect the fit of the lens. Ideally, they do this once for each eye because a person's eyes are not necessarily the same.
Getting this right is really important. The BC is measured in tenths of a millimeter, and even a slight irregularity in the fit can make for an uncomfortable 16 hours wearing the contacts. Ill-fitting lenses can also under- or over-correct the light passing through them, which affects vision quality, and at that point, the contacts are really just counterproductive.
Understanding BC in contact lenses
If it helps, think about the normal BC for contact lenses as being kind of like the curves on a country road. If the engineers have done their math right, the drive is safe, smooth, and fun. If the curves are too sharp, drivers have to slow down, or their Porsche will suddenly become an ATV. Even if the curves are too gentle, the road might not be laid out the right way, and good luck getting uphill without adding an hour to the drive.
Likewise, contacts can't have too much or too little curvature. Even a minor deviation from the prescription could result in lenses falling out or slipping, messing up the image quality, or just being uncomfortable to wear. To extend the already strained road metaphor, having the wrong BC can also restrict oxygen flow to the eyes, which is like ... uh, let's go with stuffing a sock in your tailpipe. That's close enough.
Measuring base curve in clinical settings
People usually get their base curve worked out during an initial exam by their eye care professional. Because the eyes never really settle down and stop changing shape, they should be measured again every so often, plus a special check if the lenses are causing discomfort or the image is blurry. This is most often done using a tool called a keratometer, which is Greek for "eyeball-curve-measuring thingy."
Adjusting base curve for a better fit
If an eye care professional decides the current BC isn't the right BC, it's back to the drawing board for an adjustment. This isn't often a big deal because it really just involves a tweak to an already existing prescription — and possibly a change in brand. Once the adjustment is made, people can go back to ordering contact lenses online and stay on top of their supply.
What is BC in contact lenses, and why it matters
Getting the proper fit is crucial to getting the most out of contact lenses and to basic comfort. Contact lenses are a lot like underwear that way. If you wear contacts, be sure to get your BC checked and regularly updated. Also, you should probably use some of these cool online coupons and promo codes to save on your next adjustment or fitting.
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FAQ
How do I know if my BC is wrong?
You should get your base curve reassessed during your annual eye exam. Get checked sooner if you notice any symptoms of a bad fit.
Can BC vary between brands of contacts?
BC can vary between brands. Make sure you're getting the set your eye care professional recommends.
Why would a professional change a BC?
A person's BC has to exactly match their eyes, or the lenses might not work as well anymore. This is why eye care professionals periodically check to make sure a current BC still works. Plus, it's fun to say "keratometer."