Contact lens check-up: What is a contact lens exam and fitting?
Eye tests for contact lenses include special tests and measurements that are not included in routine eye tests for glasses.
So if you are interested in contacts — or you already wear them and want to have your contact lens prescription updated — make sure you say so when you schedule your appointment for an eye test.
This will ensure your test includes extra time for your optician to perform everything required for a proper contact lens fitting or prescription update.
What to expect during a contact lens test
During your eye exam for contact lenses, your visual acuity will be tested using an eye chart, and a number of tests will be performed to determine your eye health and whether prescription eyewear is required to correct refractive errors.
Your optician will gather additional information so you can be fitted with contact lenses, or the fit of your current contact lenses can be evaluated.
You may be asked general questions about your lifestyle and preferences regarding contact lenses, such as whether you might want to change your eye colour with coloured contact lenses or if you're interested in options such as daily disposables or overnight wear.
Your optician may also discuss the option of rigid gas permeable (RGP or GP) contact lenses, which often provide sharper vision than soft lenses.
Your optician might also ask how you want to correct vision problems related to aging. After age 40, it’s normal to experience a condition known as presbyopia that decreases your ability to read small print and focus on near objects.
To correct presbyopia, your optician may offer you the choice of multifocal or bifocal contact lenses. Another option is monovision, which is a special contact lens fitting technique where one eye is corrected for distance vision and the other eye is corrected for near vision.
Measurements taken during a contact lens test
Just as one shoe size doesn't fit all, one contact lens size doesn't fit all.
If the curvature of a contact lens is too flat or too steep for your eye's shape, you could experience discomfort or even damage to your eye.
The following tests are performed during an eye exam for contact lenses to make sure you see clearly, comfortably and safely with contacts.
An instrument called a keratometer is used to measure the curvature of your eye's clear front surface (cornea).
A keratometer analyses light reflections from your cornea and determines the curvature of your eye's surface. These measurements help your optician choose the proper curve and size for your contact lenses.
Because the keratometer measures only a small, limited section of the cornea, additional computerised measurements of your cornea may be performed using an automated instrument called a corneal topographer.
Corneal topography provides extremely precise details about surface characteristics of the entire cornea. It does this by measuring how your eye reflects light.
Sometimes, corneal topography measurements are combined with wavefront measurements that provide even more specific information about the shape of your cornea by identifying tiny imperfections called higher-order aberrations. These combined measurements can help your optician determine the type of contact lenses that will give you the sharpest vision possible.
If your eye's surface is found to be somewhat irregular because of astigmatism, you may need a special design of lens known as a toric contact lens or be fitted with GP contact lenses.
Pupil and iris measurements
The size of your eye's pupil may also be measured during your eye test for contact lenses.
To obtain this measurement, your optician may simply hold a special ruler up to your eyes and compare the size of your pupils to illustrations of different pupil sizes on the ruler.
Automated instruments that measure pupil size also exist. These instruments are capable of extremely precise measurements, and some simultaneously measure the horizontal and vertical diameter of your pupil.
Similar techniques might be used to measure the diameter of the coloured portion of your eye (iris).
Pupil and iris measurements help your optician choose contact lenses that are of a proper size to fit well and look best on your eyes — especially if you are interested in coloured contact lenses.
Tear film evaluation
Contact lens fittings may also include a tear film evaluation.
Your optician will use one or more techniques to make sure your eyes produce enough tears for comfortable and safe contact lens wear, and that you don’t have unusually dry eyes.
If you have a significant dry eye condition, you might have to avoid or discontinue contact lens wear. But in cases of contact lens discomfort due to mild dryness, special contact lenses for dry eyes may enable you to wear contacts safely and comfortably.
Evaluation of your eye's surface and contact lens fit
The health of your cornea will be evaluated using a biomicroscope (also called a slit lamp). This instrument provides a highly magnified view of the cornea and other structures in the front of the eye so your optician can make sure you are a good candidate for contact lens wear or (if you already wear contacts) that contact lens wear isn’t harming your eyes.
The slit lamp is also used to evaluate the fit of a trial contact lens, because it enables your optician to observe the alignment and movement of the lens as it rests on the surface of your eye.
When trial lenses are used, you will typically need to wear them for a few minutes so that initial tearing of the eye stops and the lenses stabilise. Your optician can then make a proper evaluation of how the lenses fit without the presence of watery eyes.
In follow-up visits, your optician might apply a special dye to the surface of your eyes to make sure your contact lenses aren’t irritating your corneas or causing dry eyes.
After your initial contact lens fitting, you will be asked to return for one or more follow-up visits so your optician can confirm that the lenses are continuing to fit well and that you are seeing clearly, comfortably and safely with them. Once this is confirmed (and only after it is confirmed), your optician will then finalise and sign your contact lens prescription.
Your prescription for contact lenses will include a number of designations that are not found on a glasses prescription, including:
The brand of the contact lenses
The curvature of the optical zone of the lenses (called the base curve, or BC)
The diameter of the lenses
An expiration date for purchasing replacement lenses
Your optician might also specify wearing instructions on your contact lens prescription, such as, “Not to be worn overnight; replace every two weeks.”
READY TO GET CONTACT LENSES? Find an optician or optical store near you.
Page published on Thursday, 27 June 2019