There's more to contact lenses than their prescription strength. Though it's hard to tell because they're so small, lenses have different shapes and contours for different eyes. Just as it's important to get the right prescription, it's important to purchase lenses that sit correctly and comfortably on your eyes. Lenses that fit poorly are uncomfortable and can do your eyes more harm than good.

Get Measured

Obtain your contacts through an appointment with a certified optometrist or ophthalmologist. This improves your chances of obtaining the correct prescription and the correct size of contact lenses, as opposed to using a pair from a friend or obtaining lenses without a prescription through an overseas dealer. Contact lenses, just like shoes, gloves, belts and hats, come in different sizes. The fitting should include measurement with computerized instruments or a keratometer, which measures the curvature of the cornea. Your fitting may also include an evaluation of how your eyes form tears; eyes that are too dry may not be fit for contacts. Ill-fitting contacts cause discomfort and irritation. They can also damage the eyes, according to Dr. Burt Dubow, a Minnesota optometrist and board member of All About Vision. They may cause corneal ulcers, whose symptoms include pain, redness, eyelid spasms and discharge from the eye, according to VisionRx.

Proper Insertion

Your contacts may feel as if they don't fit properly if you insert them incorrectly. They may slide out of place or cause irritation. The most common mistake is inserting a lens that is inside out. The lens does not sit properly on the cornea, and the ridges rub against the inside of the eyelid when you blink. If you're unsure whether your lens is right-side out, hold it on your fingertip by its bottom curve. A right-side-out lens has a smooth and consistent curvature, like the letter C. An inside-out lens looks like a hat with a brim; the edges stick out. Proper insertion entails washing your hands, then placing the lens in your palm. Apply a few drops of saline or cleaning solution, pick the lens up with your index finger, open your eye as wide as possible and bring the lens to your eye slowly until it catches.

Cleaning and Maintenance

Dirty or worn-out lenses may not sit properly on the eyes. Before you insert your lenses, rinse them with solution to clean them of dust specks, eyelashes and other irritants. Take your contacts out when you sleep, and keep them overnight in airtight cases with storage solution. Replace your lenses as your prescription indicates. Disposable lenses become worn; worn lenses dry out and slide around the eye when you blink. If you have allergies or an illness that causes eye irritation (such as conjunctivitis), it is best to leave your contacts out until the symptoms clear. The sticky by-products of this irritation can cause your lenses to become dry, slide out of place or get stuck atop your eyes.

Source:

All About Vision: Q&A On Contact Lenses

VisionRx: Corneal Ulcer

Contactlenses.co.uk: The Fitting Process For Contact Lenses