Diameter in Contacts Prescription

Diameter in  contact lens prescriptions refers to the overall size of the contact lens.  The larger the diameter the larger the contact lens.  The contacts diameter is measured in milimeters.  A typical size for an average contact lens diameter might be 14.2 milimeters.  While this size is calculated by the eye doctor during the exam it's also normal for many patients to require contacts of a similar diameter and in fact many popular cosmetic contacts only come in a single diameter.  It's sort of the one size fits all approach and if you don't fit in that size then you don't order those contacts.

It's also possible, not necessarily recommended, for the same person to wear contacts of different diameters.  This is especially evident with Circle Lenses contacts which are normally very large in diameter so that they cover more of the iris and provide the illusion that the wearer has larger colored eye balls.  Many circle lenses are available in only one size diameter.  Note that they eye doctor should provide with the diameter that is most likely to be the best fit and as such the most comfortable contact lens.

Doctor's Exam and valid Contact Lens Prescription and why it's important

Everyone should have their eyes checked by a board certified eye care provider before considering wearing contact lenses.  While the frequency that an adult needs to return to their eye doctor to keep their prescription from becoming expired is debated on this site we don't discount the benefit and need to have an initial eye exam and receive that first prescription.  When and under what circumstances a person is required to return for another checkup before purchasing contact lenses online is becoming more subjective and many people are exploring other options to buy contact lenses online without a current prescription.

What happens during that first and subsequent contact lenses examination?  Quite a bit happens actually.  In addition to the doctor determining the contact lenses power of compensation required of each eye to best correct the patient's vision they also test the patient for glaucoma and other diseases.  Along the way the Doctor determines prescribes two critical variables... base curve and diameter.

Diameter is pretty much like it sounds, it's the size of the overall lens.  Typically the diameter of contact lenses ranges from the 13.5 to 14.5 mm.  Note that many contact lenses only come in a fixed diameter so the eye doctor is telling you which diameter to select for a particular lens based on your eye size but they may tell you what lenses would be a good fit and what might not be.  This is where the doctor has a role in selecting or recommending the contact lenses that may fit you the best.  Note that the primary difference between 'normal' contacts and circle lenses contacts is that circle lenses are intentionally made to have a larger than normal diameter so that the color of the lenses covers your whole iris and parts of the sclera giving circle lens wearers the appearance of larger eyes.

The other important bit of info you will get from your prescription and doctor's eye exam is what your base curve is.  As we are all humans and our eyes are all somewhat similar in shape many people have a similar base curve, the most popular being 8.6.  The base curve is again what it sounds like, it's a measurement of the shape of your eye ball.  For people that have atypical base curves a standard 8.6 contact lens may not 'fit' their eye comfortably and could result in irritation and other problems.   Fit, hence contact lenses 'fitting'.  The Doctor is literally measuring your eyes and then trying on a pair of lenses much like you would try on a pair of shoes.  Shoes that are too small hurt your feet and shoes that are too big can give you blisters.

So if diameter depends on the lens itself and most people have the same base curve and many color contact lenses are only made with a single base curve and diameter why do you need a prescription to wear plano or non-corrective contact lenses?  Because the establishment says you should.  Or because it's good sense.

Base curve in contact lenses prescription

Base curve refers to the general shape of a contact lens.  A contact lens fits over a person's cornea and needs to fit snugly and comfortably.  Of course all people potentially have a slightly different sized cornea hence the need for a contact lens prescription which is the end result of a contact lens fitting performed by a certified optician.  A person's cornea should of course not change size so if you have already been fitted and know your base curve and diameter and feel your vision has not changed in many years you may question the need to get a new examination and new contact lens prescription each and every year in order to purchase contact lenses on-line.  For most people the regular checkups give them peace of mind that their prescription has not in fact changed.  Additionally the optician will be checking the overall health of the patient's eyes including test for glaucoma and other degenerative eye conditions.  You should not buy contact lenses without a doctor prescription unless it's recently expired and you are confident your eyes are healthy and your vision has not decreased.  For some this may seem risky but consider that young adults that have yet to need eyeglasses or contact lenses are not required by law to have their eye sight checked by a doctor or a machine in order to retain a drivers license.

This page is in process and will contain links to mulliple reputable sources for more complete definitions of base curve as it relates to prescription contact lenses.   Visit this wikipedia link for more information pertaining to contact lenses and base curve.

http://en.wikipedia.org/wiki/Base_curve