Understanding Your Glasses or Contact Lens Prescription

In order to determine which type of refractive surgery, if any, that you might be a candidate for, it is imperative that you have your glasses or contact lens prescription.  You should easily be able to get your prescription by a simple phone call to your eye care practitioner’s office if you don’t have it. Just advise your doctor’s office personnel that you would like for them to read it to you over the phone and you can transcribe it.  If you wear contact lenses, the prescription is usually written on the container in which they are dispensed. 

As a refractive surgeon, I’ve noticed that when I (occasionally) ask a patient if they know their glasses or contact lens prescription, they often try to recall their uncorrected vision on the eye chart!  The answer is frequently, “Uh, I think it was 20/200 or something like that”.  This is only minimally helpful, if at all. You must realize that refractive errors are given in diopters (D) of power, not in terms of vision.  There is only a rough correlation between the two.  Let’s take a very brief look at how the refractive error is determined. 

If an eye care practitioner is seeing a patient for the first time, vision is usually checked without any correction and this is noted in the chart.  Next, a refraction is performed.  A refraction is the process of determining one’s glasses (or contact lens) prescription, which is typically completed as the patient looks through a device known as the phoropter.  The phoropter has an array of lenses that will correct just about any type or degree of refractive error.  But it is the practitioner’s job to complete this process accurately.  How does he or he accomplish this?  Well, first the patient views the chart with both eyes open while the practitioner views the retinal reflex.  The retinal reflex is the red reflection of light that bounces off the retina when light is shone directly into the eyes.  This is seen in the “red eye” that occurs with direct flash photography as well as from a cat’s eyes when viewed with your car lights at night.  Next, the practitioner moves his retinoscope up and down and side to side and the patient’s red reflex returns to the his eye based on the patient’s refractive error.  This red reflex must be “neutralized” by placing the correct lens or lenses in front of the observed eye.  This correction may contain spherical lenses for correcting either myopia or hyperopia and a cylinder lens at the correct axis for correcting any existing astigmatism.  The sphere and cylinder lenses are combined to provide the proper prescription.  The number of prescription possibilities is infinitely large.  

Here’s how the prescription will probably read: 

Sphere (D) + or – Cylinder (D) x Axis (in degrees)   

The prescription is written for the right eye as “OD” which stands for oculus dexter and “OS” for oculus sinister.  As such, an actual prescription without any astigmatic correction would look something like this: 

OD:  -4.00

OS:  -3.75

If this were the case, the patient has four diopters of myopia (the number is negative) in the right eye and three and three/quarters diopters of myopia in the left eye.  If the numbers were positive, the patient would have hyperopia rather than myopia.  Let’s take another example that includes astigmatism: 

OD:  -5.00 +2.00 x 90

OS:  -6.00 +2.00 x 95 

In this case, the patient’s right eye has five diopters of myopia and two diopters of astigmatism in the 90 degree axis.  The patient’s left eye has six diopters of myopia and two diopters of astigmatism in the 95 degree axis.   

If you need a bifocal correction because you are in the presbyopic age range (usually forty or above), the prescription will have an “add” written at the bottom to indicate the bifocal power.  It will look something like this: 

Add:  +1.75 (D) 

This indicates that you need +1.75 diopters of near focusing power, essentially magnification, to help you see clearly up close. 

If you require a trifocal correction for presbyopia, your prescription might read: 

Add:  +1.50/+3.00 (D) 

This indicates that you need +1.50 diopters of near focusing power (magnification) at about arm’s length and +3.00 diopters of magnification at near (usually about 16 inches).   

How “Strong” is Your Prescription? 

Using the above referenced formula, you should be able to determine (in most cases) whether you’re myopic (nearsighted), hyperopic (farsighted), have astigmatism, and/or are presbyopic (need a near focusing “add” power.  However, you’ll also need to understand how “strong” the prescription is because this often dictates the refractive surgical options that are available.  To understand the strength of a prescription, it would be helpful to review the meaning of the term “diopter”. 

A diopter is a term used in optics that is a unit of measurement for the focal length of a lens.  A good analogy is that a “foot” or “meter” are both units of measurement when defining the length of an object.  A diopter is defined by the following formula: 

D = 1/m 

In this formula, “D” represents diopters of power and “m” represents the focal length of the lens in meters.  For those of you who are just about to stop reading because you abhor math or physics, please… wait!  I’ll simplify! 

A lens that can bend (refract) parallel rays of light to a focal point at exactly one meter is said to have a power of one diopter.  A two diopter power lens can refract parallel rays of light to a focal point 0.5 meters away from itself and a 10 diopter power lens has a focal point 0.1 meter away (10 centimeters) from itself.   

If you are nearsighted and you have a –1.00 diopter prescription, this means that the farthest you can clearly focus in front of your eye is one meter (about three feet).  If you have –2.00 diopters of myopia, the farthest you can clearly focus in front of that eye is 0.5 meters (about 1.5 feet).  Continuing, if you have –10.00 diopters of myopia, the farthest you can clearly focus in front of that eye is 0.1 meters (10 centimeters or about four inches)! 

If you will recall, I mentioned that eye chart acuity only roughly correlates to the glasses prescription?  To give a rough idea, if you only have –1.0 diopter of myopia, you probably have vision in the range of 20/30 to 20/80 on the eye chart, which means that you could function quite well, perhaps even drive safely without glasses.  If you have –2.0 diopters of myopia, you might have visual acuity of about 20/80 to 20/200 or so.  This level of myopia would generally preclude safe driving without correction.  For those who are unfortunate enough to have –10.0 diopters of myopia, nothing on the Snellen eye chart (at 20 feet or simulated to be so) can be distinguished.  As such, the patient is usually asked to “count fingers”.  The vision for a patient with this level of correction might be recorded as “CF (counting fingers) at 3 feet”.  For this individual, just ambulating without correction might be dangerous!  

If you are hyperopic (farsighted) and your prescription is about +1.00 or less, your vision may be only minimally affected although this is age dependent.  The younger an individual with hyperopia, the more easily he or she can accommodate “through” the hyperopia.  That is, accommodation, or focusing of the lens of the eye, can actually improve the vision of the hyperope at distance.  This ability to accommodate declines with age and the individual who is about 40 to 50 years and above has minimal ability to accommodate through the hyperopia.

If you reviewed our refractive errors page, you will understand that, in the hyperopic eye, parallel rays of light coming into the eye are focused at a theoretical point behind the eye.  With accommodation, the natural lens of the eye fattens from front to back and literally “pulls” the focal point toward the front of the eye closer to the retina.

Returning to some examples of hyperopic refractive error, if your prescription is +2.00 or above, it is likely that you are moderately to even severely affected in terms of uncorrected vision and glasses or contact lenses are required most of the time (again, this is age dependent).  If your hyperopic prescription is +3.00 or above, you likely will require correction no matter what your age.  Notice that I’ve not attempted to correlate the hyperopic correction to any level of Snellen eye chart visual acuity because this is fraught with extreme potential error due to highly variable abilities to accommodate.

Finally, let me give a rough guideline regarding mild, moderate, and severe levels of myopia and hyperopia.  Please refer to the table below:   

Refractive Error Type and Degree	Myopia	Hyperopia
Mild	-0.25 to –3.00	+0.25 to +1.00
Moderate	-3.25 to –5.75	+1.25 to +2.75
Severe	-6.00 and above/font>	++3.00 and above