What Does “Power” or “PWR” Mean in a Contact Lens Prescription?
In order for us to see, light enters the eye through the cornea, goes through the pupil, and then must focus perfectly on a single pinpoint spot on the retina in the back of the eye called the fovea.
If light does not focus on the fovea, then that eye is “uncorrected” and the point needs to be corrected by moving the point of focus back to the fovea.
Two questions are critical to correcting this focus:
- “How far?”
- “Which direction?”
The power is a number on your contacts prescription that shows how far and which direction to move the point of focus to align on the eye’s fovea.
Power in Contact Lenses
The “how far?”, or perhaps “how much?”, is the number part of the power value that you would see when reading your lens prescription.
This indicates the strength of the lens that is required to move the focal point to where it needs to be. The main type of power is referred to as “sphere power”, but there are also additional powers for certain types of contacts.
Power Direction
The power direction is determined by which way the point of focus needs to move in order to correct the vision. This is indicated by a plus or minus sign on a contacts prescription.
Contact Lens Power Measurements Explained (Video)
Contacts Power Ranges
There are several different types of measurements when it comes to power values.
Sphere Power
The most common availability of contact lens sphere powers range from about -6.00 through +6.00 in quarter diopter (0.25) steps. Higher powers are less commonly available, but are usually in half diopter (0.50) steps.
For example, if the available SPHERE power of a lens went from -8.00 though +8.00, the availability in power availability may look like this…
Uncommon High Negative Power Ranges
-8.00, -7.50, -7.00, -6.50, -6.00… (notice that these powers are in half diopter steps because they are below -6.00)
Common Power Ranges
…-5.75, -5.50, -5.25, -5.00, -4.75, -4.50, -4.25, -4.00, -3.75, -3.50, -3.25, -3.00, -2.75, -2.50, -2.25, -2.00, -1.75, -1.50, -1.25, -1.00, -0.75, -0.50, -0.25, 0.00, +0.25, +0.50, +0.75, +1.00, +1.25, +1.50, +1.75, +2.00, +2.25, +2.50, +2.75, +3.00, +3.25, +3.50, +3.75, +4.00, +4.25, +4.50, +4.75, +5.00, +5.25, +5.50, +5.75, +6.00… (notice that these powers are in quarter diopter steps because they are between -6.00 and +6.00)
Uncommon High Positive Power Ranges
…+6.50, +7.00, +7.50, +8.00 (notice that these powers are in half diopter steps because they are above +6.00)
Astigmatism Additional Power Values
Lenses that correct astigmatism also have limited ranges for both cylinder power and axis.
Cylinder Power
Cylinder powers usually range from -0.75 through -2.25 in half diopter (0.50) steps.
For example, the normal cylinder powers available are -0.75, -1.25, -1.75, and -2.25. There is greater availability of powers (especially in the CooperVision brands labeled -XR, meaning “extra range”).
Axis
Axis is the number that designates the orientation of the contact on the eye, and the typical range of availability is 0 degrees through 180 degrees (as in half of the degrees of a circle) in 10 degree steps. Therefore, the typical availability of axes are 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, and 180.
ADD Power
ADD power is the additional power for presbyopia – people usually over 40 who need a multifocal.
A contact lens ADD is written based on the wearer’s daily activities and needs, usually written as LOW, MEDIUM, or HIGH.
How Does Sphere Power for Contacts Work?
During your eye exam, your eye doctor will use specialized tools to identify where the light focuses in your eye and where that is in relation to the fovea.
We do not measure light distances in millimeters, instead we measure it in diopters. Your doctor will be able to tell how many diopters the focus point must move to be repositioned on the fovea and in which direction.
Contact Lens Power for Nearsightedness (myopia)
If light focuses imperfectly in one spot in front of the fovea, then the eye is “uncorrected” and is called nearsighted (also known as myopic).
When an eye’s focus is corrected to see far away, the elastic lens in the eye is flat and while the muscles relax. When this corrected eye focuses up close, the muscles flex and the elastic lens turns into a more rounded shape to allow for near object focusing. Then, when the eye looks back to a far distance, the muscles relax, and the lens is flat again.
To accomplish this with a contact lens, the light is adjusted backwards until it reaches the fovea. We know the distance measurement needed to bring the point onto the fovea is called the power. Since the point needs to move backward, then the power needs to be “negative” (-) in direction.
An example of a nearsighted prescription would be something like “-2.25 Diopters”.
Contact Lens Power for Farsightedness (hyperopia)
If light focuses imperfectly in one spot behind the fovea, then it is also “uncorrected” and is called farsightedness (also known as hyperopic).
To move the point back to the fovea, the focus point is adjusted forward until it reaches the fovea.
Since the point needs to move forward, then the power needs to be “positive” (+) in direction.
An example of a farsighted prescription would be something like “+2.25 Diopters”.
Why is Power Important for Contacts?
Power is an extremely important part of a contact lens prescription, while some measurements like base curve or diameter aim to maximize the fit and comfort of the lens, power is critical in ensuring that your contact lens does what it’s supposed to do – to help you see!
In fact, without the power specified, your prescription would be considered invalid.
What if My Contacts Prescription has the Wrong Power Number?
Without the correct power value and direction on your contacts prescription, the focus of light through your lens would not be positioned on the fovea. This will prevent your eyes from focusing properly either up close, far away, or both. Consult your eye care professional if you believe this to be the case or have trouble focusing.
If the power number is incorrect, it conflicts with the purpose of getting contacts in the first place, right? On top of blurry vision, this can cause heavy strain on your eyes, which may lead to headaches and further vision problems in the future.
How to Find the Correct Power for Your Prescription
As we mentioned before, your eye doctor should be the one you trust to provide you with the correct prescription information for your vision. Without the proper tools, expertise, and knowledge you will not be able to determine these measurements.
It’s also important to note that a contact lens prescription should not be expected to match the eyeglass prescription. In addition to vision correction values in a prescription, it is also determined by the needs and requests of the wearer.
An eye care professional is the best person to help guide you toward the best contact lens prescription. Quite often, a trial fit with a sample pair of contacts is necessary to guide your doctor to your best fit. Shortcuts should be avoided.
What Does Deliver Contacts Do to Ensure I Have the Correct Power?
We make sure that every prescription we receive has all of the necessary details included in the script, including the power.
Some products that we carry are limited in base curve selection, but we ensure that you have the ability to specify your base curve at the time of order. This way, you can ensure that your order with us aligns with your correct prescription.
Frequently Asked Questions (FAQs)
What does “power” on contact prescriptions mean?
The power is a number on your contacts prescription that shows how far and which direction to move the point of focus to align on the eye’s fovea.
Is the power number for contacts important?
Power is an extremely important part of a contact lens prescription, while some measurements like base curve or diameter aim to maximize the fit and comfort of the lens, power is critical in ensuring that your contact lens does what it’s supposed to do – to help you see! In fact, without the power specified, your prescription would be considered invalid.
What power ranges do contact lenses come in?
The most common availability of contact lens sphere powers range from about -6.00 through +6.00 in quarter diopter (0.25) steps. Higher powers are less commonly available, but are usually in half diopter (0.50) steps.
What if my lens power is incorrect?
Without the correct power value and direction on your contacts prescription, the focus of light through your lens would not be positioned on the fovea. This will prevent your eyes from focusing properly either up close, far away, or both. Consult your eye care professional if you believe this to be the case or have trouble focusing.
