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: 

1. “How far?” 
2. “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.

Is “Sphere” and “Power” the Same Measurement for Contacts Prescriptions?

The main type of power is referred to as “sphere power”, but there are also additional powers for certain types of contacts.

These terms are sometimes used interchangeably, especially for standard contacts that aim to correct near- or far-sightedness (also known as myopia and hyperopia). This is because sphere power is the only type of power measurement in those prescriptions. 

Other types of contacts, like toric lenses for astigmatism or multifocal lenses for presbyopia, have other measurements that go along with the sphere power. We’ll get into those in more detail later.

So no, “sphere” and “power” aren’t exactly synonymous. However, in the right context you can accurately describe the prescription of certain lenses.

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.

If the prescribed sphere power is a positive (+) number, that means you are farsighted or hyperopic, which means that the light focuses behind the fovea. In this case, the lens needs to converge the light more to bring the focal point forward onto the retina.

If the prescribed power is a negative (-) number, you are nearsighted or myopic, which means that the light focuses in front of the fovea. Here, the lens needs to diverge the light to push the focal point backward onto the retina.

Contact Lens Power Measurements Explained (Video)

Power Measurements for Contact Lenses

There are several different types of measurements when it comes to power value ranges.

These include:

• Sphere Power
• Cylinder Power
• Axis
• ADD Power

Sphere Power

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.

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. 

Imagine the sphere power of a contact lens like a scale that runs from -8.00 to +8.00. This range represents the available powers to correct either nearsightedness (negative values) or farsightedness (positive values).

Now, let’s divide this scale into three sections. The outer two sections, from -8.00 to -6.00 and from +6.00 to +8.00, are for those who have relatively high degrees of nearsightedness or farsightedness. Powers in these ranges are available in half-diopter steps, like -8.00, -7.50, -7.00, and so on, or +6.50, +7.00, +7.50, up to +8.00.

In contrast, the central section of the scale, ranging from -6.00 to +6.00, is for those with more common degrees of nearsightedness or farsightedness. Powers in this range come in smaller, quarter-diopter steps, like -5.75, -5.50, -5.25, down to 0.00, and then up to +5.75, +6.00.

So, depending on your specific needs, there’s a wide range of power options available to ensure your contact lenses provide you with the best vision possible. The maximum sphere power available for contact lenses is typically +8.00 for farsightedness and -8.00 for nearsightedness.

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 contact lens.

A contact lens ADD is written based on the wearer’s daily activities and needs, usually written as one of three options:

• LOW
• MEDIUM
• HIGH

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 have a limited selection of lens power options, but we ensure that you have the ability to specify the power measurement that you were prescribed 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.

What is the maximum power contacts come in?

Typically the maximum positive (highest hyperopic) prescription power is +8.00 diopters.

What is the minimum power contacts come in?

Typically the minimum negative (highest myopic) prescription power is -8.00 diopters.