The Spherical Equivalent of Your Prescription

One of the major pains of the last few years has been the lack of any colored prescription contact lenses for people who have astigmatism. When FreshLook Colorblends Toric was discontinued by Alcon, it created a big hole in the market for all those who want to wear color contact lenses but don't have perfectly spherical prescriptions.

In this article, I will discuss how it is possible for your eye doctor to use something called the spherical equivalent of your prescription so that even if you have a small amount of astigmatism, you might still be able to order prescription colored contact lenses.

What is Spherical Equivalent?

When you receive a prescription for glasses, you can have numbers in any of the following 3 categories: 'Sphere', 'Cylinder', and 'Axis'.

When there are only numbers in the 'Sphere' category, there is no astigmatism. For contact lens wearers, this gives you the most options. You will have no problems getting prescription colored contacts.

When you have numbers in all 3 categories — 'Sphere', 'Cylinder', and 'Axis' — it means that you have some level of astigmatism. This will often require you to order contact lenses for astigmatism, but unfortunately for color contact lens wearers, color contact lenses for astigmatism have become very difficult to find.

That's where the spherical equivalent comes in. Calculating the spherical equivalent of your prescription condenses your prescription for astigmatism, which has 'Sphere', 'Cylinder' and 'Axis', into a single 'Sphere' number. Doing this can allow some people with astigmatism to order brands of contact lenses that they otherwise would not be able to.

The trade off however, is that vision through the spherical equivalent prescription will not be as clear as the prescription for astigmatism. Keep that in mind when you and your eye doctor are deciding if this is the right option for you. The higher the 'Cylinder' number is in your original prescription, the worse the vision will be through the spherical equivalent prescription.

How The Spherical Equivalent is Calculated

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This is a simplified overview of how the spherical equivalent is calculated.

Note that only your eye doctor or a prescribing optician is qualified to prescribe you contact lenses.

The axis is removed

• The axis is not part of the calculation for spherical equivalent at all. It completely disappears.

The cylinder is divided by 2

• The Cylinder power is power in your lens that is only present in one direction. In other words, it is only present in half the lens. When attempting to combine the cylinder with the Sphere, your doctor must take this into account by only taking half of the Cylinder.
• Because Cylinder powers only come in steps of −0.25, it is possible that when it is divided by 2, that the number does not end in a multiple of −0.25. That is where your eye doctor's professional judgement comes into play to decide which closest multiple of −0.25 will be the most appropriate for you based on many different considerations.

The sphere and the ½ cylinder are combined

• The Sphere and the Cylinder are then added together to give you the 'Equivalent Sphere'.

The above process will give your eye doctor a theoretical starting point for you to try contact lenses with. This does not always produce the best results, hence your eye doctor will measure your vision with the Spherical Equivalent that he/she calculated, and make additional adjustments as needed.

Examples

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Below is a demonstration of what calculating the Spherical Equivalent does. It takes prescriptions with 'Sphere', 'Cylinder' and 'Axis' numbers and produces a single number that is the closest to the combination of all 3 numbers.

Example 1: Glasses Prescription:

	Sphere	Cylinder	Axis
Right	−2.00	−0.50	180
Left	−2.50	−0.50	170

Theoretical Spherical Equivalent:

	Sphere
Right	−2.25
Left	−2.75

Example 2: Glasses Prescription:

	Sphere	Cylinder	Axis
Right	−3.50	−1.00	150
Left	−3.25	−1.25	070

Theoretical Spherical Equivalent:

	Sphere
Right	−4.00
Left	−4.00

Example 3: Glasses Prescription:

	Sphere	Cylinder	Axis
Right	+2.25	−0.75	020
Left	−1.00	−0.75	032

Theoretical Spherical Equivalent:

	Sphere
Right	+1.875
Left	−1.375

Calculating Things Isn't Your Strong Suit?

No problem! If you want to quickly look up the spherical equivalent of any 'Sphere' and 'Cylinder' combination, you can do so right here at Contacts Advice using the Spherical Equivalent Charts.

Remember that calculating or looking up the spherical equivalent of your prescription is not a substitute for being prescribed contact lenses by your eye doctor. Contact lenses should not be ordered with anything other than a valid contact lens prescription written by your optometrist.

Other Uses of Spherical Equivalent

Calculating the spherical equivalent has many practical uses. Eye doctors are skilled at performing these calculations for many different things, including:

1. Order spherical contact lenses for you

• Whether it's for color contact lenses or clear ones, contact lenses for astigmatism are more expensive and can create fluctuations in vision if the contact lenses are not stable on your eyes. If you have a small amount of astigmatism, your eye doctor may take the spherical equivalent of your prescription to order you regular (spherical) contact lenses.

2. If your 'Cylinder' number is too high for contact lenses

• In this case, your doctor would not be taking the true spherical equivalent — he/she would be applying the same principles in order to drop your 'Cylinder' number down to the highest available 'Cylinder' power for contact lenses for astigmatism. I.e., if your cylinder is −3.25 but your doctor wants to put you in a contact lens that has a maximum cylinder correction of −2.75, he/she will have to apply the principles of the spherical equivalent in order to do so.

3. To help you adjust to your new glasses prescription

• If your eye doctor detects a large change in your cylinder, it is not uncommon for them to reduce the cylinder using the principles of the spherical equivalent in order to help you adjust to your new glasses. New glasses containing a large change in cylinder can cause symptoms of eyestrain, dizziness and headaches.

When Not To Use Spherical Equivalent

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Remember that the higher your 'Cylinder' number is in the original prescription, the worse your vision will be through the Spherical Equivalent.

Generally speaking, if your 'Cylinder' is higher than −1.00, your eye doctor will most likely advise against using the Spherical Equivalent to order contact lenses, as the vision will be too greatly affected. Also remember that using your spherical equivalent with any amount of 'Cylinder' in your glasses prescription will result in some amount of blurriness. If you're that person who cannot tolerate any blur in his/her vision, do not allow your Spherical Equivalent to be used for your contact lenses. You will not be happy with the outcome.

If you have astigmatism and you're interested in ordering color contact lenses, speak with your eye doctor about what he/she can do with the numbers in your prescription in order to help you out.

Using A Contact Lens Prescription Online

Once an eye doctor has issued you a valid contact lens prescription, you can then use it to order contact lenses in stores or online. Depending on the website you choose to buy your contact lenses from, you may or may not have to provide the website with your prescription (depending on the regulations governing the site).

Regardless of whether you have to provide your contact lens prescription or not, it is always extremely important to only use a valid contact lens prescription to order contact lenses.