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How to Choose the Correct Optical Lenses for Your Snorkeling or Dive Mask

Snorkel-Mart offers pre-ground step diopter optical lenses for people requiring snorkeling and dive masks with optical correction. The lenses we offer are known as "Step Dioptre" lenses, which basically means stock or pre-fabricated lenses. Our stock prescription lenses provide what most optometrists will refer to as a "spherical equivalent". The calculation that we use(half the CYL value added to the SPH value) has worked really well for our thousands of satisfied customers to correct vision under the water. These optical lenses are in stock and ready to be installed into the prescription masks made by Deep Blue Gear.

Use Our Rx Optical Lens Calculator to Calculate Your Recommended Prescription

Enter the Sphere (SPH) and Cylinder (CYL) values from your prescription into the Optical Lens Calculator below and click the Calculate button to determine which of our rx lenses would best suit your needs. *Note: If your Cylinder value is greater than +2.5 or less than -2.5, please contact us at (888) 597-8445 to learn about obtaining custom ground lenses.

NOTE: In addition to selecting the dropdown choice for positive (+) or negative (-) lenses, please remember to insert decimal points. Sometimes doctors get in a rush and do not put the decimal point in the actual prescription, but most sphere and cylinder values have decimal points. Example: -075 is the same as -0.75 or +550 is the same as +5.50.

Eye +/- Sphere +/- Cylinder Suggestion
(right eye)
(left eye)

Send Us Your Prescription

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Learn More about How To Choose an Optical Lens

Please watch the video below if you would like to see an overview of how to use the optical lens calculator on our website.

How To Read The Information On Your Prescription

You will need your most recent eyeglasses prescription. Please note we are unable to use contact lens prescriptions! Those are a completely different prescription. If you cannot locate the correct prescription, we suggest contacting the optometrist who last tested your eyes or the company where you last bought your glasses.

We have defined some of the terms you will see on your prescription below.
OD (Ocular Dexter) refers to your right eye.
OS (Ocular Sinister) refers to your left eye.
SPH (Sphere) refers to the degree of correction needed by your eye, measured in dioptres. For nearsighted people this is always (-) negative. For farsighted people this is always (+) positive.
CYL (Cylinder) refers to the amount of astigmatism in your eye (the degree by which your eye is not uniformly round). Although step dioptre lenses do not correct for cylinder, the cylinder number does affect which lens will best suit you.

There may be other terms on the prescription such as ADD, PD and AXIS, but these values are not considered when calculating the spherical equivalent. If you need our custom lenses though, they are needed.

How our Optical Lens Calculator Selects the Best Lens for you

When you input your sphere and cylinder values into our calculator, it adds half of the CYL value to the SPH number. Note: The cylinder may be either (+) positive or (-) negative, which can make the math a little confusing. You need to be careful with the (+) and (-) signs.

Hopefully, after adding 1/2 the CYL to the SPH number, you get a nice number that ends in .5 or .0 (We carry stock lenses in 1/2 diopters from -10 to +4.0). If not, round that number down to the nearest .5 or .0. Once that is done, the number you are left with is the lens you should select! Our stock lenses can be installed in either the the Spirit 2 - Prescription Diving Snorkeling Mask by Deep Blue Gear or the Maui - Prescription Diving Snorkeling Mask by Deep Blue Gear.

Information about our Executive Bifocal Lenses

Bi-Focal Lenses

Our bifocal prescription lenses do not offer any optical correction in the top section of the lens; however they do provide a bifocal magnifying lens in the bottom portion of the lens. Although these lenses are not necessary for the majority of snorkelers who require nearsighted correction (negative dioptre), they are handy for SCUBA divers who need to read their gauges etc.

Examples of Prescriptions and Recommended Optical Lenses

Example #1
OD - 2.5 0
OS - 3.0 0

OD : Sphere of -2.5, plus ½ of the cylinder 0 : -2.5 + 0 = -2.5
OS : Sphere of -3.0, plus ½ of the cylinder 0 : -3.0 + 0 = -3.0

So in Example #1 the best Step Dioptre lens for the right eye would be -2.5, and the best lens for the left eye would be -3.0.

Example #2
OD - 5.75 -0.5
OS - 5.5 -0.5

OD : Sphere of -5.75, plus ½ of the cylinder -0.25 : -5.75 + -0.25 = -6.0
OS : Sphere of -5.5, plus ½ of the cylinder -0.25 : -5.5 + -0.25 = -5.75

So in Example #2 the best Step Dioptre lens for the right eye would be -6.0, and the best lens for the left eye would be -5.75.

Example #3
OD - 5.0 +1.0
OS - 3.5 +0.5

OD : Sphere of -5.0, plus ½ of the cylinder +0.5 : -5.0 + +0.5 = -4.5
OS : Sphere of -3.5, plus ½ of the cylinder +0.25 : -3.5 + +0.25 = -3.25

So in Example #3 the best Step Dioptre lens for the right eye would be -4.5, and the best lens for the left eye would be -3.25.

As Step Dioptre lenses increase in increments of -0.5 it may not be possible to exactly match your requirements. For Example if you are in need of a -2.75 lens, the closest Step Dioptre lens would either be -2.5 or -3.0. In this case you would always move down and choose the weaker power -2.5. Water will also serve to magnify somewhat, therefore objects viewed underwater will appear to be slightly nearer and larger than actual.

Did We Answer All of Your Questions?

We hope that this information is helpful. If you still have questions, please contact our Customer Care team at 1-888-597-8445, chat with us, or visit our Contact Us page