How to Choose an Optical Lens

Snorkel-Mart offers optical lenses for people requiring masks with optical correction. The lenses we offer are known as "Step Dioptre" lenses, which basically means stock or pre-fabricated lenses.

Enter the Sphere and Cylinder values from your prescription into the Lens Calculator below and click the Calculate button to determine which of our lenses would best suit your needs.

NOTE: The figures on the prescription may appear in this format: -075 or +550, etc.. When using the calculator, you'll need to insert decimal points, like this: -0.75 or +5.50 Stock lenses provide a spherical equivelant for the prescription in question. If your Cylinder value is greater than +2.5 or less than -2.5, please contact us to learn about obtaining custom ground lenses.

How to calculate a prescription for the best suited lens:

You will need to know what your current eyeglass prescription is. This is usually given on a piece of paper when the eyes are tested. If you cannot locate the prescription, we suggest contacting the optometrist who last tested your eyes or the company where you last bought your glasses.

The prescription may contain some terminology that we may need to clarify.

OD (Ocular Dexter) refers to your right eye.
 
OS (Ocular Sinister) refers to your left eye.
 
Sphere Refers to the degree of correction needed by your eye, measured in dioptres. For nearsighted people this is always (-) negative.
 
Cylinder Refers to the amount of astigmatism in your eye (the degree by which your eye is not uniformly round).
 
(There may be other terms on the prescription such as Add, PD and Axis, but these values are not considered when selecting a Step Dioptre Lens)

Although step dioptre lenses do not correct for cylinder, the cylinder number does affect which lens will best suit you.

To best select, you must add one half (1/2) of the cylinder number to the sphere 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.

Bifocal Lenses: Some of our lenses are offered as bifocals. These 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 dipotre), they are handy for SCUBA divers who need to read their gauges etc.

Here are some examples:

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.

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.

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.