Guide to Swim Goggles

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If you require corrective lenses to see clearly, you may also want to purchase goggles with prescription lenses for the pool.  This guide walks you how to choose the prescription goggles that are best for you.
The Lens
Prescription goggles come with ready-made spherical lenses of varying strengths known as diopters, or step diopters. This type of lens production is similar to the generic reading glasses found in pharmacies: The prescription will be a close match to your own, but may not be as precise as that in the glasses or contact lenses you use every day. For use in the water (an already-blurry environment), step diopter prescription glasses are perfectly adequate. Negative diopter lenses are the most commonly available goggles, and are intended for those with nearsighted or myopic vision. Lenses range in diopters from -1.5 to -8.0 and come in increments of 0.5. Positive diopter lenses are for farsighted swimmers, +1.0 to + 8.0
Finding Your Diopter Strength
The first step to choosing prescription goggles is determining the diopter strength that most closely matches your current prescription. Follow the below steps, or ask your doctor to help you choose the best ones for you.

    Get your most current prescription numbers.
    Calculate the strength of the diopter you need using this formula:
             1/2 of the cylinder + sphere = diopter strength
    Sphere is the degree of weakness in diopters. This is always a negative number for nearsighted people, and a positive number for farsighted. Cylinder is the degree of astigmatism in your eye. Add half of this number to the sphere to determine diopter strength.
   
   
    To choose a diopter lens, simply find the one that has a prescription closest to your own. A general rule of thumb when choosing a diopter lens is to round down — rather than up — to the prescription that most closely matches your own. This places less strain on your eyes.

For example, if your prescription is a sphere of -5.0 and a cylinder of +2.0, then (1/2 x +2.0) + -5.0 = -4.0 step diopter lens. Or if your prescription is a sphere: -2.0 and  Or if your prescription is a sphere: -2.0 and a cylinder: -1.0, then (1/2 x -1.0) + -2.0 = -2.0 (rounded up from -2.5) step diopter lens