How to determine the diopter for a SCUBA mask

Written by pamela stephens
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The sport of scuba diving has grown exponentially since its inception in the 20th century. An acronym for the phrase "self-contained underwater breathing device," scuba requires the use of specific equipment. One important piece of equipment is a dive mask. For individuals who wear eye glasses, prescription lenses for scuba masks can be purchased. Because of the lower cost, most recreational scuba divers opt for a step dioptre dive mask lenses (also known as stock prescription lenses) as opposed to custom-ground lenses.

Skill level:
Moderately Easy

Things you need

  • Eye glass prescription

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  1. 1

    Obtain a current eye glass prescription. Using an out-of-date prescription can result in an inaccurate dioptre choice for your scuba mask lenses.

  2. 2

    Refer to the eye glass prescription. Decoding the prescription is not as difficult as it might first appear. "Oculus dexter" (OD), "oculus sinister" (OS) and "oculus utrique" (OU) are Latin terms that mean right eye, left eye and both eyes. Sphere is the amount of correction needed in each eye. Cylinder is the amount of astigmatism.

  3. 3

    Locate the sphere number and the cylinder number for the OD and OS. These numbers will likely be different for each eye.

  4. 4

    Choose the numbers for one eye and calculate the dioptre. Add the sphere number and half of the cylinder number to determine the dioptre. For example, if the prescription indicates a sphere of 2.0 and a cylinder of -.50 this would be calculated as 2.0 - .25 = 1.75.

  5. 5

    Round down when possible, because water is a natural magnifier. Dioptres are in increments of .25. In the example, a dioptre of 1.5 would be chosen.

  6. 6

    Repeat the same formula to calculate the dioptre for the other eye.

  7. 7

    Purchase specific dioptres for each lens in the scuba mask. Most full-service dive shops have step dioptre lenses in stock and can easily replace regular lenses with them. You will not need to show your prescription to purchase step dioptre lenses; however, dive shop professionals can double check your prescription to make sure that you purchase the correction that is best for your sight.

Tips and warnings

  • Choose a scuba mask that fits well and meets your personal diving style.
  • Step dioptre scuba mask lenses are similar to non-prescription reading glasses. These lenses only correct nearsightedness and farsightedness.
  • Individuals with astigmatism greater than -1.00 should consider custom-ground scuba mask lenses.

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