How to Determine the Diopter for a Scuba Mask

Thanks to e-how and Pamela Stephens

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 Using an out-of-date prescription can result in an inaccurate diopter choice for your scuba mask lenses.

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.

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

Choose the numbers for one eye and calculate the diopter. Add the sphere number and half of the cylinder number to determine the diopter. 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.

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

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

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

Kathy Dowsett
www.kirkscubagear.com

Wearing Contact Lenses When Scuba Diving

Avid divers know what an amazing feeling it is to submerge into a world so different from our own. The underwater paradise is the only real frontier left to explore on the planet and even in frequently visited locations, like the Red Sea or Bunaken, you’ll have the opportunity to experience something unique, see fantastically colored fish and impossibly shaped coral reefs.

However, when you are scuba diving, you rely heavily on your sense of sight to take in all the beauty. This means that if live with a vision defect, such as nearsightedness or astigmatism, you will need prescription goggles or, if your wallet doesn’t stretch that far, a pair of contact lenses. So, there is no reason not to get the best experience every time you’re down in the water.

Using Contact Lenses when Diving

It is possible to wear glasses when scuba diving but it’s not the optimal choice when you’re also wearing a diving mask. What we would recommend is to wear contacts while diving. There are a couple of things to consider when you wear lenses underwater.
■If you experience mild discomfort, perhaps feel as if the lenses tighten a bit while you’re down, simply use lubricating eye drops before and after each session. This should relieve some of the irritation.
■Another thing to think about when diving with contact lenses is that you should blink as much as possible. In doing so you’ll prevent bubbles from forming underneath your lenses – these bubbles are in no way harmful to your eyes but they can cause minor discomfort and blur your vision.
■Also, when you clear your mask of water, remember to close your eyes so that you don’t lose your contact lenses!

Which Lenses to Wear

As most contact lens wearers know, there are two types of lenses; hard contacts and soft contacts. When diving, it is recommended to wear soft contacts, due to the fact that soft contacts contain a percentage of salt water which helps prevent them from floating off your eyes if they are open when you flood your mask. Hard contacts (gas-permeable) are more likely to simply disappear off your eye into the water, and it’s next to impossible to find a lost lens under water due to their inherent translucency. This is why you should always wear disposable lenses when doing water activities, since an eventual loss is not that great.

More to Think About

You shouldn’t worry about wearing contact lenses when scuba diving – it is perfectly safe. But, eye care experts suggest the following tips to keep in mind when using lenses
■When using contacts it’s important to ascend slower than normal.
■Wear soft contact lenses.
■Rinse lenses between dives to get the salt water out.
■Bring an extra pair if a problem should occur.
■Let your diving buddy know you’re wearing contacts so that he or she can retrieve your mask if you should lose it.

Other than this there is really nothing special to think about when hitting the water –simply dive right in!

Thanks to Scuba Diving

Kathy Dowsett

www.kirkscubagear.com

Do Things Seem 25% Closer Underwater?

Image via Wikipedia

Every scuba diver has been taught that underwater objects appear to be only three-quarters as far away as their physical (measured) distance. For objects subtending small angles at the mask and eye it is certainly true that, for any object distance, the object's in-water virtual image produced by the planar air-water interface (mask faceplate) is always nearly 25% closer to the interface. This is a consequence of the laws of refraction and is independent of the observer. It is not true, however, that the virtual image is always perceived to be at that distance.

Research into underwater distance perception contradicts the popular notion held by most scuba divers that things always look closer under water. An initial investigation by Ross (1965) found that object distances less than 39 ft. were indeed underestimated, as one would expect from the laws of refraction, but those greater than 39 ft. tended to be overestimated. Subsequent studies by Kent (1966), Luria et al (1967), and Luria and Kinney (1970) were in agreement that distance estimates for objects closer than one meter were, in fact, too small, but beyond one meter the perceived distances were too large!! Moreover, the distance overestimates increase both with object distance and water turbidity.

The correlation with water clarity suggests that the overestimates for large object distances are caused at least partly by loss of object-to-background contrast due to the absorption and scattering of light by water particles along the light path. The loss of colour and the haziness means that foreground objects are less distinct relative to the background. Studies in air (Fry et al) have demonstrated that reduced contrast effects cause object distances to be overestimated, so this trend apparently carries over into the underwater environment.

This cannot be the full explanation because the overestimates also occur in water even under conditions of high contrast. Luria and Kinney have pointed out that the major difference between scenes in air and scenes in water is that underwater there are relatively few clearly visible, familiar, and well-defined distance cues. The scenes approach the hazy, unstructured, homogeneous aspect of what is referred to in psychology as a 'Ganzfeld'. The characteristic loss of peripheral stimulation associated with a Ganzfeld impairs both object detection capability and depth of field judgments (stereoacuity). Experiments have shown that this leads to distance overestimates.

Hence, the underestimates that we all expect on the basis of refraction occur only at small mask-to-object distances.

Kathy Dowsett

www.kirkscubagear.com

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