Gone are the days when divers with less than perfect vision had to stick a pair of modified eyeglasses into their oval-window mask to see clearly underwater. And no longer is it necessary for those of us with a few years in our goody bags to carry a magnifying glass to view small creatures or read our gauges.
As the number and diversity of recreational scuba divers have increased, so have the choices for improving our underwater vision. While we may not be able to see as crisply and colorfully as on land, today’s vision correction options are broad enough to permit virtually anyone with correctable vision to enjoy a full range of sight underwater.
Why We Wear a Mask
Perhaps your experiences with the sting of salt water or the burn of chlorine in the eyes tell you that the best reason for wearing a dive mask is to protect your eyes from the water. While this is one function of a mask, the primary benefit is to allow you to see clearly. Without the pocket of air surrounding our eyes, the floating world is just a big, wavy blur.
Our eyes work much like a camera (actually, it’s the other way around). Light rays enter the eye through the cornea, a transparent dome-shaped tissue that protects the iris and the pupil. The iris is the colored part of the eye that rings the pupil the opening through which light enters the eyeball. The iris constricts or expands to accommodate changing amounts of light, similar to the aperture of a camera lens.
Directly behind the pupil opening is the lens, which focuses light rays entering the eyes from many angles so they converge on the retina at the back of the eye. Photoreceptors in the retina translate the light into nerve impulses. These impulses are then carried to the brain for interpretation. We are said to have “perfect” vision when the light rays are properly focused to create a sharp image.
Since humans are land creatures, our eyes have evolved to provide maximum visual acuity in air. Light waves act differently when traveling through water than through air, reducing the ability of our eyes to process them effectively. We need the airspace provided by a mask for our eyes to function properly.
Do I Need Additional Vision Correction Underwater?
Even though we wear a mask, the fact that water is 800 times as dense as air causes additional vision distortions. When light waves move from air into water, they are slowed and bent, or refracted. You can easily illustrate this phenomenon by inserting a pencil into a glass of water (see photo). Observe how the pencil seems to shift and bend at the air/water interface.
During a dive, light must travel from air into the water, then through the glass of your mask and across the air pocket before it reaches your eyes. Each time it enters another medium of a different density the light is refracted. Refraction is the reason that objects appear closer and larger by about 25 percent. Notice that the underwater portion of the pencil appears larger than the above-water section.
Due to the magnifying effects of refraction, some divers with mild nearsightedness (better vision close up than at a distance) or farsightedness (better vision at a distance) on land are comfortable underwater without additional vision correction. To determine if you see well enough underwater to dive safely, ask yourself these questions: Can you read your gauges and watch (or computer) quickly and accurately? How about your compass? Does your distance vision allow you to maintain orientation to your surroundings at all times?
Dr. Frank K. Butler Jr. of the U.S. Navy Medical Corps is a leading expert on hyperbaric ophthalmology and an adviser to the Divers Alert Network (DAN). In an article for Alert Diver magazine, he says, “If you see well enough to qualify for a driver’s license and operate a motor vehicle safely, then you should be able to see well enough to dive safely.”
In general, individuals who wear corrective lenses in daily life will experience a noticeable benefit from using some form of vision correction while diving.
If you wear prescription glasses, then the logical choice for diving is to take your glasses underwater not by literally sticking them inside your mask, but by incorporating corrective lenses into your dive mask. Lens and mask companies offer divers a variety of options to accomplish this; each has advantages and disadvantages.
Bonded lenses. One of the earliest methods for adding vision correction into dive masks was bonding, which is still used today. Lenses are ground to match the diver’s prescription and glued to the inside surface of his or her mask faceplate. Over the past two decades the glue has been improved to provide a transparent, permanent bond. Just like glasses, lenses can be ground to accommodate any correction, including astigmatism, bifocals and strong prescriptions. Lenses can be bonded to any mask with a flat faceplate, whether it has one or two front windows. It is even possible for the lens manufacturer to remove the bonding via a high-heat process and affix the lenses to another mask.
Typically, bonded lenses are ordered through your scuba retailer when you purchase a mask. Of course, you’ll need to know your glasses prescription; the best approach is to bring a copy of a recent prescription with you to the dive center. Once you’ve chosen a mask that makes a good seal on your face and otherwise suits your needs, the salesperson will mark the mask to ensure proper placement of the lens (see sidebar). The mask is then sent to the lens manufacturer for grinding, polishing and bonding. Turnaround time is between three and 10 days and the cost is comparable to similar prescription lenses in glasses (not including the price of the mask).
One disadvantage of bonded lenses is that they can only approximate the shape of the existing mask faceplate(s). Depending on the shape and size of the mask, the bonded lenses may not cover the entire window, reducing the in-focus portion of the field of view. Bonded lens manufacturers claim that this effect is minimal.
Another potential disadvantage is that adding a second layer of glass to the faceplate increases the weight of the mask. Since water provides additional buoyancy, the extra weight may not be noticeable unless the prescription is a strong one.
Because of the greater curvature that must be ground into the glass to produce lenses with a high degree of correction, lenses made from regular glass are limited in size creating a “Coke bottle” effect in the dive mask. Therefore, bonded lenses with correction of more than about plus or minus five may be ground from a special “hi-index” glass that is thinner but denser, allowing the lens to fill the faceplate more fully.
Prescription masks. Another option is to have prescription lenses installed directly into a mask, replacing the existing faceplates. Some lens manufacturers install custom-made lenses in your mask; others sell masks with lenses made to your prescription as a unit. Similar to bonded lenses, prescription faceplates can be ground for virtually all vision correction requirements. However, this option is only available for masks with two windows in the faceplate, as the lenses must be made separately for each eye.
A major advantage of this alternative is that there is only one thickness of glass in the faceplate, reducing the mask weight. Also, the full field of vision of the mask is preserved because the lenses are made to the exact size and shape of the original nonprescription lenses.
At least one underwater vision correction specialty company uses a chemical resin instead of glass for its lenses. Called “C39,” the resin is stronger than tempered glass so it is less likely to break. The material can be ground and polished, giving it an optical sharpness comparable to glass. Weight is an important advantage; according to one manufacturer a C39 lens is up to 50 percent lighter than a comparable glass lens more comfortable for divers requiring thick lenses. Although the resin is slightly more likely to scratch than glass, water fills in all but the largest abrasions, rendering them invisible during use.
Prescription masks with ground lenses replacing the original windows tend to cost more than bonded-lens masks. The lenses cannot be reused unless the replacement mask has the same shaped windows as the old mask.
The process for ordering a prescription mask is the same as for bonded lenses. Your dive center salesperson or vision care professional will mark the location of your pupils on the mask you choose and send it with your prescription to the manufacturer. If the lens company is supplying the mask, the prescription should also include the pupillary distance (i.e., the distance between the pupils).
Drop-in lenses. An alternative that is potentially both less costly and quicker is to buy a mask that accepts drop-in prescription lenses. Certain two-window masks are designed so that dive center sales staff can remove a restraining ring, pop out the original plain lenses and replace them with ready-made prescription lenses.
Scuba retailers that offer this option maintain an inventory of corrective lenses in a variety of diopters (strengths) so you can walk out the door the same day with a prescription mask that greatly improves your distance vision. Stock lenses are typically available for strengths ranging from a mild 1.5 to a strong 8 in half-diopter increments.
This option works well for distance vision corrections if you don’t suffer from astigmatism or need a very strong prescription. For bifocals or more complicated prescriptions, you’ll need custom-ground lenses. Drop-ins are not available for all two-window mask styles, but the range of sizes and shapes is growing.
Thus far, the vision correction options discussed have centered on distance vision. Bonded lenses as well as custom prescription masks can be manufactured to meet virtually all your diving needs, while ready-made drop-ins satisfy only distance correction. For those who just need help reading gauges or camera displays, there are additional choices that correct close-up vision.
If you prefer custom-made lenses for reading, you can have bifocals ground with plain glass in the top and your prescription for farsightedness in the bottom portion of the window(s). Or custom reading lenses called see-unders can be bonded onto the lower portion of your existing faceplate in either one- or two-window masks.
For an inexpensive fix, you can purchase a single ground lens spot. This small round lens affixes to the lower portion of the left or right side of your faceplate, like a tiny magnifying glass. Adhesive and instructions are provided for self-installation.
A similar product consists of two flexible semicircular lenses that adhere to the inside of your mask faceplate with water surface tension. These are easily removable for use in more than one mask, but may also change position during a dive if the mask fills with water.
One disadvantage shared by all mask-based vision correction options, as well as by glasses, is that peripheral vision is less sharp than central vision. This is because the lens is not situated at the surface of the eye, but at a distance. As you look to the side, the angle between your eyes and the lens is greater. Although the vertex correction (see sidebar) minimizes the degradation in sharpness when you’re looking straight ahead, it is not as effective for peripheral vision.
Also, if your mask has side and/or bottom windows, these extra viewing ports are not optically corrected, so peripheral vision remains blurry.
Contact lenses are the vision correction option that results in the best visual acuity because the lens lies right on the cornea, eliminating refractive error. As you turn your eyes toward the side, the center of the lens goes with them, so peripheral vision is also improved.
Contact lenses do not tend to float off the eye when exposed to water, as many believe. In fact, they may adhere more tightly to the cornea. This situation may cause blurred vision.
Butler recommends that divers use soft contacts rather than hard or gas-permeable lenses because bubbles forming behind hard lenses may irritate the cornea, causing pain and temporary blurring of vision. Due to the slight possibility that your contact lenses could absorb bacteria from the water, use of disposable lenses is a good alternative.
For the relatively small percentage of the population that qualifies for and chooses to wear contact lenses (estimated at 15 percent), this option is another solution for correcting underwater vision. A growing number of individuals have opted for a more permanent resolution to their vision problems: corrective surgery.
Three types of eye surgery are used to correct nearsightedness. Radial keratotomy (RK), the oldest of the procedures, involves reshaping the cornea by making radial incisions. This does weaken the cornea and, according to Butler, could result in injury due to severe mask squeeze, although he says that he knows of no such reported incident among divers who have had this procedure.
Photorefractive keratectomy, or PRK, uses a laser to reshape the cornea and presents no such risk. LASIK, a newer type of laser eye surgery, is reported to have minimal risks for divers. However, little or no formal research has been conducted on any of these procedures.
Recommendations for resumption of diving after vision correction surgery range from three months for RK to two to four weeks for PRK and LASIK. Consult with the doctor performing your surgery regarding recommendations for returning to diving.
Further Vision Improvements
In addition to correcting vision to approximate the sharpness of your sight in air, many divers are interested in improving other aspects of their vision.
Color correction. Light is a combination of all the colors of the visible spectrum, each with its own wavelength. When light enters water, it is not only refracted, but also diffused and absorbed. Those colors at the red end of the spectrum are absorbed by the water at shallower depths than those at the violet end, resulting in the blue or green cast of bodies of water. You’ve undoubtedly noticed that red, orange and yellow tones disappear soon after beginning your descent. Shine a dive light on a sponge that appears black, and you’re likely to discover that it is really bright red.
Many underwater photographers and others who desire to see “truer” colors underwater are opting for mask lenses that enhance the colors lost at depth. Lenses in various shades of pink eliminate the blue hues, making underwater colors seem more natural.
Definition enhancement. Low light or poor visibility conditions underwater result in reduced definition. Adding yellow-tinted lenses to a dive mask sharpens edges and improves contrast, much like putting a yellow lens in ski goggles reduces glare and whiteout.
Depending on the type of lens you choose bonded or prescription faceplate the pink or yellow color may be sealed between two pieces of glass, affixed to the inside of the lens or mixed as part of the resin.
Other vision-improving options include antifog lenses and demagnification lenses. The latter, which compensate for the magnifying properties of water, are useful to spearfishermen and others that must judge distance accurately. Most of these vision improvement choices can be combined with one or more of the prescription lens options.
Field of view. The refraction of light underwater significantly reduces a diver’s field of vision compared with seeing in air. Dive mask manufacturers have tried to improve peripheral vision by adding side, top and bottom windows, but have been unable to provide prescription lenses for these extra view ports.
A new mask style addresses this problem by curving the entire faceplate of the mask, creating a panoramic view. Divers who are not nearsighted must wear contact lenses to see clearly in the mask, and, unlike other masks, above-water vision is blurred.
Which Solution Is Right For You?
If you use some form of vision correction in everyday life, then you need to correct your underwater vision as well. Not only is ability to see clearly and accurately a safety issue, but your diving confidence and enjoyment increase greatly when you can see where you’re going as well as everything you pass along the way.
There are many viable alternatives. When updating your eye prescription discuss with your vision professional which option best suits your needs and preferences. Dive center sales staff will help you choose the mask that complements your vision correction choice.
Measuring For Prescription Masks
Although we are most familiar with the plus or minus numbers that indicate our degree of vision correction, these are not the only measures involved.
The sphere refers to the roundness of the cornea how much flatter or steeper the surface is compared with a perfect sphere. The “+” and “-” symbols indicate the strength of the prescription, or how much the light waves need to be bent to focus properly on the retina.
Another measurement, the cylinder, specifies the correction for astigmatism caused by unevenness in the surface of the cornea. The point where the up-and-down and side-to-side cylinder measurements meet is called the axis.
The optical center of a lens is where your vision is most acute. Lenses should be installed (bonded or prescription faceplate) so that the optical center is directly in front of your pupils when you are looking straight ahead. One way to ensure that this occurs is to mark the mask before it is sent to the lens manufacturer to have prescription lenses installed. When you purchase the mask, the salesperson will have you put it on and look straight into the distance while he or she places a mark on the glass with felt-tip pen indicating the center of your pupils. When the lens company is supplying the mask, the prescription that you send should also contain the measure for pupillary distance the distance between the pupils.
Vertex is the distance from the cornea to the lens. In eyeglasses this usually measures between 12 mm and 14 mm, but the distance in dive masks averages 50 mm. This difference can result in reduced sharpness, especially in high-volume masks coupled with a strong prescription. The lens company will make the appropriate correction for your mask based on a mathematical formula.