HydroOptix vs. Flat Masks
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| 1. |
Q |
I thought the magnification from a flat mask helps me see better? |
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A |
Not really |
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When using a conventional flat mask you only see a truly sharp image in your central 7-degree cone-of-vision when your eyes look exactly straight ahead (perpendicular) through the lens of the mask. The further off-axis you look (i.e. at an angle to the lens), the fuzzier the image, because the light rays are bent (refracted). This phenomenon is really obvious in a clear swimming pool, but even in 6-foot visibility, everything looks sharper through a HydroOptix Double-Dome lens. (top)
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| 2. |
Q |
Why does a flat mask make things look fuzzy around the edge? |
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A |
When looking through a flat mask, refraction makes everything look fuzzy at the edges -- shown here. In fact, your overall vision through a flat mask, in the clearest water possible, is 10 times blurrier than MilSpec requirements for dust goggles. By contrast, HydroOptix Double-Dome masks deliver razor-sharp vision, no matter which direction you look -- the off axis view is over 100-times sharper vs. a flat mask (10X better than MilSpec minimum requirement). (top)
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| 3. |
Q |
You claim the view through a conventional flat mask is so bad... are there independent metrics you can use for comparison with the HydroOptix view? |
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A |
Thank you for asking! |
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First, if your in-air vision were so narrowed as it is while underwater using a flat conventional mask, you'd be qualified for disability, and laws would prevent you from obtaining a driver's license in North America or Europe. |
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Second, the U.S. Military demands clear viewing through gas masks, dust goggles, pilot's helmet visors, etc. The military sets specifications (MilSpec) that manufacturers must obey. Underwater, we perform 10 times better than MilSpec for in-air pilot visors, while conventional flat masks underwater are 10 time worse! (top)
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| 4. |
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Are there military specifications to limit optical distortions? |
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A |
There are very rigid Military Specifications for in-air vision systems, but conventional flat masks distort 10 times worse than these requirements!
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• Magnification -- objects look 25% closer and 34% bigger than reality. |
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• Lateral Chromatic Aberration -- when you look where your nose points, things are sharp. But the further off-axis you look, the fuzzier the view becomes. |
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• Pincushion Distortion -- off axis lines bow inward; all shapes are distorted, worst at the edges. |
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The next time you are in a swimming pool, observe the tiles through a conventional flat mask both on & off axis.
The Double-Dome masks vastly exceed(s) MilSpecs for optical resolution. (top)
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| 5. |
Q |
How does the military measure optical resolution? |
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A |
The goal is to resolve tiny objects that occupy a small area of your total field-of-view. There are 360° in a circle, and 60 arc-minutes per each degree of arc. MilSpec requires aviation visors / dust goggles / gas masks to resolve 2 arc minutes or less. If you can resolve less than one arc-minute -- which means exceptional vision -- you are able to notice very tiny / distant objects that occupy a miniscule fraction of your total view (i.e. spotting a distant airplane). Off-axis, flat masks can only resolve objects that span 22.0 arc minutes of your view, while HydroOptix resolves an outstanding .016 arc minutes. HydroOptix resolves tiny details in the entire field-of-view 100 times better than flat masks... whether you're in 2' or 200' visibility, you'll see better than ever before. (top)
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| 6. |
Q |
Aren't there other true size or panoramic masks? |
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A |
No, we're the world's first and only! For decades, mask companies ignored the refractive index of water. Wrap-around shapes -- like ski goggles -- have been tried, but the immutable laws of optics result in distortions like fun-house mirrors, which also cause large areas to be out-of-focus. HydroOptix believes some companies today are guilty of advertising that's simply not true; we have filed complaints with the Federal Trade Commission to stop these “Optical Delusions.” |
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Judging image quality through the mask in air is no indication of how it will perform underwater. The results you see underwater are caused by the shape of the water (not the mask) merged against the trapped-air, inside the mask. |
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Check out the thumbs-down tech review of Aqualung's Sphera™ Mask for scuba by the U.K.'s "Diver" Magazine (uses the same lenses as the Seal™ goggle). |
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HydroOptix is the only company to prove our claims: we have the ray-traced results, performed by our world-renowned optical engineers. (top)
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| 7. |
Q |
The MEGA 4.5 DD mask is a moderate-volume mask, but aren't low volume masks better? |
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A |
Traditionally, there has been three reasons why flat-mask designers pursued low volume masks: |
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Wider View (Myth!): |
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Moving flat windows closer to your eyes, you will get a very slightly larger field-of-view in air, but surprisingly when underwater, the closer the flat glass is to your face, increases the angle of refraction resulting in objects being more magnified (and narrowing your view)! The very widest flat mask yields a small 71° of horizontal vision underwater; most flat masks deliver less than 65°. Look at underwater photos of divers wearing conventional flat masks; as they angle their head away from the camera, the surface of their mask turns into a mirror -- if you can’t see in, they can’t see out. |
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A related phenomenon -- when you are underwater looking up, you can only see sky through a 95.5° cone. At greater angles, the undersurface of the water acts like a mirror, reflecting back the bottom. Optical engineers call this T.I.R. (Total Internal Reflection). |
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Easier Clearing: |
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For some new divers, learning to clear a flooded mask can be a challenge. All things being equal, a low-volume flat mask is easier to clear than large-volume masks, because you're displacing less water. But HydroOptix has an even better solution: our patented Twin-Turbo™ Purge Valve system! The Twin-Turbo™ Purge Valves with its oversized purge-valves means you will get water out of your mask easier than any other mask in the world. |
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Deep breath-hold diving: |
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Gonzo freedivers prefer ultra low-volume masks; more air for their lungs rather than spent equalizing pressure on the inside of the mask to ambient water pressure. Is it worth gaining a few extra seconds of bottom time vs. enjoying almost 5X times wider view? That's an individual choice, but we suggest freedivers not to knock-it 'till they try it. (top)
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| 8. |
Q |
Are you going to make a color correcting mask with pink / orange lenses? |
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A |
Not yet. Amber and pink filters were first used 60 years ago on underwater cameras with color film to help restore color balance in shallow water -- but when you are deeper than 40 feet, that pink / orange filter behaves like a dark gray filter, making the image darker and blurry. That is because the warm wavelengths of light (similar to the color-filter) have been fully absorbed by the water above you, and that filter blocks the ambient blue light. "Blue-blockers" are great for sunglasses, not for diving! |
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In low illumination, two physiological events occur inside your eyes: |
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1. Your pupils dilate; just like widening the iris blades of a camera lens, your focus will not be as sharp. |
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2. Images are formed inside your eye -- on your retina -- where you have two kinds of imaging sensors. CONES provide color vision and high resolution, but your cones stop working in dim light. RODS are for dim light, but rods have poor resolution and cannot show color. |
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If you compare clear vs. tinted lenses at 40 feet, you will discover the image through the clear mask is more colorful, has higher contrast, and is sharper; the deeper you go the more obvious the difference gets. |
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For safety's sake, you would never drive a car at night while wearing sunglasses. Likewise, we think that wearing tinted lenses deeper than 50 feet is ill advised. (top)
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| 9. |
Q |
How well does the Double-Dome lens work with camera housings? |
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Pro -- Double-Dome lens |
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1. Physically more relaxing: it's easier and faster to position your body… and less neck strain. With a flat mask, you pull the housing firmly against your mask, thereby "locking" your skull's relationship with the viewfinder. That means to frame a shot that is above, down, left, right, either your neck or your entire body must rotate, pointing your nose at your subject. With our Double-Dome masks, your shooting eye can rotate naturally in its socket, in a VERY wide cone (about 60°), and still see into the viewfinder. Try looking through a land-camera… you'll see what we mean! |
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2. Enhanced situational awareness: for wide-angle photography, this might mean spotting a subject you would have missed while experiencing "tunnel-vision" when wearing a conventional flat mask. |
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3. Improved eye-hand coordination: useful when manipulating double strobe arms, or juggling more than one housing. |
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Con -- Double-Dome |
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Eye -relief of the Double-Dome is greater than a low-volume flat mask; your eye will be about 1/4" to 1/2" further from the viewfinder. Image size will be what you get in air, without the magnification effect of water. Many cameras and housings work great with the Double-Dome mask; some are not ideal. To test your viewfinder, put your mask on while on dry land and look through your viewfinder. If you use a low volume mask, pull the viewfinder away from your face 1/4" to 1/2". Can you live with that? |
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The Future |
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There has been more progress in the last 5 years of digital photography than the last 80 years of photochemical photography. There is a huge advantage to confirming your shot seconds after you have released the "shutter," while you are still 80 feet down. Even pro-photographers have botched exposures, unaware until reviewing their beloved Kodachrome® weeks later. In less than 4 years, as resolution and contrast latitude continue to improve dramatically, mini-LCD screens will make eye-relief a non-issue, and digital will be the rule, not the exception. (top)
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Divers with 20/20 vision
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| 10. |
Q |
I've got 20/20 vision. Can I use the Double-Dome masks? |
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A |
Absolutely! Divers who do not fit the Naked-Eye-Range of our Double-Dome masks can use contact lenses to temporarily adjust their vision to match the range of our masks. To some this may sound crazy but we currently have over 700 divers doing just that. Not sure what type of contacts you would require, if you know your eye prescription simply check out our Rx Conversion Chart. If you do not know your prescription it would be best to have your eyes tested by your local Eye Care Professional. If you discover that you would require contact lenses to dive with our Double-Dome masks you can visit on of our DEC-Pro's (Diving Eye Care Professionals) who can fit you with the latest generation of super-comfortable disposable contact lenses. If there are no DEC-Pro’s in your area contact us and with your personal eye care professional’s contact info and we will call them and inform them of what they need to do let you dive with our mask.
Our optional S.I.-Kit (Surface Interval Kit) provides you with prescription eyewear between dives and a place to store everything you'll need:
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1. Transparent Dry Box. |
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2. Mirror |
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3. Eyeglasses (choice of -4.5, or -3.0, 64mm PD) |
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4. Polarized clip-ons |
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5. Sports Retainer |
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Our negative diopter CoverLens™ snaps onto the MEGA 4.5DD mask, giving sharp above water vision for entering and exiting the water. Divers using the MEGA 3.5DD mask will have a monocle with wrist lanyard to have sharp vision above water. (top)
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| 11. |
Q |
I'm a 20/20 diver; do I need to see an eye doctor before I purchase a mask? |
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Not necessarily -- Many divers first arrange a demo of the mask with their local retailer where they compare the field-of-view with a traditional flat mask. Even though your vision will be out of focus (unless you are young) you will be able to experience how much larger the Double-Dome mask field-of-view is over a conventional flat mask. This A to B comparison helps divers become more motivated to overcome the initial awkwardness that ALL contact lens users have overcome. (top)
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| 12. |
Q |
It sounds like contact lenses are a big hassle... is it really worth the effort & cost? |
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A |
Using contact lenses is a skill that must be learned. Your body has a natural blink reflex that exists to protect your eyes by preventing foreign debris (dust, sand, etc) from entering your eye. The first time many people use contact lenses it can take upwards of thirty minutes for each eye. This is because the body is not accustomed to wearing contacts and is trying to protect itself. But with practice inserting and removing contact lenses your body adjusts and it becomes easier and easier. Soon using contact lenses will be as common a task as checking your air gage before diving. Is it worth the effort? Check out our Who's Talking page and read the letters and emails that are sent to us by divers who have learned to wear contacts just to use our mask. (top)
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| 13. |
Q |
I've great vision! Why must I wear contact lenses? |
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A |
Our Double-Dome lens allows divers to see almost 5x more underwater because of its unique shape. The domed lens is able to wrap around giving the diver a much-improved filed-of-view. Double-Dome divers see 170° horizontally and 90° vertically compared to a conventional flat mask that only sees 71° horizontally and approximately 50° vertically (depending on the mask). Having each eye centered in a dome eliminates the refraction that causes objects to appear 34% larger and 25% closer when using a flat mask. But when the Double-Dome lens is submerged in water the water forms around the lens, and because the water has a higher index of refraction than air the water gets curved into a negative lens. In order for divers with perfect vision to see properly we must use contact lenses to adjust their vision to match the range of the mask. This is very easy to do. Our Rx Conversion Chart takes a divers age and eye prescription into consideration as acts as a good guide to show people what prescription of contacts lenses they will require to dive with the Double-Dome masks. (top)
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| 14. |
Q |
I'm in my 50's; I don't wear contact lenses, only reading glasses. Can I use the Double-Dome masks?
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If you only wear reading glasses you will most likely require contact lenses to use the Double-Dome masks. Once you use the proper contact lenses to dive with the Double-Dome mask you will experience the Magic Bifocal Effect underwater. Underwater your eyes will focus close, like they did when you were in your 20's! No longer will you need bifocals underwater if the Rx strength of your bifocals or reading glasses is +2.0 or less. If +2.25 or stronger, then the latest bifocal contacts or monovision will give you great close-focus ability. The new generation of disposable soft contacts work for a wide variety of vision needs, for folks well into their 80's. There are now affordable disposable contact lenses that will work great for your eyes and the Double-Dome masks. As we get older, it is a wise idea to have regular eye-health examinations anyway... the additional cost of getting fit for contact lenses is a small price to pay for a view that's priceless. (top)
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| 15. |
Q |
Do you have any other solutions that don't require contact lenses? |
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A |
Please check out our Products Page for more information on HydroOptix products that do not require contact lenses. (top)
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| 16. |
Q |
If I wear contacts for underwater vision, how will I see above water? Won't I be "blind" above water? What will I do between dives? |
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While wearing the MEGA 4.5DD above-water, our CoverLens provides clear above-water vision. Our optional SI-Kit (Surface-Interval Kit) includes: |
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1. A pair of prescription eyeglasses. |
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2. Custom-matched polarized lenses, |
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3. Sports retainer. |
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4. Mirror,
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5. Transparent Dry Box. (top)
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| 17. |
Q |
If I'm 20/20 and don't want to use contact lenses, can't I just put eyeglasses inside the Double-Dome Mask? |
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A |
We would love the solution to be that simple. A simple dome, submerged, creates a negative-power effect... which then requires complimentary positive-power optics to net "0" power (as explained in "How it Works"). We used the world's most powerful optical design software and a team of world-renowned optical gurus to analyze hundreds of different optical solutions (i.e. different shapes of lenses and all kinds of glass and optical polymers). The computer always "preferred" putting the positive-power "on" your eye. The optically superior solution is to wear positive-power contact lenses and become "temporarily" nearsighted.
In the optical design computer, trying to move the positive-power away from one's face, even by a few millimeters, created all sorts of optical distortions. Our goal was a simple shape that used simple optical materials... ah, if only life were so accommodating! We are rightly very proud of the design breakthrough we made with the MAX, however, the mask with the widest field-of-view and highest-resolution will always be the Double-Dome masks! Bigger isn't always better; more expensive isn't always better.
The result you would get with plus-powered eyeglasses inside a Double-Dome mask:
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1. Images magnified almost as much as a flat mask. |
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2. Crummy resolution in the central field-of-view. |
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3. Awful resolution past 30 degrees from center. |
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4. Impossibly bad resolution in peripheral view area. |
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5. Terrible fogging; multiple air / optical surfaces multiply the problem. The MAX has sealed lens barrels to avoid fogging. But a sealed chamber must resist implosion; we have computer-designed the MAX barrels to withstand the industry-standard drop-ball impact test while under pressure at 450 Meters. We will not certify diving the MAX deeper than 75 Meters. (top)
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Contact lenses
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| 18. |
Q |
How safe are contact lenses? |
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If you are fit to dive you are fit to wear contact lenses. The risk of eye infection is infinitesimal when a few simple hygiene protocols are followed. If you have an existing eye infection or corneal damage (e.g. scratched/very red eyes) and you have an impaired immune system (e.g. fever) you should not use contacts due to the elevated risk of eye infection during this time – and you should not be diving.
Swimming with contact lenses does not raise the risk of eye infections. But keeping contacts in for many hours or days after swimming IS VERY RISKY. Double-Dome divers who wear "Diving Rx" contacts are highly motivated to remove their contacts after their last dive, to return to their normal vision. Timely removal of contacts virtually eliminates the bacteria found in water.
The cornea is remarkably robust and resistant to infection, but the risk to one's vision, if basic hygiene protocols are ignored, can be severe. These protocols are detailed by your eye care professional during your contact lens wear instruction. It is the individual diver's responsibility to carefully obey these guidelines for proper use and to preserve the health of your eyes.
The US Navy, DAN (Divers Alert Network), PADI and NAUI all certify contact lenses as safe for diving... as long as you discard them or disinfect them at the end of the day. Soft lenses have a great track record for not falling out during dives, unlike older hard lenses. The latest-generation of disposable contact lenses are so comfortable that many 20/20 divers don't feel anything in their eye -- most lenses are more than 50% water!
If you require contact lenses to dive with our Double-Dome masks and do not have a current eye care professional use our Zip-Code Locator to find a local DEC-Pro (Global Network of Diving Eye Care Professionals). Many of these Eye Care Professionals are scuba divers with decades of experience who understand the divers needs. (top)
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| 19. |
Q |
Can I use one-day disposable contact lenses? |
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A |
Not only can you use them but they are recommended. See previous question, #18. (top)
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| 20. |
Q |
Can I use extended-wear contact lenses? |
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A |
Yes, but we don't recommend sleeping in them overnight; they should be removed and properly disinfected at the end of the diving day. For 20/20 divers, one-day disposable lenses are the most economical and convenient method.
Swimming with contact lenses does not raise the risk of eye infections. But keeping contacts in for many hours or days after swimming IS VERY RISKY. Double-Dome divers who wear "Diving Rx" contacts are highly motivated to remove their contacts after their last dive, to return to their normal vision. Timely removal of contacts virtually eliminates the bacteria found in water.
The cornea is remarkably robust and resistant to infection, but the risk to one’s vision, if basic hygiene protocols are ignored, can be severe. These protocols are detailed by your eye care professional during your contact lens wear instruction. It is the individual diver’s responsibility to carefully obey these guidelines for proper use and to preserve the health of your eyes. (top)
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| 21. |
Q |
Which contact lenses are right for me? |
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A |
This is a question that only you and your eye care professional can answer together. To give your eyes maximum comfort, not just the diopter power is considered; lens diameter and base curve are also important. We have established our DEC-Pro Network (Global Network of Diving Eye Care Professionals) to ensure that each and every diver using the Double-Dome system is completely satisfied, and continues to enjoy razor-sharp underwater vision and almost 5X times the view! (top)
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Divers with prescription needs
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| 22. |
Q |
I'm confused about the difference between Nearsighted and Farsighted (called Shortsighted and Longsighted in the U.K.). |
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A |
Our Glossary is very extensive and should be able to find the answers to your questions there. |
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NEARsightedness: same as myopia (called SHORTSIGHTED in the U.K.); enhanced ability to see close objects, inability to focus on distant objects. |
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FARsightedness: same as hyperopia (called LONGSIGHTED in the U.K.); usually able to focus on distant objects, inability to focus on close objects. (top)
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| 23. |
Q |
Do the Double-Dome masks include corrective lenses for nearsighted divers? |
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A |
No extra "glue-on" or replacement lenses are attached on the inside of the Double-Dome lens. The Double-Dome has zero optical power out of water, but when a Double-Dome mask is submerged, the water itself, curving around the domes, magically creates a negative diopter lens. Unlike all conventional Rx lenses that have specific diopter power, a Double-Dome mask corrects for a wide range of nearsighted prescriptions.
If you are not a Naked-Eye-Match, just use affordable disposable contact lenses to adjust your vision to match the Double-Dome optics.
You will need contact lenses if:
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A. You are nearsighted, but outside the required accommodation range.. |
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B. Farsighted. |
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C. 20/20 and more than about 22 years old (we all lose our ability to focus close as we age according to this chart). (top)
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| 24. |
Q |
I am nearsighted. Do I have to see my Eye Care Professional in order to purchase a mask? |
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A |
Not necessarily. The MEGA 4.5DD works, without contact lenses, if your prescription is generally in the range of -2.5 to -5.0 diopter, and astigmatism less than ± 2.0. Please email us a copy of your prescription, along with your age - we will get back to you quickly. If you do not know your prescription, your Eye Care Professional should provide you that info free of charge. If you do not currently have an Eye Care Professional, may we recommend one of the members of our Global DEC-Pro Network (Diving Eye Care Professionals)? (top)
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| 25. |
Q |
I don't wear contact lenses; I wear glasses (nearsighted with astigmatism and reading). Can I use the MEGA 4.5DD mask? |
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A |
Depending on your prescription. If your prescription is within the Naked-Eye-Zone on the Rx Conversion Chart then you will not require contact lenses. If you are out side the range you will need contacts to bring your visions inside the range of the mask. The new generation of disposable soft contacts work for a wide variety of vision needs, for folks well into their 80's. There are now affordable disposable contact lenses for those of us who need bifocals. And disposable contacts to correct astigmatism (toric lenses). The odds are very good that you can be fit with latest-generation comfortable soft lenses, and you will be able to use our MEGA 4.5DD mask.
If you have less-than-favorable memories of the "old-style" hard contact lenses we're sure you'll have a much better experience with modern disposable/soft contact lenses.
For maximum comfort and your own enjoyment, we'd suggest you first try above-water vision contacts; we think you'll be delighted at your improved vision. Then it is easy for your Eye Care Professional to modify your prescription to match the optics of our MEGA 4.5DD mask.
If your current Eye Care Professional dispenses contact lenses and you want to remain with that provider, you can e-mail us his or her name / phone or e-mail and we'll contact them on your behalf and explain your needs to them; that saves you time.
If your current Eye Care Professional does not dispense contact lenses, we'll refer you to one of our DEC-Pro members.
Our DEC-Pro Network is growing daily... we'll refer you to the contact lens provider who's geographically closest and also an expert with:
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1. The tremendous range of new-generation disposable soft contacts. |
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2. Your special needs as a scuba diver. |
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To help you decide on what you'll need just click here and follow the instructions. (top)
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| 26. |
Q |
I'm in my 50's; I don't wear contact lenses, only reading glasses. Can I use the Double-Dome masks? |
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A |
If you only wear reading glasses you will most likely require contact lenses to use the Double-Dome mask. Once you use the proper contact lenses to use the Double-Dome mask you will experience the Magic Bifocal effect underwater. Your eyes will focus close, like they did when you were in your 20's! You will not need bifocals underwater if the Rx strength of your bifocals or reading glasses is +2.0 or less. If +2.25 or stronger, then the latest bifocal contacts or monovision will give you great close-focus ability. The new generation of disposable soft contacts work for a wide variety of vision needs, for folks well into their 80's. There are now affordable disposable contact lenses that will work great for your eyes and the Double-Dome masks. As we get older, it's a wise idea to have regular eye-health examinations anyway... the additional cost of getting fit for contact lenses is a small price to pay for a view that's priceless. (top)
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| 27. |
Q |
I already have an eye doctor. Do I have to see one of the members of your DEC-Pro Network (Diving Eye Care Professionals)? |
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A |
No. If your Eye Care Professional is not a member of our DEC-Pro Network but you think he or she might be interested in joining, please send us his or her name and phone number, so we may explain your needs. Your Eye Care Professional will immediately understand the simple optical principal behind the Double-Dome System. Please download this 3-page PDF for your eye doctor. (top)
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| 28. |
Q |
If the Double-Dome masks correct my nearsighted vision underwater, but has no optical power above water; won't I have fuzzy vision above water? |
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A |
Yes, you will have fuzzy vision above water, but we have two solutions: |
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1. Our Monocle comes with Double-Dome masks. It has a standard -4.5-diopter lens -- custom diopter power lenses are available through HydroOptix. The Monocle easily slip-locks around your wrist on a lanyard, so it's quickly available. |
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2. SCUBA DIVERS: our patented CoverLens™ comes standard with the MEGA -4.5DD mask and temporarily clips to the front of the mask, giving you sharp vision above water. After you enter the water, unclip the CoverLens from the front of the MEGA 4.5DD mask and stow it in the provided Stowing-Plate. During your safety-stop at the end of your dive, snap the CoverLens back onto the MEGA before surfacing. |
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The CoverLens comes standard with -4.5 diopter -- custom diopter power CoverLens inserts are available through HydroOptix. |
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Our optional SI-Kit (Surface Interval Kit) provides you with prescription eyewear and a place to store everything you'll need: |
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1. Transparent Dry Box |
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2. Mirror |
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3. A Pair of prescription eyeglasses |
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4. Custom-matched polarized clip-ons |
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5. Sports Retainer |
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4. Protective eyeglass case. (top)
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| 29. |
Q |
How can the Double-Dome Lens act as a universal lens for a range of nearsighted divers? |
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A |
The prescription or strength of your eyeglass or contact lenses are determined by three things, the inside curve (R2), the outside curve (R1) and the thickness of the lens. Changing any one of these values changes the prescription of the lens. If the curves remain unchanged but the thickness is increased the lens become more powerful, if it is made thinner the lens becomes weaker. When using a Double-Dome lens mask the mask lens has zero optical power but once underwater the water forms around the lens and water has a higher index of refraction than air, the water gets curved into an optical lens. Now the inside curve (R2) is where the water curves around the domed lens and the back of the lens (R1) changes depending where you are focusing. If you are looking at your dive buddy 30 ft. away you are looking through a 30ft. column of water. This creates a –4.5 diopter lens in the water and allows you to focus far away. If your buddy starts swimming towards you and the distance becomes less the power of the lens created in the water becomes weaker. When you look at your gauges, which may only be twelve inches away, you are looking through a very short column of water. This water lens is actually weaker than -4.5 diopters allowing your nearsighted vision (na |
