WHEN FUNCTIONING PROPERLY, light coming in through the pupil in the front of the eye is refracted so that an image is focused on the retina at the back.
The cornea is responsible for two-thirds of the refractive power of the eye. The lens accounts for the remaining third, and focusing is achieved when ciliary muscles change the shape of the lens, in a process called accommodation.
These muscles weaken as we get older so that, by the time most of us reach our 40s, the lens can no longer change shape to accommodate nearby objects.
You can spot people with this condition, presbyopia. No matter how far they hold a book out in front of them, the words won’t come into focus.
So people in middle age swell the ranks of those who already need vision correction.

Your prescription
A dioptre (D) is a unit of measurement for the optical power of a lens. In humans, the total optical power of the relaxed eye is around 60D.
The reciprocal of this number gives you the focal length of the eye, which is 1.5cm – about the length of the eye.
If you have a cylinder correction, it means you have astigmatism. This is when focusing is uneven so that, for example, horizontal lines appear more focused than vertical lines.
Myopia (short-sightedness) is a sight defect where the image is formed in the eye in front of the retina because the eye is too long, or its focusing power too strong. Negative correction in terms of dioptres is required.
In hyperopia (long-sightedness), the image is formed behind the retina because the eye is too short, or its focusing power too weak, so positive correction is required.
I now wear varifocal spectacles because my prescription is different for near, intermediate and distant vision, and a small correction for astigmatism.
I need a +3D correction for distance vision as well as +1.75D for reading.
I can read my gauges if I wear +3D contact lenses, suggesting that wearing a mask under water is equivalent to adding about +1.5D to the focusing power of the eye. This is not surprising, because under water objects appear about a third bigger and 25% closer than they actually are.
If you have a prescription of less than +1.5D, it may be worth putting off buying underwater correction until you have tried wearing a mask without it.

The case for prescription masks
Stick-on magnifying lenses are fastened onto the inside of the tempered glass of
a mask, but they have a reputation for fogging up or falling off and are, in my opinion, a false economy.
Most scuba-mask manufacturers offer off-the-shelf single-vision prescription masks. There is more choice for negative dioptres, and these typically cost £20-40 for each lens, so up to £80 added to the price of the mask itself.
If your vision problem is simple, without too much astigmatism (max 3 or 4D), these off-the-shelf lenses will do a good job.
Bifocal (though not varifocal) masks are also available. The IST Mirrage filled the gap in the market left by the discontinued Sherwood Magfour.
The top two-thirds of the lenses are plain glass. The bottom third has +1.75D lenses and this is what you look through to see your gauges more clearly – ideal if you have presbyopia or are longsighted and have a problem with close work. They cost around £40.
Bifocal masks with lenses ground to your individual prescription, including corrections for astigmatism, are also available. As usual, a good fit is essential so, if possible, have prescription lenses fitted to your favourite mask.
But most companies that fit lenses for your prescription do so to a limited range of masks, so it is essential that you try these on to find the best fit before commissioning the work.
Without securing the mask, hold it to your face, breathe in gently and hold your breath. Let go of the mask.
If it stays on your face, it has passed the most basic test for fit.
Because masks change the shape of your face, this should be done with your regulator in place and wearing your hood, if you have one.
Your dive shop can help you out with a prescription bifocal mask, or try one of these online traders:
Simply Scuba (www.simplyscuba.com) offers a mask with zero power at the top from £60. Prescription Swimming Goggles (www.prescription-swimming-goggles.co.uk) offers both zero power at the top from £165 (including the TUSA Ceos mask) or different powers top and bottom from £180 (including the Unidive 9447 mask).
Different powers top and bottom with bonded lenses are offered by both DiveLife (www.divelife.co.uk) for £191 and DiveSight (www.divesight.co.uk) for £96.
DiveSight says there is no additional cost for having the cylinder and axis corrected too.
It is also worth bearing in mind that, for optimum results, the optical centre of each lens should be lined up with each pupil. For this, the manufacturer needs to know your pupillary distance (PD), the distance between your pupils. Ask your optician, or work it out at home.

The case against prescription masks
Though it will instantly double costs, divers should consider taking a back-up prescription mask, particularly on trips to remote locations where sourcing a replacement would be difficult, if not impossible.
The back-up would be of limited use to other divers in an emergency.

The case for contact lenses
Soft daily disposable lenses are the best option for scuba. They significantly reduce the chance of infection, but also are gas permeable.
This is important, because the cornea has no blood supply, so the only way for it to obtain oxygen and get rid of waste products is through the tears or direct exchange with the air covering the eye.
Contact lenses are classed as either hard, rigid gas-permeable (RGP) or soft.
Hard lenses, though good for dusty environments, should be avoided for scuba because they are not gas- permeable and prevent nitrogen gas escaping. This “may cause blurry vision when air bubbles become trapped between the lens and the eye” warns Diver’s Alert Network (DAN).
This happens when bubbles become larger in diameter than the thickness of the tear film and create indentations on the surface of the cornea, and is why super-slow ascents are recommended for divers wearing hard lenses.
The UK Sports Diving Committee adds that “hard contact lenses have been shown to cause corneal oedema (ie swelling) during decompression and after dives… caused by the formation of nitrogen bubbles.”
Hard lenses are also smaller than the area of the iris (the coloured part of the eye), increasing the risk that they can be washed out if the mask floods.
Varifocal (or multifocal) contact lenses are available, even as daily disposables. They combine reading, intermediate and distance prescriptions, so that wearers have simultaneous vision for far away and close up. They can also include correction for astigmatism.
Not everyone gets used to them, so it’s worth taking advantage of a free trial.

The case against contact lenses
The most significant hazard to contact lens-wearers is microbial keratitis.
Though extremely rare, this disease, caused by microbes such as the Acanthamoeba and the bacterium Pseudomonas aeruginosa, can lead to blindness if not treated.
Research in 2006 revealed that more than 85% of acanthamoeba keratitis cases occur in wearers of contact lenses. This has been traced to poor lens hygiene. Although amoebae were found in daily disposable lenses, none of the potentially pathogenic strains were observed.
Before handling contact lenses, you need to wash your hands with soap and water, then rinse and dry them, preferably with a clean, lint-free towel.
Given that your mouth is not sterile, rinsing lenses with saliva is not a good option, though I’ve seen it done.
Every year some 5% of contact lens-wearers experience complications. Poor hygiene aside, this is mainly because people wear them for ridiculously longer than the prescribed time.
Mishandling can also cause problems. For example, scratching the cornea with long fingernails can lead to infection.
Except for those wearing daily disposable lenses, protein in your tear film accumulates on your contact lenses, creating a thin haze fogging your vision.
Eventually, the deposited proteins denature (change form), tricking your immune system into thinking that the deposit is something foreign. So you produce antibodies that can lead to eye irritation, itching and redness.
DAN also warns that increased pressure under water can cause contacts to stick to the front of the eye, though this is very rare. Divers are advised to stow re-wetting drops to help dislodge stuck lenses.
It is also thought that osmosis can cause soft lenses to change shape. Sat on the front of the eye, the lens is bathed in tear fluid, which has a salt content between that of fresh and sea water.
If wearing lenses, it is important to close your eyes if your mask floods, making it highly unlikely that water will enter your eye.
But if your eyes were flooded with fresh water, the lenses might contract, which would cause discomfort and make the lens difficult to remove.
If splashed with salt water, the lens might swell, making it more likely for them to be flushed out.
Another disadvantage of soft contacts is that they can absorb the defogging chemicals some divers use in the mask, which can irritate the eye.
Either avoid these chemicals, rinse them out carefully, or use spit.

Daily disposal
Soft daily disposable contact lenses come in a blister-pack and, provided the seal is not broken, each lens should remain sterile immersed in its own solution.
Sceptical about the stated shelf life of disposable contact lenses, a couple of scientists investigated in 2008.
15% of all lens solutions tested positive for bacterial contamination, whether or not the lenses had expired.
This looks alarming, but bacteria are ubiquitous and a viable population of microbes must be present before infection is likely. Good lens hygiene will reduce the number of microbes entering the eye, but they cannot be eliminated.
The results may suggest that we can safely ignore the expiry date but I would err on the side of caution and dispose of “expired” lenses. They are cheap in the context of an expensive hobby.
My lenses have a shelf-life of four years, and 32 pairs cost me about £12.50.
If fresh ones were used for every dive, this would equate to 40p a dive, but
I keep mine in for the second dive, as changing them at a typical dive site is likely to introduce more microbes.
Until recently, I worried about how I stowed spare lenses with kit left topside while diving. The inside of vehicles can get very hot, particularly overseas, but apparently contact lenses are sterilised after sealing in their individual containers at temperatures above 100°C.
However, I will still store them in a sealed Perspex box for security and keep them out of direct sunlight.
Silicone hydrogel contact lens material has improved the comfort and wearing time, particularly for those with dry eyes. Silicone is gas-permeable and has now been developed with so much oxygen permeability that these lenses are approved for overnight (extended) wear.

Orthokeratology (Ortho-K) temporarily improves vision during the day by reshaping the cornea with the use of oxygen-permeable contact lenses worn overnight.
This is most beneficial for those with myopia (up to -5D) and -1.5D of astigmatism. Though more complicated and expensive, long-sighted people (up to +3D) might still be suitable candidates for this procedure.
The European Academy of Orthokeratology (www.eurok.eu) has more information, or watch Philip Schofield’s experience on YouTube.
In 2005 this procedure cost £200 for the initial consultation and then £40 per month for lenses, an annual bill in excess of £500. It has the advantage of being reversible.

Because the cornea provides most of the focusing power and is easily accessible, a laser can be used like a knife to cut away sections of it to correct vision.
Laser treatment such as LASIK typically costs £2000, and is not recommended for those with dry eyes.
Refractive lens exchange, also called lens-replacement surgery, is the same procedure as cataract surgery, which is performed on 300,000 patients a year.
In cataract patients, the cloudy natural lens is replaced with a silicone lens. It is touted as being a permanent solution to sight defects and ideal for those who are long-sighted and who would be left needing reading spectacles after laser treatment.
I was offered this treatment for £4000 but felt the tiny risk of complications was too big for me, after a friend helpfully quipped: “Does it come with a free guide dog”
Any problems, such as ghosting of images, are extremely difficult or impossible to correct. Whether you choose this option comes down to your bank balance and your attitude to risk.
Diving is possible after most types of corrective eye surgery. However, a surgical procedure that involves cutting the eye (as opposed to laser surgery) and treatment for serious conditions such as glaucoma may weaken the eye.
Divers must not dive until they get the green light from their consultant, to give time for their eyes to heal.

Some divers wear a distance contact lens in one eye and a reading lens in the other. You could wear contact lenses for distance vision and a bifocal mask with a blank lens at the top and a reading prescription in the bottom.
Or you could invest in a prescription mask and have daily disposable lenses as a back-up, or vice versa.