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Read answers to some of our most frequently asked questions.

What types of lenses are available for dive masks?

There are three types of lens available for dive masks. These are Single-Vision, Bifocal and Reading Segments. Single-Vision is used primarily for correcting distance vision and the lenses will fill as much of the lens aperture as possible, within the limits of the lens blanks we have to use, the style of the mask and the individual prescription.

Bifocals use most of the lens area for correcting distance vision, with an area at the base of the lens for correcting close vision, for reading gauges, camera settings etc. Read segments are placed in the lower portion of the lens aperture and are used to correct near vision, where no correction is required for distance. For cost purposes, they are classed as single-vision lenses.

There are no varifocal lenses available for dive masks. This is simply because there are no flat-fronted, lens blanks made that would enable us to do this.However, bifocals probably account for some 50% of all the orders we receive. These look like one single piece of glass, with a small, virtually indiscernible, 28mm reading segment at the bottom. This segment is well out of line of sight when you are swimming ahead, but positioned to be exactly in line of sight when you want to read your computer or gauges, check camera settings or look at anything up close. We carefully set the height of the top of the segment depending on the make and model of mask.

For specific applications, such as photography, we can provide a much larger read area if required.However, it is only possible to supply larger read segments in those instances where a read segment is all that is required. With no correction for distance vision, the read segment is, in effect, a single-vision lens, and can be as large as we care to make it, within the confines of a 65mm diameter lens. However, this will only be a viable solution if your distance vision is good without any correction. Modern bifocals are made using 'fused lenses', whereby the read segment is set into the overall lens. This gives it fixed dimensions of approximately 28mm wide X 15mm high. We can adjust the height at which the segment is positioned, but we cannot change its size.

The only way we can give you both distance correction and a LARGER read segment, is by resorting to older style 'Franklin Split' lenses. This entails having two separate pieces of glass in each aperture, one for distance and one for reading, butting together with a horizontal join near the centre of the lens aperture. Not the most attractive solution, but nevertheless effective.

How long does it take?

We normally deliver in the UK within orders within 14 days of receipt of order.

However, we do ask that you allow 28 days, from receipt of order to delivery, to allow for eventualities such as lens breakages.

Obviously, overseas orders also have to allow delivery time. Depending on service availability, we dispatch via 'Airsure' or Royal Mail 'International Signed-For'.

Will you be replacing the complete lenses?

The short answer is ‘No’. This isn’t a failing on our part, nobody can. There are no flat-fronted, corrective lens blanks made that are large enough to fit the entire lens aperture of any dive mask.If there were, the edge thickness of higher correction minus lenses and the centre thickness of plus lenses, would make the lenses very heavy. We endeavour to make the lenses as thin and light as we possibly can, for any given prescription. Some mask manufacturers do supply replacement, corrective lenses,“off the shelf”. These are often for minus corrections only, they make no correction for astigmatism ( Cyl.) and are only available in half, as opposed to quarter dioptre increments. As specialists in ‘precise prescription’ glazing… exactly the same as the prescription in your everyday glasses or contact lenses, we do not supply ‘off the shelf’ lenses.
Moreover, many masks do not have removable lenses.

How are the prescription lenses attached?

The lenses, which are made from high-quality optical glass, are permanently UV bonded to the inside surface of the mask’s original, plain ( plano ) glass faceplates. Unlike the self-adhesive lenses that are available over the counter, our lenses will not come off.

Are all masks suitable for all prescriptions?

As a general rule, all the masks shown on our website are suitable for the majority of prescriptions. The only exception to this relates to masks described as ‘Low Volume’, which are not recommended for higher prescriptions – especially with plus correction. This is because the thickness of the lenses ( at the centre with plus correction and at the edges with minus prescriptions ), will tend to place the glass too close to the eyelashes, which can prove very aggravating to the user.

Can you recommend a mask?

This is a question we try to avoid answering, purely because the fit and feel of a mask is such a personal thing. All faces are different, all masks are different and one person’s favourite is another person’s pet hate.

All the masks we show on our website are good quality, known brands, from reputable companies. We can tell you which ones are ordered most often and we can tell you what other people say about them… but we cannot make a firm recommendation as to which you should buy.

Do I need to clean the lenses with toothpaste before using the mask?

No, any film on the glass will have been removed by our bonding process and we clean the lenses thoroughly before dispatch.

Does being under the water alter the prescription?

Yes it does… Water being denser than air, it has a higher refractive index. However, this is countered to a large extent by the fact that the lenses are further from the eye than they would be with spectacles. We always glaze to precisely the prescription supplied by any customer as we find this achieves the best result in every case.

Why not just wear contact lenses?

No reason whatsoever… except, of course, for a little protozoa, called

Acanthamoeba. This lives in both salt and freshwater and can cause Acanthamoeba Keratitis, which is an infection of the cornea - the front part of the eye. This is a painful, sometimes permanently damaging condition which can result in a corneal graft… or even the loss of an eye!

A contaminated contact lens greatly increases this risk.

Secondly, if your mask floods, there is the risk of a contact lens being displaced… which is inconvenient to say the very least. But this can have an even more serious outcome… The higher osmolarity of seawater,

or lower osmolarity of freshwater, when compared to tears, can cause soft contact lenses to adhere to the eyes. To ‘unstick’ them will require irrigation with isotonic saline, something that is not readily available on most beaches or dive sites!

Soft contact lenses can also absorb proprietary ‘de-fogging’ solutions, which can cause eye irritation. Another good reason not to wear them.

Last but not least, some divers who wear hard or gas-permeable contact lenses, can experience blurred vision after diving. This is because gas is under pressure in the mask, causing the tears to absorb nitrogen. When the diver surfaces at a normal rate, the tears and cornea are unable to eliminate the inert gas as quickly as they would without the contact lenses in place, causing bubbles to form between the lenses and cornea, which will affect the surface of the cornea, causing the blurring.

Can’t I just get “off the shelf” lenses from my dive-shop?

Certain mask manufacturers offer a limited range of replacement ‘corrective’ lenses, which actually fill the entire lens aperture. However, as these come in a limited range of prescriptions, go in half dioptre, as opposed to quarter dioptre, increments and make no correction for astigmatism ( the Cyl. Part of a prescription ), they are only suitable for a small percentage of people.

It’s a bit like buying ‘off the peg’ clothes… If you are lucky enough to have a prescription of, for example, +2.50 right eye sphere and -1.00 left eye sphere, there will be dive-shop lenses that ‘fit’ you perfectly.

However, if your right prescription contains figures like -2.25 Sph. with -1.25 Cyl. an Axis of 135 and a +2.25 add, you are going to need precise ‘made to measure’ lenses. Moreover, as sight4sport is run by a qualified optometrist, you can be confident that DiveSight will give you the best advice and that the prescription is correct and the lenses are set to your optical centres (inter-Pupilliary Distance ), an important factor in giving you optimum underwater vision.

We are also able to advise and cater for very high corrections ( above +/-10.00 Sphere and 4.00 Cyl ) and to allow for prism, where applicable.

Can you glaze my existing mask?

In a word, 'Yes'. Whilst we are always more than happy to supply a mask, we are able to glaze a mask you already have - with the proviso that it has tempered-glass faceplates and is in good condition. We will require your mask for a minimum of two weeks. 

Can I come and see you and try some masks on?

In a word, 'No'. It's not that we are unfriendly, it's simply that you would have a wasted journey... As ophthalmic specialists, we are not actually a 'public-facing' company - we do not have retail premies and, more importantly, we do not carry a stock of masks. We have arrangements with all the major agents and distributors, to buy masks as we need them. Many of the mask manufacturers have minimum order requirements and, given the range of models and colour options, it is simply not a viable or economic proposition to carry stock.
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