Most lenses are made from tempered glass, which minimizes distortion, cuts down fogging and offers maximum scratch resistance. In most cases, corrective lenses, to your precise prescription, will be permanently bonded to the rear of the mask’s original flat ( plano ) lens, though, in cases of very high prescriptions, they may be bonded to the front surface. In some instances, depending on mask construction, corrective lenses may be fitted in place of the original, non-prescription lenses. Whilst most lenses are made from toughened glass, plastic ( CR39 ) or polycarbonate are increasingly being used. Both these materials have the advantage of being available with tints, which enhance colours in different waters and at different depths. It should be noted, however, that plastics and polycarbonate do not offer the level of scratch resistance found in glass.
Some Technical Bits...
We need to get a little technical to explain the problems which can be encountered when lenses to the nearest, approximate power are used, as found in the ‘prescription’ masks offered on many other websites. First, a few definitions and explanations...
These are used to correct ‘long-sightedness’ ( Hypermetropia ) This describes the condition when distant objects can be seen clearly but near objects are blurred. The higher the power of a plus lens, the greater the effect of ‘tunnel vision’. This can necessitate a special choice of lens for higher + powers.
These are used to correct ‘short-sightedness’ ( Myopia ) This is when near objects can be seen clearly but distant objects appear blurred. The higher the power of a minus lens, the thicker the lens edges become, giving peripheral distortion’. This sometimes requires lenses to be made to a specific size or in a specific material.
The 'cylinder' value of a lens is used to correct astigmatism, the condition where one part of the eye has an irregular curve ( instead of being round, it is shaped more like a rugby ball ). To further complicate matters, astigmatism may also occur at an angle ( this will be the case if you have an ‘axis’ value on your prescription.) If these problems are not corrected your eye cannot compensate, other than to a small degree, so your vision will be distorted and blurred. The only way to achieve correction is by using lenses with a cylinder value. 'Off the shelf' lenses do not have any cylinder correction. This is one of the reasons why we do not recommend their use.
Bifocal ( Read add )
As we get older, usually starting around the mid 40’s, the mechanism that allows the eye to focus ‘up close’ becomes increasingly less efficient and ‘reading glasses’ become a necessity, even though other spectacles are not generally required. Obviously this can present a problem when diving, as some correction will be needed to read gauges, or observe small objects without touching a delicate environment, such as a reef. To address this issue, we can supply masks with 'bifocal' lenses, whereby plain lenses or distance prescription lenses have a small, reading lens segment incorporated into the lower part of the lens.
Precise Prescription - Optimum Vision
DiveVision is a professional optical service and our primary aim is to give the underwater vision you need in order to get the optimum satisfaction and enjoyment from your diving. Any mask supplied by us will be glazed to your exact prescription. We do not use 'off the shelf' lenses with the nearest, approximate power.
The ‘power’ of the spherical and cylindrical correction ( your prescription ) is measured in dioptres. Ready-made, 'off-the-shelf', ‘prescription’ dive masks or swimming goggles, have lenses which come in 0.5 dioptre increments. As a result, they will only be to the nearest approximate power to your prescription, relying on your eye’s flexibility to try and adjust for the uncorrected error. Moreover, these lenses only correct the spherical part of the problem, with no correction for any cylindrical (astigmatic ) element of the defect. In order for us to supply you with the correct lenses, it is essential that the prescription information you give us is totally accurate and up-to-date ( please refer to Supplying your Prescription Details page )
If you are unhappy with your purchase, we will take it back and refund the cost of the mask, but we cannot reimburse you for the cost of glazing as the lenses we fit will be to your specific, personal prescription and the mask cannot be offered for resale with these lenses fitted. ( See Terms and Conditions )