Top Things to Look Out For When Choosing Spectacle Lenses
Choosing the lens is as important as, if not more so than, choosing the spectacle frame. While the frame helps you make a statement about yourself, visual quality and benefit is derived from a pair of accurately prescribed and dispensed lenses.
Progressives / Bifocals / Single Vision Lenses?
If you are about 40, and having difficulty focusing on books, computer and basically wishing your arms could be longer, you are probably experiencing the onset of a condition known as Presbyopia/"Lao Hua Yan". It is when the crystalline lens which used to be flexible to allow focusing at different distances begins to stiffen. The stiffening of the crystalline lens makes it progressively more difficult to change focus and thus to see clearly at all distances. When presbyopia begins, people who already wear glasses may need bifocals or progressives in order to see comfortably up close. People who have never worn eyeglasses find that they need reading glasses or bifocals or progressives in order to read and see up close. It is important to note that Presbyopia affects everyone including those who have cataracts and also those who are short-sighted.
The most common way to correct for presbyopia is to use spectacles with bifocal or progressive addition lenses. Bifocal lenses have two points of focus: the main (top) part of the spectacle lens contains a prescription for distance vision, while the lower portion of the lens holds the stronger near prescription for close work. Progressive addition lenses are similar to bifocal lenses, but they offer a more gradual visual transition between the two prescriptions, without a visible line between them. With progressives lenses, you can look up and see clearly across the room and far into the distance. You can also view your computer in the intermediate zone and drop your gaze downwards to read and do intricate work through the near zone of the lenses.
A corridor of optimum power will progress vertically down each progressive lens. There will be slight image distorsion especially at the side of the lens because all the residual power will be pushed to the sides. Your eye care professional should take careful measurements to ensure that you are looking through the centre of the lens when you are relaxed and looking straight ahead, so that your eyes can naturally access the various powers within the lens.
With modern technology, it is relatively easy to adapt to progressives. Adaptation can last from a few minutes to two weeks. However, many progressive lens users have commented that vision is more comfortable when the depth of the frame is wider. This is so because the progression of powers can be more gradual down the lens.
Photochromatic Lenses / Photogrey / Transitions
These are lenses that darken when exposed to ultraviolet (UV) radiation. They adjust automatically to changing light conditions to allow just the right amount of light to reach the eye. The higher the intensity of the UV, the darker the lenses will turn. Photochromatic lenses are clear indoors and at night and get as dark as sunglasses outdoors, helping to reduce glare, eyestrain and fatigue. They also block 100% of harmful UV rays, to help protect the health of your eyes. Long term UV light exposure is thought to be associated with cataract formation and retinal degeneration. Transitions lenses are the predominant brand of photochromatic lenses sold today. Previous Transitions Lenses blocked UVA (315 to 380nm) and UVB (290 to 315nm), and the recent Transitions VI has expanded protection up to 400nm. UVB is accountable for sunburns and UVA is more harmful causing damage to the eyes. One thing to take note of is that photochromatic lenses do not turn dark inside the car because the windshields block most of the UV rays that cause photochromatic lenses to activate, or darken. If the intention is to block out the glare when you are driving on the road in the day, it is advisable to get dark tinted sunglasses instead.
Plastic lenses without coatings transmit about 90% of light to the eye. The rest of the light is lost as it is reflected off the front and back surfaces of the lens. Having an anti-reflection coating allows more light to be transmitted to your eyes, which will benefit your vision in many aspects. Anti-reflective(AR) coatings provide better night driving vision than identical lenses without AR coatings by eliminating the reflections of streetlamps and headlights on your lenses. People also tend to be more comfortable working at a computer when wearing lenses with an anti-reflective coating. It will help to reduce eye strain and fatigue.
Another advantage is that other people can now see your eyes more clearly because lens reflections are now eliminated. However, lenses with an anti-reflective coating may require more frequent cleaning than uncoated lenses because any smudges on the lenses will now be easily observed.
Anti-reflective coatings are especially important if you wear high index plastic lenses because lens materials that have a high index of refraction reflect more light than regular plastic or glass. Anti-reflection coatings also help to minimise internal reflections which make the lens look thicker than it actually is, and external reflections which will make the person's eyes less visible to others.
Anti-reflective coatings are highly recommended for all spectacle lenses.
Scratch-resistant coatings are created on the lens during the manufacturing process. This coating does not affect how the lens functions and does not interfere with vision. It contributes to the durability of the lens. Although it is not 100% scratch proof, it helps to prevent minor scratches that will occur on a regular lens. These minor scratches over time can damage the surface of the lens and impair vision.
Impact resistant lenses
When choosing lenses for children, the top priority considered should be safety. Children are generally more easily excitable and they also tend to be more prone to accidents. As such, there is a significant risk of a glass lens shattering with pieces entering the child's eye. The same applies to workers in an environment whereby they are exposed to flying particles, dust, metal shavings, fibreglass and glass. Impact-resistant lenses are also important for sports especially when they involve objects travelling at a very fast speed, like a squash ball. Another potential danger with squash is that the racquets themselves move at high speed in a confined space and often make contact with one another.
The best recommended impact resistant lens material in the market now is polycarbonate. Polycarbonate material is manufactured in a different way as compared to other lens materials. The raw material is compressed by an enormous amount of pressure. The resulting product holds up so well under impact not because it is hard and unyielding, but more because it is flexible and gives slightly under pressure without breaking. Studies have been made detailing exactly what happens when an object travelling at high speed hits a polycarbonate lens versus a traditional plastic lens. Using film that is slowed down and viewed frame by frame, one investigation demonstrated that a fast-moving ball coming into contact with a plastic lens broke the lens and impacted the eye. In contrast, the same ball travelling at the same rate of speed impacted the polycarbonate lens and flexed the lens but did not break it.
However, one of the very few weaknesses or drawbacks of polycarbonate lenses is that they are not as good optically as high index or plastic lenses. They are associated with higher 'chromatic aberrations', which means that 'rainbow fringes' may be noticeable around objects with high contrast especially with higher power lenses made with this material. This effect is also more prominent when one is looking out the side of such a lens.
At Jerry Tan Eye Surgery, we will assess your visual expectations and take into consideration your lifestyle, occupational and leisure needs, to best determine the type of lenses most appropriate for you.