Eyeglasses can correct numerous refraction
problems. The most common are nearsightedness,
astigmatism and presbyopia.
|Concave and convex lenses compensate for defects in the eye, bending
light rays to place them precisely on the retin and correcting vison.
A concave lens is thinner in the center and thicker on the edges. It bends
light so that it converges further back in the eye, reaching the retina
and correcting nearsightedness. A convex lens is thicker in the center and thinner
on the edges, opposite from a concave lens. It bends light to converge further
forward in the eye, landing on the retina and correcting farsightedness. Concave
and convex lenses can be used separately and in combination to correct a variety
of vision problems.
Lens varieties include:
- Single-vision lenses - Single-vision lenses have the same focusing
power over the surface of the whole lens. They may be prescribed for distance,
near or reading-only vision.
- Bifocals - Bifocals meet dual vision needs. The lenses are sectioned
into two or three parts that correct vision at different distances. The upper
part generally has the power you need to see in the distance (or has no correction),
while the bottom portion has the strength you need to read. The line between
the sections of the lens bothers some people, as objects appear to jump when
eyes cross the line.
|Bifocals provide near vision correction, and may alo provid distance
correction in the upper portion.
- Trifocals - Trifocals meet vision needs at three distances. Similar
to bifocals on the top and bottom, there is a middle section designed for
vision just beyond reading distance.
|Trifocals correct vision at three times the distances.
- Progressives - These are similar to bifocals and trifocals but the
variance in the lenses is more gradual, correcting vision at every distance
and eliminating visible lines
|Progressive lenses correct at multiple distances and provide a gradual
transition instead of visable lines and abrupt shifts. Some distortion may
be noticed in the lower corners.
Choosing Lens Materials
Lenses are made from several materials. There are a great many lens formulas
and coatings with different performance characteristics.
- Glass - Original material used for glasses, these lenses are made
mainly of sand (silicon dioxide) plus various elements (such as titanium dioxide
for thinness). Glass lenses are long lasting and naturally scratch-resistant.
They are heavy and breakable but can be treated for shock resistance.
- Standard plastic - Developed in the 1960s, plastic lenses are made
of a hard resin that is light and impact resistant. It is thick and easily
scratched unless treated with a protective coating.
- Mid-index plastic - First used in the early 1990s, higher index refraction
means this material reduces lens thickness without compromising optical performance.
It is recommended for light to moderate prescriptions. A protective treatment
can reduce scratches.
- High-index plastic - The third wave of plastic materials provides
an even higher index of refraction. It is ideal for those with moderate to
severe correction needs because higher power prescriptions can be made lighter
and thinner. A protective treatment can reduce scratches.
- Polycarbonate - The newest lens material is an acrylic-like resin
that is used for airplane windshields. It is the thinnest, lightest and most
impact-resistance of all lens materials, and comes with a scratch resistant
coating. Polycarbonates are also inherently good at filtering UV rays.
Selecting Lens Treatments
All of these materials can be treated to better protect your eyes, improve your
vision, increase the durability of your lenses, and enhance appearance. Scratch-resistant
coatings, ultraviolet light blockers, anti-reflectives, tints and edge treatments
can be combined to meet your individual needs.
- Scratch-resistant coatings - Plastics and polycarbonates are usually treated with anti-scratch varnish. The coating can be used on the front, back, or both sides of the lens.
- Ultraviolet light blockers - While some materials are better than
others at filtering damaging ultraviolet light, the only way to get 100 percent
protection is with specific UV treatments.
- Anti-reflective - Anti-reflective treatments (AR) have two purposes:
eliminate the glare others see when looking at you, and eliminate reflections
and glare that you experience when looking through your lenses. Together,
these effects improve your appearance, comfort and safety. Users typically
notice less glare from fluorescent lighting, computer screens and car lights.
Contrast and detail are improved, reducing eye strain and headaches. AR-coated
lenses used to attract more dust and show dirt and smudges more easily than
non-AR coated lenses. But today, many AR treatments include a final coating
that better repels dust and dirt and makes lenses easier to clean.
- All-in-ones - The newest option is an "all in one" coating, combining
the advantages of scratch-resistance, anti-reflection, and smudge resistance
to offer the ultimate lens treatment.
- Tints - Lenses can be tinted or colored for a variety of reasons.
Most often tints are intended to filter out some light, as in sunglasses. The right tint
for you will depend upon your vision needs and your sensitivity to light.
- Clear - Lenses with no tint allow the maximum amount of visible
light to reach the eye.
- Color - Lenses are available in virtually every color imaginable.
The color chosen can be for sun protection, personal style and to enhance
sight. Sun protection colors are usually grey or brown. Polarized lenses
eliminate the most reflective glare and are recommended for driving and
- Gradient - Gradient lenses have a darker tint on the top and gradually
get lighter toward the bottom of the lens. They are usually chosen for appearance
and sun protection.
- Photochromic - These lenses are clear when indoors and dark when
in the sun. The tint is activated in only a few moments. They are convenient
and comfortable, minimizing the need for both clear and sunglasses.
- Edge treatment - Lenses are cut to fit the shape of the frame. This
leaves a frosted, rough edge. This is especially noticeable with higher prescriptions
that result in thicker lenses. The edges can be polished to provide a finished
look, or left with the frosted look.
"Polycarbonate," L'Opto85, Mar 1997, pp 23-25
Benjamin, WJ, ed. Borish's clinical refraction. Philadelphia, PA: WB
Saunders, 1998, pp. 868-76
Fowler,C. "Why Not Make Your Own Varifocal Lens?" Optician, 1998, vol.
216, no. 569, pp. 18-20
Malaval, C, ed. "Seeing the World Past," Essilor, Paris: Creapress,
1997, pp. 42-29 & 70-89
"Markets," SPC, 1999, vol. 99, pp. 3-6 & 31-32
"Markets," SPC, 1999, vol. 102, pp. 3-6
Mercier, J-L, Miege, C, LeSaux G, Chauveau J-L. "The Design Loop for Progressive Lenses," Points de Vue, Apr, 1996. pp. 22-27