Monday, July 28, 2008

Prescription Eyeglasses

If you have thought carefully about what lifestyle needs your glasses will have to accommodate and have purchased a durable set of frames, then you will probably only need to purchase a new set of eyeglasses when your prescription changes. Many people like to purchase a new set of glasses once or twice every 10 years or so simply to update their look. Scratch-resistant lens coating and shatter-resistant lenses are two product options that can easily extend the life of your glasses. Spring hinges are another good feature for improving the durability of your prescription eyeglasses.

If you don’t like the idea of regularly purchasing new eyeglass frames, avoid selecting very trendy eyewear. The hot fashion of today will become the fashion faux pas of tomorrow, and you may be stuck feeling behind the times while you wait for your look to come around again.

Another good way to avoid the trouble of having to buy eyeglasses later on is to buy two or more pairs now. When purchasing new eyewear, take advantage of the two-for-one specials that many frame dealers periodically offer. Your life, like that of most people, is multifaceted. A single pair of glasses may not be able to be both sporty and professional, but with two or more pairs of glasses to fulfill your optical needs, buying a new pair of glasses every couple of years won’t be necessary.

If you wear prescription eyeglasses, it is very important that you regularly schedule a complete eye examination. There are two main reasons for this. First, your prescription can change gradually overtime, decreasing your quality of vision almost imperceptibly. This means that having your eyes examined by an eye care specialist is the only way to ensure you are getting the maximum amount of vision correction out your glasses. Second, people with pre-existing eye conditions, even minor ones, often have a greater likelihood of developing more severe eye disorders. In their earliest stages, these eye diseases have few to no visible symptoms. Thus, if you wait until you think there may be something wrong, it may be too late to effectively treat the eye disease without losing some quality in your vision.

If your prescription has been stable for a number of years and you do not have a personal or family history of serious eye disorders, then you can probably get away with going to the eye doctor every other year. Children and seniors should get their eyes tested at least once a year because they are more vulnerable to eye problems than people in other age groups. Those who have been treated in the past for serious eye conditions such as glaucoma or cataracts need to be examined by a trained ophthalmologist at least once a year, and more frequently if a doctor recommends it. Persons with severe diabetes have a higher risk of developing diabetic retinopathy and other anterior eye problems. Diabetics should consult their primary physician about how often they should see an ophthalmologist.

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Less Magnification or Minification of the Eyes

Also, the flatter aspheric lenses end up positioned closer to the face. This is a major benefit for anyone wearing a strong correction.

Aspheric lenses lessen the magnification of the eye usually seen in strong plus lenses.

For strong farsightedness, aspheric lenses reduce magnification of the eye. The eye at left is seen through an aspheric lens. Through a conventional lens, the eye at right appears larger, even though the lens is the same power.

Strong farsighted lenses have a tendency to enlarge the wearer's eyes, producing an unattractive magnified look. Strong nearsighted lenses do just the opposite: they minify the wearer's eyes so they tend to look small and beady. Positioning a strong correction closer to the eyes lessens this magnification or minification, for a more attractive, natural look.

In minus lenses for nearsighted people, the front surface curves steepen away from the center and toward the lens edge. As a result of this gradual curve change, lenses end up with thinner edges. From an appearance standpoint, the thinnest possible edges for a strong nearsighted lens are produced when the aspheric lens is made of a high-index material.

An even newer type of aspheric lens for strong nearsightedness has aspheric curves on the back side of the lens. This produces the thinnest possible edges for very strong minus prescriptions.

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Buying Eyeglasses With Aspheric Lenses

Most popular lens designs can now be ordered in aspheric form, including bifocals, progressive addition lenses (no-line bifocals) and single vision. Some are made from high-index materials, too.

For several reasons, frame selection is important with aspheric lenses. In general, the best looking eyewear results when the frame is not overly large and when the eyes are centered in the middle of the frame opening. Your eye doctor or optician will guide you in selecting the best type of frame to use with your new aspheric lenses.

Taking measurements for aspheric lenses requires greater care and skill on the part of the optician, but this only requires an extra minute or two. Creating the complicated curves used in aspheric lenses makes them a little more expensive than conventional lenses. But the outstanding cosmetic and visual benefits of these marvelous lenses make them a good investment.

Since aspheric lenses are flatter and positioned slightly closer to the face than conventional lenses, some wearers may notice more reflections off the flatter back surface of the lenses. The best way to eliminate these reflections is to order an anti-reflective coating, which also improves vision through the lenses.

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Friday, July 4, 2008

Bifocal glasses

Bifocal glasses are used to correct vision at two distances—a prescription on top for far away and a different prescription on the bottom for near. Most people think of bifocals as reading glasses for people over forty who lose their ability to focus up close as they age. But children can also need reading glasses.

Many children have not developed sufficient control over their focusing systems, the natural lens inside the eye that keeps images clear, especially up close. Some children lack the ability to sustain sufficient focusing over an extended time period, so after a while print begins to blur. Others can’t make fast focusing shifts from one distance to another, like from the board to their desks, so any time they look away, everything is blurry. Some children have a tendency to over focus, and the additional stress causes eyestrain and headaches. If they over focus too much, the additional tension on the visual system can make the eyes to turn too far inward, causing double vision. Finally, near work at school places much more stress on the visual system than distance viewing, and some young children respond by translating the visual stress into physical and emotional symptoms—back and neck tension, headaches, constriction of their perceptual fields and a reduction in their visual space, a tendency to develop nearsightedness, and avoidance of the reading tasks that are causing the physical and visual discomfort.

Prescribing reading glasses effectively treats many of these problems. A convex plus lens relaxes the child’s focusing system, relieving much of the visual stress. In fact, prescribing a low power plus lens is so effective in keeping children’s visual system comfortable during extended close work at school that they are often called “learning lenses.”

Reading glasses that use a bifocal are a good option for school-aged children who only need the additional correction up close. The bifocal gives them the lens support they need for deskwork but doesn’t change their distance vision. Sometimes vision therapy is also prescribed when the focusing problem is severe enough that additional interventions are also required.

New advances in lenses allow children flexibility in the type of bifocal they choose. Many children still prefer the flat-top bifocal because the line separating the two powers helps them tell exactly where their distance prescription ends and their near prescription starts. However, some children or parents don't like the look of the "line", so for them progressive no-line bifocals are a good option. The lens is made so that the change between prescriptions is so gradual no line appears. Another very popular option is the "half-moon" bifocal. It has the advantage of a clear delineation between powers liked lined bifocals but when the glasses are on the child's face, the bifocal is invisible like progressive lenses.

When bifocals or reading glasses are prescribed, it is important that children wear them for all close work, especially at school and during homework. Sometimes children will only need the bifocals for a few years as they develop control of their focusing system. Others may need the additional near-point support for as long as they are in school and spending a lot of time reading.

Bifocals are an important tool for optometrists when working with children who spend up to eight hours a day using their eyes for reading and school work. By adding an additional lens power for up close, optometrists are able to adjust children’s focusing system to give them better control and eliminate eyestrain, blurred vision, headaches, and fatigue.


Transitions bifocals

Q: I had a patient come in today asking for transitions lenses. He wanted the distance to be photochromic, but the seg he wanted to remain clear.

I was curious as to if such a lens was possible with what little I know of the technology, and if it was, does it exist?

A1: The patient probablly had a glass Photo gray ST 28, probably wants same again, but doesn't know the difference between transitions and photo gray. Glass photo gray ST 28's have clear segs.

As to transitions made this way, I don't think there are any available, but since transtions is a dip, lens could be masked in segment area before dipping.

By: chip anderson

A2: segs on glass lenses are not photochromic, not very appealing, cosmeticly. The seg is a different RI from the rest of the lens that was cast from silver halide laden molten glass. This seg "button" is attached and becomes part of the glass lens in a very interesting process. If you ever have a chance to see glass lenses, especially segmented products being produced at a factory, it is worth seeing.

It is still hard for me to understand why anyone would want that type of look....clear or dark segs and all, with all the great PALS out there, why would anyone want a seg line! I just hit 50 and have had to wear glasses for about 3 years now. The folks I grew up with all hit that age where their arms became to short. I am happy to say only one person among my peers bought bifocals....but his next pair will be a PAL, now that he knows that they exist. His ECP never presented the option to him. Beat that.

I do want to clear up one misconception in the prior posting.

Transitions are not produced by dip coating. Transitions Lenses are produced by placing the photochromic package into the front surface of the lens (imbibition technology) or by bonding a photochromic layer (Transbonding) on Trivex, Polycarbonate and 1.67 Hi Index lens materials.

Both Transitions Lens processes are patent protected.
Thus leaving the dip coating (photochromics disolved in a liquid carrier, lens dipped in the solution, allowed to dry then heated to permenantly place the pc chemicals on the surface of the lens) along with cast in place (photochromic package mixed with monomer (just like glass pc process) and cast in desired lens mold design) as the other options to make a lens photochromic.

By : Jim Schafer

Bifocals for kids

Executive-style lenses for children are overkill. For one thing, these lenses are often prescribed to reduce accommodative esotropia, which often occurs in highly hyperopic children (who need a high plus power). These lenses are excessively thick, heavy, and limited in material availability.

A FT-35 should provide more than enough width for a child (I can't imagine a child turning his/her gaze out that far to read anything -- it's almost a 40-degree eye rotation). Also, a FT will leave a little temporal vision for seeing the ground in the periphery more easily. It should be pretty easy to get a FT-35 in several materials, including polycarbonate.

As far as progressives for children... I would be tempted to use a short-corridor progressive, myself (and only if cosmetics are a serious issue). Most children wearing bifocals do so because of binocular vision disorders, not because of reduced accommodative amplitudes. This means that children can often see clearly anywhere through the distance, corridor, or near zone of the lens. However, if the child doesn't reach the full power of the near zone the benefits of the lens are not realized. This is exactly why bifocals are positioned so high with children. Progressive lenses, which already require more depression of the eyes while reading, must be positioned several millimeters above the pupil on children. You should also be aware of reduced distance and near utility.

On the other hand, certain "computer" lenses might show more promise for children. I believe there was actually a study conducted to evaluate such lenses for children, although I do not have the results handy. Blended bifocals, fit high, might also be an affordable option.

By Darryl Meister