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The eyeball adjusts its focus by changing its spherical shape. It flattens somewhat to focus near, and bulges out a bit to see far. People who are “near-sighted” (can see well near) are those whose eyeballs bulge less than they should, thus optimizing their vision for near objects.
Young people have more elastic eyeballs, and are more easily able to compress and stretch their shape to be able to adjust focus near and far. As we age, we lose this elasticity, and the eyeball settles in at a particular focus distance. (Thus, vision-correcting eye surgery (“Lasik”) in most people over 40 years of age will only be able to correct both eyes for a single distance. You will still need glasses to to read. Or, you can have one eye corrected for near-vision, and one for far vision.
A lot of women do this. But while this might mean you never need glasses, you will also not be able to see as well as when both eyes are in sync.)
The optics of producing an eyeglass lens with variable focus is complex. The resulting lens does not provide a uniform change in focus from top to bottom, i.e., from near to far. Because the eyeball also needs correction for some swiveling left-to-right viewing (rather than only straight ahead), the layout of the progression of focal variation has a shape like an hourglass, with the top distance-viewing area being the largest, the middle distance area being narrow, and the reading area a bit wider than the middle distance area. This hour glass shape is also tilted, with the lower reading area being closer to the nose. (As we focus on a nearby object, our eyes swivel inward to converge on a nearby object.)
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One example of how distant, intermediate, and near vision fields are mapped out in progressive lenses. This particular design is the Percepta Lens by SOLA. Note that the area to the left and right of the "intermediate Vision" is always out of focus.
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When only a modest correction is needed, progressive lenses work really well. The youthful first-time user will put them on and briefly notice an occasional mild blurriness which disappears in a day or so as his eye-brain learns to adapt. In no time the progressive lenses feel completely natural. But not all eyeglass wearers are youthful, and not all have mild prescription strength. And with progressives, you can't as easily follow an moving object by only swiveling your eyeballs--you have to swivel your head more to keep the glasses centered on the object. Yet this is something you quickly get used to.
There are a number of factors that influence the ease of adaptation to progressive lenses:
- Prescription strength. The weaker the prescription, the easier it is to adapt to it. People with strong prescriptions will have narrow fields of view, and may find it difficult–or nearly impossible–to adapt.
- This width of sharp focus changes with pupil (iris) opening. Wearing progressive sunglasses indoors, the field of view for reading may be too narrow; outdoors, as the pupil closes down, the reading field of view will widen and may become acceptable. Again, this is more noticeable on stronger prescriptions.
- Accuracy of alignment of the narrow reading portion. This is a critical and too-often overlooked factor. If the narrow reading correction is not placed exactly in the right spot on both lenses, so that one is looking through the center of this correction with both eyes simultaneously when reading, one eye will always be getting an annoying soft-focus view.
Although I have not yet done this, the next time I will insist that the
optometrist not mount the lenses until I'm there to maneuver the blanks around
the frame so that both are perfectly placed for near vision.
- It is essential to buy eyeglass frames that fit perfectly without moving about and without fitting so sloppily that they can rest on the head in various different positions. If your optometrist won’t spend the time to fit the frames perfectly, you’re in the wrong store. And if you insist on buying designer frames that don’t fit perfectly, you’re making a big mistake. (It is almost
scandalous that most opticians do not make any but the most perfunctory effort to properly fit eyeglass frames. Demand that they make them fit so they don't move around (and don't press or pinch your head). This will generally require them to make numerous trial-and-error adjustments of the length and point-of-bend-point of the stalk where it curves over the ear. A few minutes spent hectoring the optician for a good fit will mean the difference between years of re-propping your eyeglasses back up, and having eyeglasses that fit like they're a part of you. (But it's amazing how reluctant most optometrists are in spending time performing this essential task.)
- After finding frames that can be fitted perfectly and won’t move about, it is probably best if you yourself mark the eyeglass blanks with a marker to locate exactly what part of the lens each eye is looking through when each is viewing the same word on a page at reading distance. Even so, optometrists routinely fail to locate the reading sweet spot correctly in the final lenses . Do not hesitate to return the lenses for a re-do if that is the case with you. Generally it will only be one lens that must be re-made.
- People who are sensitive to motion sickness (and especially those who are far-sighted) will have a greater difficulty in adapting to progressive lenses, especially if their prescription is strong. Putting on the new glasses results in a sense of the “whirlies” which may take a while to overcome. (The best way to overcome it is to not take the glasses off–to wear them as much as possible. Do not go back to your old glasses to "rest your eyes." Your are retraining your brain. Backsliding will prevent the training from taking hold. Eventually the brain will adapt.) Three days should do it for most people; a week to 10 days for the the more difficult cases.
Photochromic Eyeglasses
Progressive lenses are also available in photochromic versions, that is, they change from clear to dark in the presence of sunlight. These, too, are made to sound like ideal solutions, but they have their practical problems as well.
Photochromic (e.g., “Transition”®, "Sun-Sensor"®) lenses become darker when exposed to ultraviolet light (UV) from the sun–not by bright light. And, they are also inversely temperature sensitive. Thus in cold weather they become very dark in bright sunlight–an advantage for skiers–but in hot weather they don’t darken nearly as much. In fact at 85oF (30oC) they are only about half-dark--not really enough to be called sun
glasses. Most importantly, they don’t darken at all inside a car, no matter how bright and glarey it is outside (because very little UV can pass through window glass). You will still need conventional sunglasses for driving (but they don’t need to be progressives).
On cloudy days, it can be fairly dark outside and yet the photochromic lenses darken due to a still significant UV lightfall. They can become too dark for the brightness of the day. I keep three sets of eyeglasses about; one with normal clear lenses for cloudy days, the other in exactly the same perfectly-fitting frame that are photochromic. The third, a dark bifocal sunglasses for driving and reading at the beach. In cold weather, photochromics take about 5 to 10 minutes to lighten their darkness to one-half depending on their temperature when you come indoors. To speed up the lightening a little bit, dip the glasses in warm water.
(But wait a second: we all know from our chemistry classes that chemical reactions are faster when warm than cold. Why does the photochromic darkening effect work better when cold than warm? Dick Cease of Camden, Maine gave me the answer. The darkening of the lens is an electromechanical effect. Energetic ultraviolet rays knock electrons off the silver halide and cause the darkening. But the return of the electrons to their desired orbits is a chemical reaction. This return reaction is hindered by the cold, so the electromechanical effect causes a lot of darkening. Clearing it up, being chemical, takes longer in the cold, so the photochromic lenses get darker, get darker quicker, and stay darker in the cold.)
Another shortcoming of photochromics is that they don't darken much at all when the sun is low on the horizon . Even though this can be a very glary experience, hard to shade with a hat brim, etc. Low-angle sunlight passes through much more atmosphere which
absorbs most of the UV, but not the bright visible light. Nevertheless, photochromics are enormously popular, and most people who try them are very happy with their purchase.
Photochromic lenses are available in "neutral" gray
(which can look a bit magenta) and brown shades. Click here for a link to a more detailed description. And here for additional photochromic information
What about lens coatings? All plastic lens users should opt for the quartz-coating that reduces abrasion damage. Glass lenses are hard enough (but heavier). (This hardness results in a stronger ability to bend light, which is why the same prescription in a glass lens can be thinner than in a softer plastic lens.) Some of the high-index plastic lenses can also be more difficult to get used to. If you are sensitive to wooziness, be wary of ultra-thin "high density" or Lexan plastic lenses--or ask for a satisfaction
guarantee.
Anti-reflection (AR) coatings work really well
from an optic`ally point of view, but they are a headache to keep clean. AR
coatings make the lenses appear almost invisible, as if you didn’t have any lens in place. Coupled with
borderless frames, it can give the appearance that you are not wearing glasses at all. However the 4+ layer multicoating (on layer for each color that is to be reflectively cancelled) is soft. I take modest care of my glasses, cleaning them carefully. Nevertheless, I have yet to have the multicoating to last much more than a year. However, "ScotchGuard" is a new coating said to improve durability. Ask for it. Some people may want to forego AR coating because the natural reflection of uncoated lenses helps disguise the bags under their eyes. Anyone with a strong prescription really should get the AR coatings. But the best reason for getting an AR coating is that it eliminates annoying reflections--most noticeable when you're inside reading something by outdoor illumination. The light coming in over your shoulders hits the inside lens surface and--if it is not AR-coated--bounces 2-4% of the brightness back into your eye. It then hits the inside of the front lense surface, and bounces another 2-4% back to your eye. This annoying glare cuts contrast which is one key element of seeing a good image.
Cleaning AR-coated lenses is not as easy as cleaning non-coated lenses because the thin-film AR coatings are very sensitive to other thin films--i.e., grease
and soap. Breathing on them to fog them up and then wipe them clean with your necktie rarely works well--it just
smears the grease layer around. If you use liquid dishwasher soap, you can leave a thin detergent layer on the lens which also interferes with the thin AR layers. The best bet is to buy a box of those little alcohol swabs. Clean with them and dry the lens with a soft cloth or towel.
But. after having a dozen glasses with and without AR coatings, I have finally
settled on non-coated lenses. The slight advantage of greater contrast is not
worth the greater headache of constantly cleaning them.
There are a number of various manufacturers of progressive lenses, each with his own secrete formula for making the most comfortable lenses. I have tried several different brands, and for my money the “Panamic” progressive made by Essilor, seems to be the most advanced.
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Additional stuff for eyeglass wearers.
1. Don't buy the bifocals in which the reading portion cuts all the way across the lower part of the eyeglass lens from left to right. The result is a much more noticeable bifocal line, the reflective glint of which can be really quite annoying.
2. The eyedrops that are being advertised to improve visual acuity for older people--especially in gun magazines--do not work at all, according to one report from an eye surgeon, who thinks this false claim should be reported to the FTC.
3. If ever there was a miracle eye drop, it is "Bleph," which literally cures superating eye infections in minutes. (Never seen anything like it.)
4. The biggest difficulty people have adjusting to contact lenses is that of "dry eye." Dry eye is when the tear ducts do not supply enough tear fluid to keep the eyes lubricated and to clean the eye. When contacts are worn, the tear fluid also needs to supply the surface of the eyball with oxygen cut off by the contact lens. People who are marginally dry-eyed may adjust to contact lenses fine initially, but then notice that their eyes start to sting the longer they wear them. Contacts have got much better in this regard, but the problem of dry-eyed people not getting used to contacts--ever--is still a problem for many.
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