Medical Forum / General / Vision / November 2004
Question about PAL's
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Joe Stella - 21 Nov 2004 04:38 GMT I recently purchased glasses from Lenscrafter's which had progressive lenses. I returned them after 1 week and got lined bifocals, because the PAL's had a very narrow field of view. My prescription is -4.5, -4.25 with +1.5 add on for reading. The glasses are not really "narrow", they measure 35mm vertically.
I have seen descriptions of PAL lenses which imply that the entire top half of the lens should be suitable for far vision. This was not the case with the lenses I got. There was a very narrow vertical corridor on the lens where I could see clearly. The corridor got a little wider at the top and the bottom of the lens, but not much. In general, there was only one spot on the lens where I could see sharply for any given distance. I felt like I was looking down a pipe.
So, why were my PAL's so limited? Is my prescription not well suited for them? Did Lenscrafter's screw up somewhere? Or is this just the way PAL's are?
Mark A - 21 Nov 2004 06:12 GMT > I recently purchased glasses from Lenscrafter's which had > progressive lenses. I returned them after 1 week and got [quoted text clipped - 15 lines] > well suited for them? Did Lenscrafter's screw up somewhere? > Or is this just the way PAL's are? In general, PAL's do have an hour glass shape in terms of useable area. But the size of the "hour glass" can vary quite bit based on several factors:
- the lens design - the lens material (CR-39, Spectralite, Polycarb, 1.60, 1.66, etc) - the lens power (the higher the power and the higher the add power, the more distortion in the lens) - the fitting (the placement of the frame and the placement of the lens within the frame).
It is generally difficult to find a good optician who selects the right product and does the fitting correctly for PAL's. That is one reason almost all PAL's have a money back guarantee (although LensCrafters has money back guarantee for all their glasses).
Most Lenscrafters sales people don't even know what the material is they are selling you, other than it has a name like "Featherwaites". Most of the Lenscrafter lenses are not "premium" lens designs (even if they are from major manufacturers).
Lenscrafters tends to push polycarbonate material, which has the worst optics of almost any material available. That is probably your biggest problem. If you want to try PAL's again, skip the chain stores and go for a brand name lens (although you can get name brands at Sam's, Costco, and Wal-Mart optical). Expect to pay a bit more than your Lenscrafter PAL's for a high quality lens. It does take some time to adapt, and the entire lens is not useable (although your lenses were probably worse than average).
For your Rx I would look for a premium PAL lens like a Seiko 1.60, Varilux Pananic 1.60, Rodenstock 1.60, AO Finalite 1.60, etc. Avoid polycarb like the plague.
Mike Tyner - 21 Nov 2004 06:59 GMT > polycarbonate material, which has the worst > optics of almost any material available. Why does polycarb deserve such a bad review?
-MT
Scorpion - 22 Nov 2004 03:16 GMT > > polycarbonate material, which has the worst > > optics of almost any material available. > > Why does polycarb deserve such a bad review? > > -MT A year ago I purchased a pair of polycarb PALs from my local independant optician and they were awful, especially when trying to read through the bottom part of the lenses. Less than sharp vision.
My prescrition: R: -2.25 L: -3.50 Add: 1.75
I took them back and replaced them with the Trilogy lenses. A big difference. They provide excellent optics. They're more expensive but worth every cent. I'll never get polycarbs again.
Mike Tyner - 22 Nov 2004 04:12 GMT > A year ago I purchased a pair of polycarb PALs from my local > independant optician and they were awful, especially when trying to [quoted text clipped - 7 lines] > difference. They provide excellent optics. They're more expensive > but worth every cent. I'll never get polycarbs again. No question - one brand of PAL can be better than another.
I'm looking for the reason why polycarb material is optically so inferior.
It has higher chromatic dispersion. Was your near vision obscured by colored fringes around print? If you didn't see rainbows, then the PAL design was at fault, not the material.
-MT
Robert Martellaro - 29 Nov 2004 22:34 GMT >> A year ago I purchased a pair of polycarb PALs from my local >> independant optician and they were awful, especially when trying to [quoted text clipped - 17 lines] > >-MT Mike,
Trans chromatic aberration (TCA) is created by the prismatic effect at the periphery of a lens when looking off axis. The formula for TCA is cF/V, where c is the distance from the OC in centimeters, F is lens power and V is the Abbe number. Although color fringing might be noted, the most common complaint is loss of acuity. According to Torgersen, acuity is not affected until TCA reaches 0.16 prism diopters and then drops by one line on the Snellen chart.
My concern is not so much with distance peripheral vision (that's already compromised by most moderate to high add PAL designs) but with the near point visual acuity. With PAL reading depths ranging from 13mm to 18mm, it doesn't take much power to induce .16 prism diopters of TCA if the abbe is low. Please note that 1.67 has an abbe value of 32 and should receive a similar amount of criticism. I suppose poly outsells 1.67 by a factor of 100 making poly a more convenient target for criticism.
Regards,
Robert
Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical robopt@execpc.com ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman
Mark A - 22 Nov 2004 11:43 GMT > > Why does polycarb deserve such a bad review? > > > > -MT Because polycarb (1.59 index) has the lowest abbe value (30) of any material generally available. Abbe value is a measure of a measure of chromatic aberration, the lower the number, the more chromatic aberration there is. This seems to be more noticeable the stronger the Rx is (either plus or minus, or large add powers in PALs).
Polycarb is widely dispensed because of potential legal issues of lens safety if an accident occurs, because it is fairly thin and light, and because the original patent has expired (making it relatively cheap to make).
As noted, Trivex (Hoya Phoenix or Younger Trilogy) has much better optics and very similar safety properties (tensile strength and impact resistance), although it is not quite as thin and light as polycarb.
However, lens breakage that causes eye damage from similar index (1.60) plastic lenses is extremely rare. Probably more people are seriously injured or die in traffic accidents from bad vision caused by poor polycarb vision than are injured from broken non-polycarb lenses. However if you are working in an industrial or lab setting, or playing certain sports with glasses, some type of safety lens is only prudent.
Mike Tyner - 22 Nov 2004 12:56 GMT >> > Why does polycarb deserve such a bad review? >> > [quoted text clipped - 6 lines] > This seems to be more noticeable the stronger the Rx is (either plus or > minus, or large add powers in PALs). In the original post, Joe had a narrow corridor, with peripheral blur even in the top of his progressive. He didn't mention rainbows.
I've noticed this problem in other LC PALs and I don't think it has anything to do with birefringence. The net near power in Joe's PALs was -050 OD and -175 OS.
-MT
Mike Tyner - 22 Nov 2004 13:03 GMT > The net near power in Joe's PALs was -050 OD and -175 OS. Sorry.. it was -350 OU.
-MT
Joe Stella - 23 Nov 2004 04:18 GMT >>> > Why does polycarb deserve such a bad review? >>> > [quoted text clipped - 9 lines] >In the original post, Joe had a narrow corridor, with peripheral blur even >in the top of his progressive. He didn't mention rainbows. Yes there was also noticable chromatic aberration when I looked at black text on a white page. My current bifocals also show the same effect, mostly noticable through the reading lenses. It's not really all that bad though, and I have already gotten used to it. It was the "looking down a pipe" effect that I really couldn't stand about my progressives. I had sharp distance vision through only one spot directly in the center of the lens. If I tilted my head downward even slightly, while keeping my eyes focused on the same object, my vision began blurring. Turning my head right/left only made it worse.
If I ever get progressives again, I will be looking for a much larger field of view for distance. If I could see distance clearly through most of the top half of the lens, that would be acceptable. Otherwise, I'll just stick with my lined bifocals.
Mike Tyner - 23 Nov 2004 04:29 GMT > It was > the "looking down a pipe" effect that I really couldn't stand about [quoted text clipped - 3 lines] > my vision began blurring. Turning my head right/left only made > it worse. I agree.. with a +1.50 add it's reasonable to expect the distance vision to be clear entirely across the lens, or at least as much so as single-vision. I'd have taken them right back.
-MT
LarryDoc - 21 Nov 2004 17:22 GMT [text deleted]
> Lenscrafters tends to push polycarbonate material, which has the worst > optics of almost any material available. That is probably your biggest > problem. ......... > For your Rx I would look for a premium PAL lens like a Seiko 1.60, Varilux > Pananic 1.60, Rodenstock 1.60, AO Finalite 1.60, etc. Avoid polycarb like > the plague. Lenscrafters uses poly because it's cheap, can be fabricated in-house "in about an hour" and enhances their bottom line profit, not because they are evil and "pushing" poor product.
Although poly may contribute to PAL optics issues, the design and fitting is more likely the problem. Your recommendations for premium PAL lenses are certainly fine choices.
Meanwhile, IMHO, polycarbonate is best suited for airplane and spacecraft windows and shipping containers, not spectacles.
OK. That's a bit harsh----in some lower power Rx's, poly is a fine choice for inexpensive light weight lenses. The problem is that there are different "grades" of poly and, in 2004, we have Trivex, a poly "cousin" with (arguably) significantly better optics while maintaining the superior impact resistance and light weight of the old poly. We have mid and high index lenses with better optics and light weight albeit at a sometimes significantly higher cost.
I haven't dispensed but a handful of polycarb rx's in two years. No need.
--LB
Dr. Leukoma - 22 Nov 2004 00:31 GMT > [text deleted] > [quoted text clipped - 28 lines] > > --LB Larry,
The rather abstract notion of 'quality' is becoming more abstract all the time. At one time, the 'quality' of vision was all that mattered.
Keep the faith.
DrG
Joe Stella - 21 Nov 2004 18:30 GMT >In general, PAL's do have an hour glass shape in terms of useable area. But >the size of the "hour glass" can vary quite bit based on several factors: [quoted text clipped - 10 lines] >all PAL's have a money back guarantee (although LensCrafters has money back >guarantee for all their glasses). So is there any good way for me to find one? Or is it just hit or miss, always making sure that I can get my money back?
>Most Lenscrafters sales people don't even know what the material is they are >selling you, other than it has a name like "Featherwaites". Very true. I got the impression that they knew as much about glasses as the people behind the counter at MacDonald's know about raising cows. :-) I don't think I will go back there for my next pair.
>Most of the >Lenscrafter lenses are not "premium" lens designs (even if they are from [quoted text clipped - 12 lines] >Pananic 1.60, Rodenstock 1.60, AO Finalite 1.60, etc. Avoid polycarb like >the plague. Is there some place on the Web where I can learn more about those? I tried doing a web search but all I get is marketing and advertising hype and not much real information.
Dana - 21 Nov 2004 21:35 GMT Go to a bunch of opticians and ask a lot of questions. You should be able to get at least decent idea whether they know what they're talking about and whether they're willing to spend time with you.
Here's some info and links to a lot more: http://vision9.home.comcast.net/
- D
> >It is generally difficult to find a good optician who selects the right > >product and does the fitting correctly for PAL's. That is one reason almost [quoted text clipped - 7 lines] > tried doing a web search but all I get is marketing and advertising > hype and not much real information. Mark A - 22 Nov 2004 01:34 GMT > Is there some place on the Web where I can learn more about those? I > tried doing a web search but all I get is marketing and advertising > hype and not much real information. You can go to the manufacturer web site and find the "professional" links. But I would try Google Groups and search the archives of this forum. These questions have been discussed many times. http://groups.google.com/groups?num=50&hl=en&lr=lang_en&newwindow=1&q=sci.med.vi sion&btnG=Search
Strangely enough, I have found the opticians at Wal-Mart to be above average in knowledge. I think this is partly because of their age. It is best to find someone who is old enough to wear PAL's themselves. IN any optical shop, ask to speak to the head optician, even if it means coming back later. Qualify them by asking questions to see if they know the answer.
Just keep in mind that there is no one lens that works best for everyone.
Dennis Rekuta - 23 Nov 2004 05:10 GMT <snip>
> >It is generally difficult to find a good optician who selects the right > >product and does the fitting correctly for PAL's. That is one reason almost > >all PAL's have a money back guarantee (although LensCrafters has money back > >guarantee for all their glasses). ,snip>
> >Lenscrafters tends to push polycarbonate material, which has the worst > >optics of almost any material available. That is probably your biggest [quoted text clipped - 10 lines] > > Is there some place on the Web where I can learn more about those? I would also recommend a general site like All About Vision.com. The page on progressives is at:
http://www.allaboutvision.com/lenses/progressives.htm
They also have a lot a general information about lens materials, shapes, as well as medical conditions on other pages.
After almost 40 years of wearing glasses since the third grade, I got my first set of PAL's last week. I did do a lot of reading first, but based on my ophthamalogist's suggestions and a lot of reading of web sites and this group, I felt confident in trying them. I still have to remind myself to "look down" when reading, but I was able to "see" without consciously having to think about not looking to the periphery by the end of the third day.
My prescription: OD -6.50/-1.25 x008 (R) OS -7.75/-0.50 x180 (L) Add +1.50
Based on the archives of this group and my ophthamalogist, I decided to go to 1.67 lenses from the 1.6 lenses that I got seven years ago.
The following prices are in Canadian dollars, trading at about $1.00 US to $1.20 CAN.
I tried a higher end private optometrist/ophthalmalogist practice in town that my dad had used when he needed help as his eyesight deteriorated due to a tumor. It was obvious that the staff and product were first rate. They suggested Rodenstocks with Crizlar coating. With $140 CAN frames, their PAL's totalled about $650 CAN ($540 US). Most of their customers seemed to have insurance plans. I did not.
Lenscrafters, with similar frames, and after a HALF OFF sale on the lenses, quoted me a total of $390 CAN ($325 US) for frames and 1.67 PAL's. They promptly told me that they did not do the lenses and would shop them out. However, they would not tell me the name of the manufacturer of either the lens or of the AR coating. "We use all of the top brands" was the most I got out of them. Not very confidence inspiring.
My "regular" ophthalmalogist works in a mixed practice with optometrists and other ophthalmalogists. I only see her for eye exams and field vision tests, as she has farmed me out to another specialist for my glaucoma. She came back with similar frames, Crizlar coating, and 1.67 Seiko Proceed PAL's for a total of $430 CAN ($360 US). Essilor was the other lens brand she recommended, but they apparently were not available in the frame lines that I wanted. No problem with trust here.
I will leave Lenscrafters to the people who have modest myopia.
Dennis
Mark A - 27 Nov 2004 22:42 GMT > So is there any good way for me to find one? Or is it just hit or miss, > always making sure that I can get my money back? Make sure you talk to the lead optician in the store, preferably one who is old enough to wear PAL's themselves. Ask them quesitons and see if they know the correct answer. If you get advice before hand, you may not need to very best advice from an optician about which product to buy. But you will need someone who is skilled at fitting PAL's. Money back guarantees are a must.
> Is there some place on the Web where I can learn more about those? I > tried doing a web search but all I get is marketing and advertising > hype and not much real information. If you find the manufacturer's web site, always go to the links for "professionals." You may not get all the info you want, but more info than is supplied for the consumer sites. Keep in mind that they vast majority of opticians do not have college degrees, and the information provided to them by manufacturers (or their employer) is geared accordingly.
Dom - 23 Nov 2004 07:58 GMT > I recently purchased glasses from Lenscrafter's which had > progressive lenses. I returned them after 1 week and got [quoted text clipped - 15 lines] > well suited for them? Did Lenscrafter's screw up somewhere? > Or is this just the way PAL's are? To me it sounds like your problems are with the fitting measurements and/or manufacturing of the lenses. With PALs the optical centre of the lenses has to be exactly in front of your pupil; if not, problems like yours arise. As other posters have said, it's very easy to get this measurement wrong.
The oldest PAL design fitted accurately will be MUCH better than the latest hi-tech PAL design fitted poorly.
Maybe go back to where you got them and ask them to check the centres against your pupils. Or if you don't feel you trust them, ask the same question at another establishment.
Dom
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