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Medical Forum / General / Vision / August 2007Lenscrafters and the base curve: What is the problem?
I am a 33-year-old male. In 2004, I bought a pair of -5.25, -5.25 glasses at Lenscrafters. These glasses were meant to be used for driving and other "long distance" activities. These glasses have a base curve of 2.0 - 2.75 (depending on who is measuring the base curve). The base curve was not specified in the prescription. Thus, Lenscrafters itself determined what the base curve would be. These glasses have been comfortable for me. Also, for years, I have been using -4.25, -4 glasses for "up close" activities, such as reading, computer use, etc. These glasses have a base curve in the 3.0 - 3.75 range (depending on who is measuring the base curve). These glasses are old, and I can't remember which store made them. I also don't know who determined what the base curve would be. However, these glasses are comfortable for me. In the early part of this year, I bought -5, -5 glasses from Lenscrafters. Lenscrafters determined that the appropriate base curve was in the 3.5 - 4.0 range. Well, these glasses gave me headaches, and my doctor said that this base curve was too high. So, I returned the glasses for a refund. ----- In the last few weeks, I bought -4.25, -4.25 glasses from Lenscrafters. I asked that Lenscrafters make the glasses with a base curve of 3.5, which is the base-curve measurement that my doctor came up with for the -4.25, -4 glasses. The doctor said that, because these prescriptions were similar, they should have the same base curve. The Lenscrafters optician who took my order acted as if Lenscrafters would comply with my base-curve request. Well, the -4.25, -4.25 glasses gave me headaches. An optician at the Lenscrafters store where I bought the glasses (NOT the optician who took the order) told me that only a doctor can ask Lenscrafters for a certain base curve. So, when I specified a base curve with my order, Lenscrafters disregarded my specification and gave me a base curve that Lenscrafters thought was appropriate. This base curve was in the 4.75 - 5.00 range. I believe this base curve also is too high. If -4.25, -4 glasses have a base curve of 3.5, then a slightly stronger pair of glasses should have a base curve that is 3.5 or slightly smaller. The base curve should not be a lot higher than 3.5. ----- Lenscrafters opticians tell me that Lenscrafters uses computers to determine the appropriate base curve. Well, my question is as follows: Why has Lenscrafters given me two pairs of glasses with incorrect base curves? Lenscrafters got the base curve right with the -5.25 glasses. So, why is Lenscrafters all of a sudden getting the base curve wrong? On Aug 25, 1:22 am, midwest...@yahoo.com wrote: > I am a 33-year-old male. > [quoted text clipped - 48 lines] > curves? Lenscrafters got the base curve right with the -5.25 glasses. > So, why is Lenscrafters all of a sudden getting the base curve wrong? what kind of lens? hi index? aspheric? same rx in different materials can have different curves. an optician is perfectly competent to order you lenses with the same base curve as your previous pair and does not need a doctor for this (especially given how the 2 rx's are nearly identical). doctors almost never specify a base curve or lens material for glasses on prescriptions- they leave it to the dispenser to decide. > what kind of lens? hi index? aspheric? same rx in different materials > can have different curves. [quoted text clipped - 3 lines] > never specify a base curve or lens material for glasses on > prescriptions- they leave it to the dispenser to decide. The -4.25, -4 glasses (old glasses) are made of some kind of glass. The -4.25, -4.25 glasses (new glasses) are made from photo mid-index plastic. As for the optician's competence, the optician stated (rather rudely, I might add) that the optician will not honor any requests that *I* make. He will follow doctor's orders. If there are no doctor's orders, then he will have the lab computer determine the base curve. On Aug 25, 5:30 pm, midwest...@yahoo.com wrote: > > what kind of lens? hi index? aspheric? same rx in different materials > > can have different curves. [quoted text clipped - 13 lines] > make. He will follow doctor's orders. If there are no doctor's orders, > then he will have the lab computer determine the base curve. just curious, why photochromic for computer glasses? is the frame more wrap-around? sometimes a higher base is used to accommodate more curved frame. > just curious, why photochromic for computer glasses? > is the frame more wrap-around? sometimes a higher base is used to > accommodate more curved frame. Prior to 1997, I used the -4.25, -4 glasses for "long distance" activities like driving AND for "up close" activities like reading, using the computer, etc. That is, I used these glasses for everything. These glasses are photochromic. ---- Since 1997, I've been using them for "up close" activities only. ---- "Up close" activities include talking to people face-to-face. I have been in situations where I have been face-to-face with people in the outdoors, in the sun. It is in these situations that photochromic glasses in the -4.25 range are useful. >I am a 33-year-old male. > [quoted text clipped - 48 lines] > curves? Lenscrafters got the base curve right with the -5.25 glasses. > So, why is Lenscrafters all of a sudden getting the base curve wrong? The base curves are specific to each manufacturer's lens design/material and are not transferable to other lenses or materials. So an OD would not specify that on an Rx unless they specify the exact lens it applies to (not likely unless they are dispensing the lens also). Although the base curves are supposed to be predetermined for a specific Rx (for that specific lens) based on the sphere and cylinder power, sometimes someone in the lab may use a different base curve if they don't have the correct one in stock. This is more likely to happen at an in-store lab like Lenscrafters because their lens stock is limited compared to a large lab which services many retail outlets (for chains or independent retailers who do not have in-store labs). In other cases, if you are "right on the border" between 2 different base curves they may choose to use a different base curve than the charts tell them to use because they think they know better (maybe in some cases they do). If you purchase glasses at Lenscrafters you are likely to be sold polycarb material. Polycarb has the worst optical properties (measured as abbe value) of any commonly dispensed lens material, although it has very good tensile strength and impact resistance. If you must have these features try Trivex lens material (available as Hoya Phoenix material, and also sold by Younger Optics). Otherwise, I would go with a 1.60 material for your lenses. If you think those are too thick, than 1.67 is OK, but with slightly less optical quality than the 1.60. > Although the base curves are supposed to be predetermined for a specific Rx > (for that specific lens) based on the sphere and cylinder power, sometimes [quoted text clipped - 3 lines] > which services many retail outlets (for chains or independent retailers who > do not have in-store labs). The -4.25, -4.25 glasses that I bought recently are made from photo mid-index plastic. Therefore, the Lenscrafters store sent my order to an outside lab. I waited for my glasses for about 10 days. So, I'm guessing that this outside lab had all the base curves in stock. The -5, -5 glasses that I bought earlier this year (and later returned) were also made from photo mid-index plastic. If I remember correctly, those were made by an outside lab as well. Also, the -5.25, -5.25 glasses that I bought in 2004 were made from photo mid-index plastic. I can't remember whether these glasses were made at an outside lab. As for the -4.25, -4 glasses, they are made from some kind of glass. > The -4.25, -4.25 glasses that I bought recently are made from photo > mid-index plastic. Therefore, the Lenscrafters store sent my order to [quoted text clipped - 12 lines] > > As for the -4.25, -4 glasses, they are made from some kind of glass. They went to a Lenscrafters lab. If that particular lens in not widely in demand (which we can assume if the in-store lab did not have it) then there is a chance that the lab did not have the correct base curve in stock either. However, the more likely explanation is one of these: 1. Your Rx is right on the border of two base curves, which means that neither one of them will be optimal, or 2. Your problem has nothing to do with bases curves. > They went to a Lenscrafters lab. If that particular lens in not widely in > demand (which we can assume if the in-store lab did not have it) then there > is a chance that the lab did not have the correct base curve in stock > either. I believe that the out-of-store lab takes the photo mid-index plastic in the lab's possession and *gives* the plastic a certain base curve. The plastic with the appropriate base curve is then sent to the store, and the store technicians give the plastic the prescription written by the doctor (-4.25, -4.25, in my case). Is that correct? The base curve is not something that is in stock. The plastic is in stock, and the outside-of-store lab *gives* the plastic a certain base curve. This is the procedure that was described to me by a Lenscrafters employee. Besides, the optician (at the Lenscrafters where I bought the new glasses) did say that, if my doctor had specified a base curve, that base-curve specification would have been honored. > However, the more likely explanation is one of these: > > 1. Your Rx is right on the border of two base curves, which means that > neither one of them will be optimal, or Well, my doctor measured the base curve of my old glasses (-4.25, -4) at 3.5, and Lenscrafters gave my new glasses (-4.25, -4.25) a base curve of 4.75 - 5.00. Those two base curves (3.5 and 5.00) are far apart, are they not? > 2. Your problem has nothing to do with bases curves. I should point out that the optician (at the Lenscrafters where I bought the new glasses) did adjust my new glasses in one way: He altered the angle of the lenses with respect to the temples. He made the angle of my new glasses match the angle of my old glasses. That seems to have alleviated some of the discomfort produced by the new glasses. However, when I use the new glasses to read printed words in newspapers, magazines, etc., I have trouble focusing on the words. After reading for a few minutes, I have to look away from the words. Reading with these glasses is difficult. This problem with focusing may be a base-curve problem. ---- Also, for my old glasses, the optical centers are about 4 - 5 mm below the centers of my eyes. For my new glasses, the optical centers are at the centers of my eyes. In a different thread in this newsgroup, an optician named Robert Martellaro comments that the optical centers in my new glasses should also be 4 - 5 mm below the centers of my eyes. You can see this thread here: http://groups.google.com/group/sci.med.vision/browse_thread/thread/116916f1b706d 45f/5444865502febde2#5444865502febde2 > I believe that the out-of-store lab takes the photo mid-index plastic > in the lab's possession and *gives* the plastic a certain base curve. [quoted text clipped - 11 lines] > glasses) did say that, if my doctor had specified a base curve, that > base-curve specification would have been honored. The lens manufacturer gives the labs the base curve stock lenses. Please see page 2 of this document which shows the base curves for Sola One Lens in various materials. This is a progressive lens, but the same concept applies to SV except there is no add power in a SV lens. http://www.zeiss.com/4125682000258738/EmbedTitelIntern/SolaOne_FitGuide1105.pdf/ $File/SolaOne_FitGuide1105.pdf > Well, my doctor measured the base curve of my old glasses (-4.25, -4) > at 3.5, and Lenscrafters gave my new glasses (-4.25, -4.25) a base [quoted text clipped - 15 lines] > > This problem with focusing may be a base-curve problem. I am not sure we are talking about the same thing (base curve). What I am talking about is the base curve of the blank stock before it is custom ground to your Rx. A particular lens usually has about 5-7 different fixed bases curves to choose from that they use as a starting point, and then custom grind it in a lab to your exact Rx. See the document I referenced above for more info. Base curves are not transferable from one lens to another as you can see from the document. There are some high-tech lenses that are custom ground from scratch (without a base curve) but so far as I know these are only available in certain very expensive progressive lenses. You can also get a completely finished lens from the manufacturer for certain model lenses. These lenses are also made from base curves but the manufacturer stocks every combination of finished lens (since lenses powers are prescribed to the nearest .25 diopter). I don't know if this is available for photochromatic materials. > Also, for my old glasses, the optical centers are about 4 - 5 mm below > the centers of my eyes. For my new glasses, the optical centers are at [quoted text clipped - 5 lines] > > You can see this thread here: Robert is probably the most competent optician in the USA, so I would listen to what he says. >> I believe that the out-of-store lab takes the photo mid-index plastic >> in the lab's possession and *gives* the plastic a certain base curve. [quoted text clipped - 66 lines] >Robert is probably the most competent optician in the USA, so I would listen >to what he says. Mark, I'm certainly the most experienced optician in my shop! Robert "An expert is a person who has made all the mistakes that can be made in a very narrow field." - Niels Bohr Well, I went to see the optician who works in my eye doctor's office. First of all, this optician expressed surprise that Lenscrafters will not honor my base-curve request without a doctor's note. Furthermore, this optician believes that the base curve in the new glasses is too high, and I received a written order from the doctor's office that asks that the base curve from my old glasses be put into my new glasses. As for the optical centers, the optician stated that, because my old glasses have very big lenses, it is a good thing that the optical centers of these glasses are 4 - 5 mm below the centers of my eyes. However, because my new lenses are small, the optician recommends that the new glasses' optical centers align directly with my eyes. If that does not work, then we can always move the optical centers down 4 - 5 mm. Also, Robert Martellaro said that I should use the distance PD, not the near PD, for my "up close" or "reading" glasses. However, the eye doctor's optician said that distance PD should be used for "long distance" glasses and that near PD should be used for "reading" glasses. As for my new glasses, I know that the optician at Lenscrafters measured my distance PD with the PD device, but I don't know whether my near PD was determined. > As for the optical centers, the optician stated that, because my old > glasses have very big lenses, it is a good thing that the optical [quoted text clipped - 3 lines] > does not work, then we can always move the optical centers down 4 - 5 > mm. The is a big difference between 5mm and 0mm below center, especially if you have an aspheric lens that is more sensitive to placement than a spherical lens. Maybe try a compromise of 2-3mm ? |
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