…somehow molded to fit each patient’s Rx?
Plastic and glass lenses, as you all know (or would surmise) are ground
from blank lenses to meet the specs of the scrip. At least they used to
be, way back when.
But I really can’t imagine how this can be done with contacts, particularly
soft contacts. I have the idea that “1 size fits many”—but not all. I am
probably flat out wrong on this and would like to learn the answer to
my OP.
Yes, they’re ground or cut and polished from an optical blank in a similar manner to glasses, only the blank is obviously smaller. “Soft” contacts are made from a plastic which is hard when dry, and only becomes soft when soaked in water. They are shaped and polished while dry, then soaked in saline.
Well, you can start to understand what is going on by reading Wikipedia: Eyeglass Prescription.
Salient information: a prescription corrects in part by uniformly modifying the focal point of light entering the lens. This is measured in diopters, and there is a uniform set of diopter correction steps; I believe they are every .25 diopters. The grinding process involves incorporating into a blank lens the proper corrective features, keeping in mind the actual spot under the lens where the pupil’s center will be. You can read about the process here.
The process for contacts is described here. I had been under the impression that contacts were much less personalized, which is why they didn’t used to correct for astigmatism well, but perhaps that aspect has changed in the 20 years since I wore mine. 
Contact lens prescriptions come with not only a strength value, but also values for the lens diameter and base curve which are specific variables for the patient’s eye, so you’re quite right in that one size does not fit all eyes.
I used to wear contacts, and was told that I have an unusually high curvature (whatever that means exactly) which meant an awful lot of the time they had to specially order my size, because they didn’t carry them in the office.
I always wanted to get some blue ones just to wear to freak out my family, but they don’t come in my size, so while I could probably get away with wearing a pair for a little bit for fun, they wouldn’t be at all comfortable. Drat.
A veritable font of excellent information.
Thank you all, very much!
At the Wal-Mart where I got my last pair of contacts, the optometrist had all kinds of eye-color-changing overlays and fancy effects (like cat’s eyes, stars, spirals, etc.) that he would custom-apply to your contacts if you paid an extra $85. I was surprised because I had always been under the impression that eye-color-modifying contacts and the like came manufactured that way.
I didn’t read the links and so this information may be covered already.
I switched from hard to soft contacts 20 years ago, so I presume anything I know about hard one is obsolete.
With my two-week or daily wear soft ones, they are only specified by the correction. I presume they have enough flexibility to handle normal differences in the eye. They come in correction “steps” so unless all the planets align, your vision is a little softer.
Anyone who has worn contacts knows that this is wildly incorrect. Soft contacts come in a variety of standard sizes and strengths but they are by no means custom made for each individual.
QED’s post isn’t “wildly incorrect”, and is pretty close to the truth. I’ve worked with companies and even individuals who manufacture contact lenses. The “soft” contact lenses are made from a hydrogel plastic. The exact blend and refractive index is a jealously guarded secret by each manufacturer. God knows why. As QED says, the lens is manufactured from plastic that is non-hydrated and still hard, although it is not cut from a blank. Some companies injection-mold the lens, while others use a spin-mold process. Sometimes one surface is precision cut after molding. They are not cut from a blank, however – the lens is manufactured to the correct size, which is pretty standard for contact lenses, so there’s no need to make a blank and size it later.
Some contact lenses ARE made individually, by hand. One individual I worked with was a little gnome of a man who sat in a lab/office all day, grinding down and polishing polymethyl methacrylate lenses by hand. These were invariably weird prescriptions that you wouldn’t be able to buy off-the-shelf, but they weren’t all really weird dimensions. We were able to measure his lenses on our standard lens-measuring device.
The wildly inaccurate part is that he answered in the affirmative to the question of whether they were custom made for individuals.
I’m not surprised that some are made by hand, but the vast majority come off the shelf.
treis, how do you know so much? I ask this respectfully—and somewhat groggily since it’s way past my bedtime.
Mostly from experience and talking with the eye doctors. When you get your eyes examined for contacts you get a pair right at the doctors office, which they then check for fit. You can buy contacts online, for example here. The base curve is the shape of the contact before it becomes a lense. Diameter is the size of the contact. The brand I linked to only comes in one Diameter, but some different brands come in multiple sizes. Power is how much they correct your vision.
I’m not sure that this is done for standard scripts. My wife’s script is more expensive because it requires custom lenses. My understanding is that for most people, lenses in standard powers are cut to fit the chosen frames.
I believe both spectacles and contact lenses tend to follow the same approach, which is that ‘standard’ prescriptions (i.e. -2.5 with no astigmatism correction) are made by the truckload and stored in a warehouse until someone orders them. Whether they are made in gas-permeable plastic (contact lenses) or light-reactive plastic (spectacles) they’re basically off-the-peg. If you have a slightly odd prescription (e.g. very strong, in an unusual material, or with some odd astigmatism) then it’s much more likely that they will be custom made, but you might still get them off-the-peg if the manufacturer has the volume to support this. For exotic requirements, it’s hand-made all the way.
e.g. for my contact prescription (-11.00/-10.50) I can have ‘permanent’ lenses no problem because making them to order isn’t such a big deal. However for daily disposables I am limited to exactly one brand, because custom-making them is not practical and only J&J Acuvue manufacture up to that power. Similarly when I have glasses made (-12.50/-11.25 or something) it takes forever and costs a fortune since they need to order the superexpensive Zeiss glass to be specially ground - anyone keeping large quantities of stuff like that on the shelf would go bust.
Me too. Huh. Well, even so, for unrelated reasons I won’t go to WM anyway, but I never heard of that. I had an optometrist in LA some years ago who did TV (and possibly other industry, but I don’t know) work – if you ever watched Babylon 5, for one, check out Narn eyes – but I was under the impression those sorts of things had to be specially made to order. Whaddya know.
Toric contact lenses are available to correct astigmatism, but they can be very expensive. I’m astigmatic in my left eye, but not enough to worry about getting a toric lens just for one eye.
-Plnnr, who just picked up his new contact lens perscription on Friday.
but there’s one significant difference between spectacle lenses and contacts – the spectacle lenses can be made for astigmatic eyes with the required prescruiption of spherical and cylindrical power, and they don’t need to worry about the angle that the cylindrical power lies at – they can simply rotate the (circular) blank before they cut to to match the frames.
Contact Lenses for astigmatic eyes can’t work that way – they have to be “ballasted” with a slightly thicker part on the bottom so they’ll be properly otreiented in your eye*. You can’t make up a warehouse full of them and then choose the one you want for a patient and then weight one side so that the astigmatic correction lies at the correct angle – you have to make them thicker on one side from the start. So you have to warehouse a lot of contact lenses, with not only the various spherical and cylindrical powers, but also with those cylindrical powers at a variety of angles as well.
*The ballasting doesn’t work by making the lens heavier on the bottom, so that its weight swings the lens doen that way – the difference in weight is negligible, and it would never work. The slightly greater thickness at the bottom ensures that the motion of the eyelid pushes that side preferentially down. It’s the constant closing and opening of the eyelid, pushing the contact lens that way that keeps it properly oriented. A good thing, too – without some asctive means of correction, the contact lens might lie along any direction in the eye, and you couldn’t guarantee that it would sit the right way to compensate for astigmatism.
Actually, no, I didn’t. I thought it was pretty clear from my post that I was addressing the OP’s notion that soft contacts aren’t ground or cut and then polished like eyeglass lenses. Apparently it wasn’t as clear as I thought.
Apologies for the lack of clarity.
I have a slight astigmatism in both eyes and my eye doctor told I could get toric lenses or stay with standard ones. I tried a sample pair of the toric lenses and found the difference to be worth the extra money. Especially for working at the computer.
YMMV,
Rob