I think the opticians typically buy uncut lenses (60mm diameter?) and then cut them to fit the frames using a special machine. They don't actually grind the optical surfaces of the lens.
A quick web search finds this:
Google book search will let you look at some pages inside: ISBN-10 0750688947 ISBN-13 9780750688949
(try this url:
You can get very inexpensive eyeglasses from China these days, made to your prescription.
Opticians generally have a retail business to run, insurance often pays for the glasses (so cost isn't important unless it rises much above $500 or so), and frames are often essentially a jewellery item which people are willing to splurge hundreds of dollars for. The actual cost of generic lenses and frames is very, very low, but if you want Armani + Oakley etc. you must cross their palms with silver.
I've seen lenses roughed out on CNC lathes using diamond tooling, but they have features not normally found on Model Engineers' lathes like optical interferometers used in place of the glass scales of the DRO and slides which float on air for smooth movement.
It's normally much easier to use a generational technique as this can achieve exceptionally good results with quite basic equipment. I can recommend Twyman's "Prism and Lens making: A Textbook for optical glassworkers" as one of the few really good books on the practical aspects.
At least for simple prescriptions this is not too hard. The main form of surface used in single vision lensens is the "toric" surface. This has two different radii of curvature on orthogonal axes and can viewed as part of the surface of a torus (or ring donut).
Usually to keep costs reasonale one surface is spherical and the other toric. So that leaves only one design decision which is how to split the curvature between the front and back surface.
Therefore you take the most significant aberration for spectacle lense which if my memory is correct is astimagism of oblique pencils and balance the radii to minimize this.
If you're used to designing lenses for more conventional purposes such as telescope objectives, it may seem surprising that this beats the usual axial spherical abberation (there is not much you can do about chromatic abberation other than choosing a suitable material as nobody wants doublet spectacle lenses). It is important not to forget that the eyeball rotates to look in the direction of sight, which means that the centre of the field of view is not the optical axis of the lens.
Using the simple thin lens approximation for a spherical lens to give an example
1/f = (N-1) (c_1 - c_2)
where N is refractive index of lens and c_1 and c_2 are the curvatures of the front and rear surface. So to achieve a given power 1/f (measured in dioptres) it is only the difference in radius of curvature which affects the power, so there is one free parameter which is chosen to try and make the power of the lens viewed off axis as similar to that on the axis as possible.
I can recommend M. Jalie's Opthalmic Lenses and Dispensing as a possible source of further information. Conrady's two volume set on optical desiga don't consider spectacle lenses, but cover all the basics of lens design. The main drawback is that he considers a computer as a person and spends a lot of time on how to minimize the number of multiplications this person will have to make!
Read an article this last year that compared several sources for prescription lenses on price and how close the lens matched the prescription. Since I don't remember for sure, the name of the big chain outfit included, I won't mention/guess. But Costco came out well on top both for price and accuracy in meeting the prescription. Cost savings on one pair of glasses could easily cover a basic membership.... Don't know if UK has any Costco outlets, but there's plenty of them here in the States.
I would have thought this is about the most daft and pointless way to try to save money you could possibly envisage. If you go direct to a manufacturer like I do (Fairplay Optical in Watford, look them up in Yell.com) lenses are not dear (or frames for that matter). £15 or so for lenses for a standard prescription if you send them your frames. They are also horrendously complicated to make from scratch. Contrary to common belief lenses are not made to suit your frames by grinding and polishing a flat sheet of something to the required curvature. They come already pre-formed in every possible prescription as 3" round blanks and all the optician does is scan them and cut them to the shape of the frame on special CNC grinders about the size of a large microwave oven. Fairplay had five of those working flat out last time I was in there 18 months ago.
Fairplay have their own range of frames now at minimal cost, will mend any other fault like broken fishing line in half-frames, broken ear piece plastic covers or missing nosepieces and will deal by post if you send them your frames. They even soldered a nosepiece holder back onto the frame for me once after it broke off after being adjusted too many times and it's still there very solidly 20 years later. It broke off within only a couple of years of me buying what had been a 200 quid pair of supposedly top end specs from the optician in the village. I haven't made that mistake again since. I go direct now.
My most common problem is the fishing line breaking which seems to happen every few years on one side. There's probably a sharp bit on the frame somewhere which cuts through it. My last visit was to get that fixed again after the lense fell out plus the plastic shroud covering the ear bend part of the metal frame had gone hard and cracked on one side. They replaced both plastic shrouds to keep both sides looking the same, fitted the lense back in with new nylon line and charged me a measly 3 quid for what took one of the guys at least 15 or 20 minutes never mind the materials. The petrol to get there cost me more than the repairs and they're only 20 minutes away. If I'd gone to an optician those same repairs would have been marked up many times to probably £20 or more.
Frankly someone cannot see the wood from the trees!
I have just had my first cataract operation which meant that my prescription changed in the left eye. I now have a second op on the 21st Jan but my consultant suggested a new test. OK, I took my new prescription into my local place- a wholesale place ( On-Spec Optical co) and they offered to put a new bifocal in for =A315
- in an hour! During all this, my duff eye has changed and I got new lenses in for =A330.
Frankly, I cannot see what the fuss is all about( pun intended)
Yup that's them. I did get a bad set once, one lens was totally wrong, rang them up and explained and they aked me to email the pescription thru again and sent a new set out by return, no extr chage and told me to throw the old set away. Can't fault that for service, I always think it matter how someone gets over a problem that counts.
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