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How much of a rip-off are varifocals?

  1. Dec 5, 2014 #1

    sophiecentaur

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    When you buy varifocal spectacles, you can choose between basic, better and 'best' performance. The more you pay, the wider the field over which they work satisfactorily. I once bought some of the cheapest and found that they only worked when looking straight ahead. I have since paid more and my present ones are much better; I can scan a scene by just moving my eyes rather than having to use my neck muscles. They are very good, actually - just a ludicrous price.

    What could be the reason for the various available 'qualities' - apart from an artificial commercial ploy to increase company profits. I assume that a computer program will generate the appropriate profile and the lenses can be produced on a mill (?) of some sort. Are the posh ones made of a different plastic?
     
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  3. Dec 5, 2014 #2
    Wiki calls them progressive spectacles, I would imagine they could be 3d printed by now?
     
  4. Dec 5, 2014 #3
    The biggest difference I can imagine is rather than accommodating 1 axis of differential refraction, you have to procure lenses which have a field across the lens conforming to the levels as they vary from moving your eyes.

    That might entail better materials to achieve this distinction as well.
     
  5. Dec 5, 2014 #4

    sophiecentaur

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    Yes but where is the difference in individual 'design' cost"? Does the shaping of a better lens take longer than making a cheaper one?
    I could appreciate that higher refractive index plastic could justify a higher cost - although they should all be produced for pence, surely.
    3D printing is certainly a good idea - for the future if not now. I do wonder how suitable it could be for optics (clarity is essential).
     
  6. Dec 5, 2014 #5
    The structure on the printer would obviously have an advantage, a slight buff and they are perfect, theoretically. It is just whether you can print the material, I've seen shapes formed out of clear liquids, but I don't know the resolution and how well theory meets physics in printing.

    I have nothing to add retail value wise but perhaps the efficiency of the processor running the printer.
     
  7. Dec 5, 2014 #6

    Danger

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    I've never heard of such an issue. Here, the optometrist or ophthalmologist prescribes a specific grind for the lenses. The lensmaker is obliged to create it. I need what used to be called "tri-focals", but now are considered "progressive" lenses. I have clear vision at one specific distance about 53cm away, which I'm glad to say coincides with my face's distance from the computer screen. My huge (to me) 26" CRT television (for some reason, they're still measured in Imperial units) is quite acceptable at the 2.75 metre range. That works out nicely for me as well, because both of us have our backs against opposing walls in my cave. There's a very long and boring story that I could get into, involving the loss of my very expensive glasses, but I won't.
     
    Last edited: Dec 5, 2014
  8. Dec 5, 2014 #7

    Drakkith

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    Take a look at figure 1 in the following paper: http://www.researchgate.net/publication/236923914_Comparison_of_Progressive_Addition_Lenses_by_Direct_Measurement_of_Surface_Shape [Broken]

    The figure compares the surfaces of various progressive (varifocal) lenses. It show that the surfaces typically have a very complicated, non-symmetric curvature which is generally very difficult to grind and measure. A key point the paper makes is that all of the lenses they compared were made up of a single material. Using two or more different materials allows for better correction of aberrations, but these lenses are generally more expensive to design, manufacture, and test.

    Also, while optical designers do use computers to design the lenses, you can't simply hit a button and have the software create the design by itself. There's simply too many factors to take into account when designing lenses, and most of the time the final product isn't ideal, but a compromise between cost, complexity, and performance.

    Absolutely. Cheaper lenses probably have less complicated curvatures of the lens surfaces, which should be easier to make, but also perform worse. Take a lens for a telescope as an example. Ignoring the chromatic aberrations, a single lens can easily be made with one flat surface and one spherical surface. But this lens will suffer from severe monochromatic aberrations in addition to the chromatic ones. A more complicated surface, like two aspherical surfaces, can correct these aberrations, but are much harder to manufacture than spherical and flat surfaces.

    Well, if they can reliably create surfaces to within about 100 nm of the correct shape, they should work great.
     
    Last edited by a moderator: May 7, 2017
  9. Dec 6, 2014 #8

    CWatters

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    Interesting thread. Here in the UK I have never been offered a choice of "basic, better and 'best' performance" progressive lenses.

    As I understood it progressive lenses have one focal length at the top and another at the bottom and somewhere in between they blend together. Years ago they didn't blend them at all - there was a very sharp transition and the join was very visible - these were called bifocals.

    Clearly there are lots of ways they could blend the two lenses together to make a progressive. I can imagine some people prefer a "fast" transition and others a "slow" transition between the two. Perhaps the rate of blend or even the height at which the blend occurs is different for near/middle distance glasses compared to middle/far?

    I can well imagine that if they get the height or rate of the transition wrong it would mean you had to move your neck rather than just your eyes to scan a scene that consists of a range of objects at different distances.
     
  10. Dec 8, 2014 #9

    sophiecentaur

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    The problem I had with the cheapos was the side to side performance. It was easy to get used to the progressive change of focal length from top to bottom but they were so bad from side to side that I needed to turn my head to see any detail (near of far). I can appreciate that computing the right shape would be hard but there is no difference in actual cost of running the right program (Once we have paid for the development).
    Does anyone know whether 3D printing is ever used for spectacle lenses? - or in any high quality optics.
    I guess I would be daft to suggest that it could lead to lower prices.
    @CWatters I am surprised you haven't come across variations in cost / quality in the UK. SpaceSavers, Vision Express, D&A and Boots all offer different grades. (I have sampled several over the years). The most expensive are definitely the best experience.
    Something that amazes me is the way the brain copes with the truly weird geometry that varifocals present you with. Square tables morph all over the place as you move your head - or at least, they used to. No longer, though. My brain has neatly edited that problem out for me.

    Stop press. I just Googled this link. Interesting.
     
  11. Dec 9, 2014 #10

    Drakkith

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    Nice link. Thanks, Sophie.
     
  12. Dec 9, 2014 #11

    DrClaude

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    Just to be sure: you guys realize that this is basically an advertisement from a company selling 3D-printed lenses? To quote from another source:
     
  13. Dec 9, 2014 #12

    Drakkith

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    Of course.
     
  14. Dec 9, 2014 #13

    sophiecentaur

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    Problem with things like this is that there is potentially a load of money to be made and, consequently all the information about state of the art is under wraps. The rate of acceleration of things these days is very impressive and I'd bet it won't be long before something appears on the market that's of suitable quality for glasses. I guess it has to be true that the optical quality for glasses is nothing like as critical as for cameras and binoculars - if only because it involves just one lens and low powers.
    I reckon that 3D printing is, in general, very much over-sold. "You can make anything with 3D Printing" Just try a simple drop forged spanner and try to get the head off the engine in your car. The words Chocolate and Teapot come to mind. (Comment dated today, of course and things could change 'ere long)
     
  15. Dec 9, 2014 #14

    CWatters

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    I'm 55 but have only needed glasses for a few years. Perhaps they only offered me the expensive ones :-)

    Thanks for the link on 3d printing.
     
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