1.7 vs 1.53 refractive index in glasses ??

Discussion in 'Glasses' started by carrera d'olbani, Mar 14, 2007.

  1. Hi there,

    I finally decided to get new glasses (after having the previous pair
    for around 10 years). The modern frames are small and narrow which is
    good for people who have long-sightedness but no good for the people
    with miopia like myself. Finally, I was able to choose one. I tried
    the plastic lens (polycarb ?) , and did not like it because the image
    at the edge of the lens was annoyingly less sharp than in my glass
    lenses (refractive index 1.53 ?). I think that the higher-index lenses
    have the higher optical aberrations (which is seen as a less sharp
    image especially at the lens edge) because the light has to bend to a
    higher degree. I went to another optometrist, and told him I would
    accept only glass lenses (he nevertheless tried to sell me plastic).
    He suggested me to get the high-index glass of the 1.7 refractive
    index instead of 1.53. Is this going to get me more aberrations ? What
    are the people's experiences about the 1.7 glass compared to 1.53 ?

    (I value the quality of the image above of everything. I do not care
    about the weight. I do not want to get anti-reflective coating because
    it will scratch. My lenses are -4 dioptre. The lens frame is 50 mm
    across.) Thanks.
     
    carrera d'olbani, Mar 14, 2007
    #1
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  2. carrera d'olbani

    Mark A Guest

    Please post your Rx so that a proper recommendation of a lens can be made.

    Polycarb is 1.59, not 1.53. If it is 1.53, then it may be Trivex or some
    other material.

    1.70 should only be used for a very strong Rx to help reduce the thickness
    of the lens. As already noted by another post, the higher the index, the
    worst the optical qualities of the lens, except for polycarb which has the
    worst optical qualities of any commonly dispensed lens.
     
    Mark A, Mar 14, 2007
    #2
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  3. I disagree with that. Smaller frames are good for anyone with a
    moderate to high power Rx so long as the eyes are fairly well centered
    in the lenses. That's because the lenses are thinner and the amount of
    your visual space that is covered up by the edges is narrower, so you
    actually get more vision.
    You are right, and polycarb is the worst of all.
    Anything is better than poly, and glass is the best.
    Anti reflection on glass lenses is very very durable. If you take
    halfway good care of them, they will be fine, and the "quality of the
    image above of everything" is definitely better with the AR coating.
     
    William Stacy, Mar 14, 2007
    #3
  4. a good 1.7 glass (abbe 40 such as "fit 40") is probably not going to
    cause any noticeable chromatic abberation.
    polycarb is really bad. Good 1.6 index plastic (MR-8 material) should
    be fine if you go the plastic route (for less weight and higher impact
    resistance than glass).
     
    michael toulch, Mar 15, 2007
    #4
  5. Here is the article from Wikipedia. It says that lenses from the
    materials with the higher refractive index have the higher chromatic
    aberrations. The higher chromatic aberrations are perceived by people
    as a higher "blurryness". Perhaps this is what I was witnessing. The
    materials with the higher dispesion are said to have the higher Abbe
    number (calculated according to the formula). The higher the number
    the less is the dispesion, therefore, the better is the optical
    quality. In the link below, it is stated that the Abbe number of
    plastic (CR 39) is 59.3, which is on par with the low (1.53)-
    refractive index crown glass (58.5). It is much higher than the Abbe
    number of a high-refractive index glass of a 1.7 refractive index,
    namely 36 for Hoya glass.

    This should mean that the polycarb should have a very good image
    quality from the point of view of chromatic aberration -- the same as
    that of glass, in fact. But you and others say that polycarb is bad.
    How come ?

    http://en.wikipedia.org/wiki/Corrective_lens#Plastic_.28CR-39.E2.84.A2.29
     
    carrera d'olbani, Mar 15, 2007
    #5
  6. You managed to find a glass that is just as BAD as polycarb. Stick with
    regular crown glass.

    high ABBE=good

    low ABBE=bad
     
    William Stacy, Mar 15, 2007
    #6
  7. carrera d'olbani

    Mark A Guest

    Because polycarb (1.59 index) is a fundamentally different material than
    other "plastics" and has a completely different molecular structure. The one
    advantage of polycarb is high impact resistance and high tensile strength.
    It has an abbe value of 30, which is the lowest (worst) of any commonly
    dispensed lens material.

    Trivex has about the same impact resistance and tensile strength as
    polycarb, but is 1.53 index, and has a very good abbe value of about 42.
     
    Mark A, Mar 15, 2007
    #7
  8. OK, here is the full story. I went to the optometrist for new
    spectacles (in Oz). He offered me the polycarb lenses, and I said I
    did not want them, because they produced too much blur in the
    peripheral vision, and thus I wanted glass. He recommended me the
    glass of a 1.7 refractive index because it was lighter than the
    standard glass (1.53 crown). I did not know why the 1.7 glass should
    be optically worse or how much it would be worse, so I reluctantly
    agreed.

    When I came home, I did research on the Internet (and found the above-
    mentioned article on Wiki). It said that the 1.7 glass had the
    dispersion as bad as the one of the polycarb. The 1.53 crown glass had
    much lesser dispersion (its Abbe number was ~60 vs. ~30 of the
    polycarb/1.7 glass). I became unhappy about my future glasses :-( .

    When I came to pick up the ready glasses, I saw that my new glasses
    produced as blurry image on periphery as the polycarb-fitted glasses
    did. When I moved my gaze 1 cm away from the lens centre towards the
    periphery (what, 20 degrees ?), the image became very blurry. It was
    totally inacceptable to me. I said to the optometrist about it. I
    showed him the print-out from Wiki with the data on Abbe numbers of
    1.53 and 1.7 glass and of polycarb.

    The optometrist told me that he wanted to keep me happy, and I did not
    have to buy the glasses. I asked him to fit the ordinary crown glass
    lenses into the frame (to replace the 1.7 glass), and said I would pay
    for the old and new lenses. The old chap said he already offered the
    best spectacles for me, and he would not be doing anything more for
    me. He suggested I take my prescription and go to another
    optometrist.

    I could not believe my ears. I decided that the old guy was not in the
    business for money any more (he was practising in this prime downtown
    place for 50 years); he was doing the business just for job
    satisfaction. I decided that I wanted to try these low-Abbe number
    lenses anyway, so I bought the glasses. As an afterthought, the
    optometrist told me that I could bring the glasses for refund within 2
    weeks if I find them unsuitable.

    I now have these glasses. I calculated the transverse chromatic
    abberation (TCA) according to your formula above, and actually found
    that the Torgersen criterion was correct (the TCA was around 0.33 D
    and the acuity actually dropped by about 2 lines). Interestingly, my
    old glasses with the 1.53 crown glass lenses also had an acuity drop
    at the same peripheral angle as the new glasses, however the drop was
    not as much (maybe 0.5-1 line ?). Heck, today I was watching for the
    babes walking past me, and I had to turn my head to see them better
    (thus, it became evident to my companion that I was actually watching
    them) (with my old glasses, I did not have to turn my head and still
    saw them clearly at the periphery of the lens !).

    Also, the 1.7 glass lenses give a purple or green reflection
    (dependent on the viewing angle). This tells me that the lenses have
    an anti-reflection (or any other) coating. The optometrist told me
    that the lenses had no coating just like I requested. Is there a
    coating on my lenses ?

    Thanks for your attention.
     
    carrera d'olbani, Mar 24, 2007
    #8
  9. carrera d'olbani

    Salmon Egg Guest

    It would be good to get a review of modern glasses and the links between
    density and index of refraction. In the old days, light (low density) glass
    was crown glass with a low index of refraction and dense glass was flint
    glass with a relatively high index of refraction. High density material had
    a high density of bound electrons (complicated atoms) that contributed to
    index of refraction. Because the power of a lens was proportional to n-1, it
    was never clear to me that you could get lower weight optics (spectacles) by
    shifting to higher index material. That is, there is no guarantee that
    thinner also was lighter.

    These days, with fancy materials such as rare earth glasses and who knows
    what kind of plastics, the relationship between density and index can be
    more obscure. Can anyone supply a link that will clarify these
    relationships?

    Bill
    -- Fermez le Bush--about two years to go.
     
    Salmon Egg, Mar 24, 2007
    #9
  10. You may or may not be correct on that; I'd be interested in knowing
    where you get your data. However, even if it's true that things like
    spherical aberration and coma vary inversely with refractive index, I'd
    have to hazard a guess that chromatic aberration must be the most
    bothersome of all aberrations in eyeglasses...
     
    William Stacy, Mar 26, 2007
    #10
  11. carrera d'olbani

    Mark A Guest

    Dear unhelpful person:

    Chromatic aberration is the most insidious kind of aberration in eyeglasses
    since it makes images appear as a blurry (under normal lighting) as the
    different colors in the light spectrum focus in different positions (even
    for black text on white paper).

    Other kinds of aberrations such a pincushion distortion, barrel distortion,
    etc, etc may cause straight lines to appear slightly curved, but these
    distortions do not generally make the image blurry.
     
    Mark A, Mar 26, 2007
    #11
  12. carrera d'olbani

    Mark A Guest

    Yes, I have heard of most of them. None of them are nearly as important to
    eyeglasses as is chromatic aberration.

    Chromatic aberration blurs the image (not just distorts it) and the
    aberration is generally higher (abbe value is lower) as the lens index of
    refraction increases. There are exceptions, especially Polycarbonate lens
    material (1.59 index) which has an abbe value of 30, even though there are
    many higher index lens materials (1.67 etc) that have an abbe value of 32 or
    higher (the higher the abbe value, the lower the chromatic aberration).

    Almost every lens manufacturer will specify the abbe values of their
    materials to help ECP and consumers make the proper choices and trade-offs
    for the patient. I have never seen a lens manufacturer specify values for
    the other aberrations you have mentioned. That does not mean the other
    aberrations you mentioned don't exist, but they are not significant enough
    to worry about in modern lens designs for eyeglasses.
     
    Mark A, Mar 27, 2007
    #12
  13. carrera d'olbani

    redbelly Guest

    Spherical aberration and coma will also blur the image. Here the
    situation actually improves with higher index, as less curvature is
    required for a given lens strength.

    Mark
     
    redbelly, Mar 27, 2007
    #13
  14. carrera d'olbani

    whitely525 Guest

    You remind me of the picture frame I once purchased with 'low
    reflectance glass'

    In fact, there was no glass in it at all. Effective, mind.
     
    whitely525, Mar 30, 2007
    #14
  15. I posted it in the very first message. It is -4 dioptre.
    My previous glasses lasted me for 9 years. I wanted my new glasses to
    last for about the same. Sometimes I work in dusty conditions. When I
    wipe the glasses, the coarse dust can scratch the AR coating off. In
    fact, I just noticed a tiny scratch on the AR coating.

    I guess I could bring the glasses back to the optometrist for a
    refund, but... I doubt that I could get anything better anywhere else.
    It is all the same. I am just recalling the reason why agreed to order
    the glasses with the high RI, and then buy them when it came out that
    the glasses got AR coating (contrary to what I ordered) was that I
    wanted to try something "modern" and "high-tech". Oh well, you gotta
    pay for the "toys".
     
    carrera d'olbani, Apr 1, 2007
    #15
  16. carrera d'olbani

    Toller Guest

    You are both wrong.
    My polycarb lenses produce an annoying chromatic aberration that is not
    present in my plastic (I can't quite think of the name right now, the
    ordinary stuff) do not. I just got a pair of plastic glasses and polycarb
    safety glasses; when driving the difference is quite obvious on the tops of
    trees.

    However, they are not particularly blurry, and you get used to it. My old
    lenses were polycarb and I stopped noticing the colors after a while. I
    only notice it now with my safety glasses because I don't often wear them
    outside.
     
    Toller, Apr 1, 2007
    #16
  17. carrera d'olbani

    walla18j14 Guest

    Robert,

    I have been reading with great interest the information you & others
    here have provided regarding the differences in lens choices for
    progressive lenses. Being in my mid-40's, I am now in need of getting
    my first pair of progressive lenses filled. My Rx is currently a: -50/
    PL with a +1.75 for reading.

    Working as a waiter during lunch in a fine dining establishment, it
    would be far too inconvenient for me to resort to putting on/off my
    non-Rx readers every time I'm about to write an order while standing
    at a guests table! Hence, the need for progressive lenses.

    Yet, after getting quotes with 3 different national optical chains
    today I can only see NOW how each one was 'pushing' so strongly for me
    to go with polycarbonate lenses (Panoptic-short by Duralite???) over
    plastic C-39, never mind the dramatic cost difference!

    Being this is my first time trying progressive lenses and my Rx, Do I
    need the polycarbonate lenses for safety reasons of impact resistance
    or would C-39 do as well? Also, if I go with the plastic lenses
    should I opt for either the UV, Scratch, or AR coatings?

    Thanks in advance for any help you and the others can lend....
     
    walla18j14, Apr 24, 2007
    #17
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