Plastic lenses

Discussion in 'Digital Photography' started by Alfred Molon, May 28, 2006.

  1. Alfred Molon

    Alfred Molon Guest

    Would it be possible to make plastic lenses with the same optical
    quality and performance as glass lenses? The advantage would be lower
    weight.
     
    Alfred Molon, May 28, 2006
    #1
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  2. Alfred Molon

    Bill Funk Guest

    SINCE glass and plactic have different optical qualities, no.
    However, several lenses already include plastic (well, epoxy or some
    such) now.
     
    Bill Funk, May 28, 2006
    #2
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  3. Alfred Molon

    Alfred Molon Guest

    I meant with the same performance in terms of sharpness, aberrations,
    MTF etc. Just curious. Or if not plastic what about other materials
    which weigh less than glass?
     
    Alfred Molon, May 28, 2006
    #3
  4. Alfred Molon

    RW+/- Guest

    No.
     
    RW+/-, May 28, 2006
    #4
  5. Alfred Molon

    RW+/- Guest

    refractive index is not the same.
     
    RW+/-, May 28, 2006
    #5
  6. Assuming some appropriate material existed, the refractive index being
    different wouldn't matter. Don't think of it as a drop-in replacement
    for the glass elements in an existing lens design. Instead, think of it
    as an alternate material from which new lenses could be crafted.

    Such a material wouldn't have the same refractive index, but it would
    have a *known* index, which would be taken into account when designing
    lenses, just as the refractive index of glass is now.
     
    Eric Schreiber, May 28, 2006
    #6
  7. Refractive index of lots of things isn't the same. In fact that's
    *why* Nikon uses ED glass and Canon uses (used?) fluorite elements in
    some lenses! And optical glass is available in a wide range of
    refractive indexes.

    Furthermore, plastic has essentially completely replaced glass in
    eyeglasses, and the resulting eyeglasses are much thinner as well as
    lighter, suggesting a higher refractive index than the glasses
    previously used. This also suggests that (at least with coatings) the
    plastics are durable enough to use even as the front element of a
    lens; a pair of glasses gets a LOT more abuse than the front element
    of a lens, since it's used a lot more hours a day and not treated with
    as much care. (And of course most of the elements are internal,
    where they're not exposed to such wear and tear).

    So I very much doubt that the available refractive indexes are the
    limiting factors in use of plastic lens elements. In fact I remember
    reading that some consumer-grade lenses are using molded plastic
    aspheric elements, apparently getting more benefit from the cheap
    aspheric elements than they lose in the lower precision
    (hypothetical).

    I'm curious to know what constraints there are, and why plastics
    aren't more widely used.

    Obvious possibilities include:

    1. Refractive indexes available. (Just because I don't like your
    suggestion doesn't mean I can rule it out out-of-hand!)

    2. Ability to manufacture to necessary precision.

    2.5 Cost to manufacture to necessary precision (no incentive for change)

    3. Temperature stability -- may shrink/expand too much over the
    relevant temp range.

    4. Flexibility -- may not hold shape well enough to retain the
    desired shape with the necessary precision.

    5. Toughness -- surfaces may be too easily damaged (seems unlikely
    with the eyeglass experience, but).

    6. Outgassing -- may give off stuff that fog the system over time.

    7. Prejudice -- they may believe the market would reject plastic
    lenses.

    And since I'm not a lens designer, I could easily have missed the real
    problem.
     
    David Dyer-Bennet, May 28, 2006
    #7
  8. Alfred Molon

    RW+/- Guest

    If such a thing were feasible it most likely would have been done by now.
    The quest for lighter eye glasses has been going of for years.

    Another thing that has to be considered is the stability of the material
    used.
     
    RW+/-, May 28, 2006
    #8
  9. Yes, and has resulted in essentially all eye-glasses now being made
    out of plastic.
    Yes, that can be an issue -- both strength to resist deformation (and
    remain held in position), and also change in size with temperature.
     
    David Dyer-Bennet, May 28, 2006
    #9
  10. Alfred Molon

    J. Clarke Guest

    That doesn't mean that a skilled lens designer can't crank up his computer
    and come up with something with equal performance, just that the details
    have to be different.

    Now it may be that designing in plastic he may run into limits that prevent
    equivalent performance from being achieved, and it may also be that there
    are manufacturing problems inherent in making precision optics from
    plastics that prevent the necessary tolerances from being held, but
    "different index of refraction" per se should not be a problem.
     
    J. Clarke, May 28, 2006
    #10
  11. Alfred Molon

    RW+/- Guest

    Fortunately it hasn't. Unless the plastic glasses are colored, as is
    typical for sunglasses, I cannot wear them. Most people do not notice the
    difference which is lucky for them. Just try and get safety glasses made
    out of glass which pass OSHA tests. :)

    The newest plastic glasses are thinner and so it offsets the RI differences
    but does not eliminate it.

    Your post addresses some key issues all of which requires consideration.
     
    RW+/-, May 28, 2006
    #11
  12. Alfred Molon

    RW+/- Guest

    Most, far from all.
    Temperature is one key for sure. Another thing to consider is resistance to
    Ozone, and other chemicals. The Sun does an excellent job of destroying
    plastics along with natural and/or unnatural airborne pollutants.

    If you'd like a rather simplistic test for light transmission of materials
    try and start a fire with them on a sunny day.
     
    RW+/-, May 28, 2006
    #12
  13. Alfred Molon

    J. Clarke Guest

    It also does an excellent job of destroying Leica shutters if you aren't
    careful about how you set the camera down and are forgetful about lens
    caps. Does it an awful lot faster than it will damage a plastic lens.
    Some plastics will hold up for decades in outdoor exposure without any
    significant loss of optical performance. Others won't. But camera lenses
    generally don't get that much direct sunlight unless you shoot all day
    every day at the beach or something.
    With what? I'm sorry, you've lost me. Never had any problem starting fires
    with plastic magnifiers as long as the diameter was sufficient, if that's
    what you're suggesting.
     
    J. Clarke, May 28, 2006
    #13
  14. So, if an "equivalent" lens could be designed that was, say 80% of
    the current weight, but that required 125% of the current SIZE
    (i.e., primarily length), would that be a "good thing"?
     
    Philip Bailey, May 28, 2006
    #14
  15. You're missing another property: dispersion. This measures how much
    refractive index *changes* with wavelength. Glass with no dispersion
    would allow making lenses that have no chromatic aberration, but that
    doesn't exist. On the other hand, having materials with a wide range of
    dispersions available allows mostly correcting for chromatic aberration.

    The acromatic doublet is the simplest example. You combine a strong
    positive lens made out of a material with low disperson with a weaker
    negative lens made out of a material with high dispersion. If you do it
    right, you get a lens that still has positive power, but where two
    colours are brought to exactly the same focus and chromatic aberration
    is considerably reduced through the whole visible wavelength range.

    The value of some exotic materials like fluorite and rare-earth glasses
    is that their combination of refractive index and dispersion is unlike
    regular glasses which allows the lens designer to make corrections that
    they simply couldn't do using standard glasses in the same number of
    elements.

    If plastics don't provide the same variation in dispersion
    characteristics as glass, there is a strong reason to keep *some* glass
    elements in a complex lens.

    Dave
     
    Dave Martindale, May 28, 2006
    #15
  16. Alfred Molon

    J. Clarke Guest

    It would for Colin Fletcher but not for James Bond.
     
    J. Clarke, May 28, 2006
    #16
  17. Alfred Molon

    grruffbowwow Guest

    plastic is severe chromic aberations. If you look at a light source (
    like a fluorescent tube) through these lenses there's a yellow ghost on
    one side and a green or magenta ghost on the other. And that's with
    coatings to prevent internal reflections. You only really notice it
    when you first switch to the high-index plastic, and quickly become
    used to it.
     
    grruffbowwow, May 28, 2006
    #17
  18. Alfred Molon

    RW+/- Guest

    I have yet to see any plastic hold up for decades under your scenario.
    You got it, all by yourself, eh? You also seem to understand that there is
    a significant difference. Good for you. :)
     
    RW+/-, May 29, 2006
    #18
  19. I believe it is a matter if the ability to, not a question of theory.

    It has proven very difficult to produce high quality, long lasting
    lenses of plastic.

    Overall with the exception of very large lenses, the weight of the glass
    vs plastic is nothing to worry about. Changing out much of the metal parts
    with plastic has worked (take a look at some of the digital SLR kit lenses).
    Making very large plastic lenses of high quality is not practical with the
    technology available at this time.
     
    Joseph Meehan, May 29, 2006
    #19
  20. Here's an interesting article that discusses this:

    http://www.p2pays.org/ref/34/33158.pdf
     
    Eric Schreiber, May 29, 2006
    #20
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