Minus-Violet filters

Discussion in 'Digital Photography' started by g n p, Jun 22, 2005.

  1. g n p

    g n p Guest

    Do these lens filters sharpen up digital pix??
    Seems they would eliminate the chromatic distortion (violet fringing) so
    common to many
    digi-cameras.
    They work wonders for non-apo astro refractors (lens, not mirror,
    telescopes).
    (Check out www.lumicon.com where the 58mm costs $79.95)
    Any experience??

    TIA
     
    g n p, Jun 22, 2005
    #1
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  2. g n p

    Marvin Guest

    The cause of violet fringing seems to be either a lens defect (somehting like chromatic
    aberraion perhaps) or it comes from the way the camera constructs the image fromthe sensor
    data. Either way, a violet filter (which is a haze filter) isn't likely to correct the
    fringing.

    I'd have to see it to believe it.
     
    Marvin, Jun 23, 2005
    #2
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  3. g n p

    Nostrobino Guest

    My guess is it's the latter. Many people including reviewers refer to purple
    fringing as chromatic aberration, but that doesn't seem to me to be a likely
    explanation for it. Chromatic aberration is either primary (a lens brings
    different wavelengths to different focal lengths) or secondary (different
    wavelengths produce different image sizes at the focal plane). While
    secondary chromatic aberration implies that there will be some color
    fringing, why always purple? That is certainly not the case with lenses used
    on *film* cameras, so why on digital?

    I think it must be something that happens at the sensor itself, perhaps
    something to do with the angle at which the image-forming rays strike the
    primary-color filters.

    Well, that's an ultraviolet filter you're talking about. He's asking about
    *minus*-violet filters, which I presume are supposed to remove the purple.
    I've never heard of any such thing before.

    Me too. I would be surprised if any filter could magically remove the purple
    fringing without making the rest of the image greenish.

    N.
     
    Nostrobino, Jun 23, 2005
    #3
  4. g n p

    g n p Guest



    Like I said in the OP, check out www.lumicon.com
    The minus violet is essentially a high pass filter allowing RGBY to pass but
    nothing below blue.
    It DOES work miracles in refractors (telephotos) but I wanted to check its
    performance in everyday digital photography.

    Thanks again.
     
    g n p, Jun 23, 2005
    #4
  5. g n p

    Nostrobino Guest

    I've looked at the product on that site, but I don't think it will do what
    you want it to do. It appears from the description to be designed primarily
    for astrophotography, where I can see that eliminating far-violet and
    ultraviolet wavelengths will indeed reduce chromatic aberration and
    resulting image degradation as far as points of light are concerned. My
    guess is that color accuracy is not of much importance in such photography,
    as it is, of course, in terrestrial photography.

    Again, I just don't believe that purple fringing in digital photos is the
    result of the lens passing too much purple, which it seems to me is the only
    thing a minus-violet filter could correct.

    It's an interesting idea, though.

    N.
     
    Nostrobino, Jun 24, 2005
    #5
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