Can't get rid of purple hue?

Discussion in 'Digital Cameras' started by Brett, Nov 30, 2005.

  1. Brett

    Brett Guest

    I have an Olympus c2100z (2.1 pixel). In this photo,
    http://pg.photos.yahoo.com/ph/cygen...dnm=29f8scd.jpg&.src=ph&.tok=phmuGBEB4x1p2UzO, I
    can't get rid of the purple hue on top of the apple. The apple was very red
    there but that isn't nearly reflected in the photo. I adjusted white
    balance in all four areas available to me. The purple never went away. I'd
    just end up with more orange photos. This was taken at the highest JPEG
    quality. I'd use TIFF but it is super slow on this camera.

    Here are the photo specs:
    1600X1200
    72dpi
    24 bit depth
    Focal length: 24mm
    F/8
    3.2s
    ISO100
    Light Source: Tungsten

    Any suggestions?

    Also, I will be getting a 20D soon. I'd imagine the photo would be more
    accurate as to what I'm actually viewing with the naked eye. Will taking
    this photo in RAW at a higher pixel count make it more accurate? General
    comments?

    Thanks,
    Brett
     
    Brett, Nov 30, 2005
    #1
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  2. Brett

    Paul Allen Guest

    I thought maybe you were talking about the purple fringing that small-
    sensor cameras sometimes show around strong highlights. I have a c700
    that's of similar vintage to your c2100, and it's got that problem
    pretty bad. But I don't see anything like that in your image. There
    does seem to be a vague semi-uniform purple irridescence over the lit
    part of the apple. Did the apple have a waxy coating? Perhaps the
    camera captured exactly what it saw.
    From what I've heard, you'll like the 20D. I'd sorely miss the long
    zoom, but I'm not you.

    Paul Allen
     
    Paul Allen, Dec 1, 2005
    #2
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  3. Brett

    Paul Mitchum Guest

    If the apple is outdoors, then the 'purple hue' is actually a reflection
    of the blue sky. Adjusting the white balance won't make it go away.

    Try shooting it indoors, or shooting it in the shade where you'll still
    have diffuse sunlight, but no blue reflection. Or open it up in
    Photoshop and kill the blues in the apple.
    You're getting a 20D so you can photograph apples? :)

    I betcha a 20D will register the blue sky reflection as well.
     
    Paul Mitchum, Dec 1, 2005
    #3
  4. Brett

    Brett Guest

    That's what I'm wanting to understand. Why? I see a complete red apple but
    the camera interprets some purple around the reflecting part - where the
    light is hitting it. This is indoors by the way. Not sure if the apple has
    a waxy coating. I bought it out of the store ready to eat. I just want to
    know why I'm seeing one thing and what is going on with the camera that is
    it seeing another. I want to understand technically what is going on? In
    other words, what may be going on with the camera, lens, processing, light,
    etc with this particular camera. I doubt it is there with the 20D. This is
    just a lesson in technical digital photography. Why does one camera do this
    and not the (better) other camera. I have many ideas as to why that is
    (better processing, more pixels, better lens) but "better" is a loaded word.
    I'm interested in the specifics as to what is causing this paricular
    problem.

    That's what more lenses are for. : )
    Thanks,
    Brett
     
    Brett, Dec 1, 2005
    #4
  5. Brett

    Hunt Guest

    I'd guess that there is some window/sky light hitting the top of the apple,
    which contains UV light. Our eyes will not see the color difference, but the
    camera will. Mixing UV and the red from the apple yields purple. If you had
    placed the apple in a tent and photographed it, with only a single color light
    source, say tungsten, I'll bet that you would get closer to what you saw, than
    this image. Also, any coating on the apple may well reflect UV, as well. For
    accurate color, you have to control ALL of the lighting.

    Hunt
     
    Hunt, Dec 1, 2005
    #5
  6. I guess you are not asking how to get rid of the purple in an
    editor..? But I just did it and you can see the result here:
    http://00-00-00.org/apple.html I use PhotoImpact 10, choose
    "Adjust" and "Color replacement". Then you can pick a color in
    the photo, select a range, and then adjust hue, saturation and
    lightness for that color range alone. Nice and easy.
     
    Vidar Grønvold, Dec 1, 2005
    #6
  7. Brett

    Paul Allen Guest

    Well, the camera's sensor is almost certainly not sensitive to the
    exact same spectrum range that your eyes are, but the pixels it produces
    are RGB triples. So there's room for surprises in certain
    circumstances. Is this what's really going on? Damfino.
    Lets be really precise about how we use language here. The apple is
    illuminated by a strongly directional light source at the upper left.
    I see a slight purplish irridescence over most of the illuminated half
    of the apple. There's also a strong specular highlight at a small spot
    on the apple where it is reflecting the light source directly. When you
    say "some purple around the reflecting part", are you talking about
    that small highlight or the entire illuminated portion of the apple?
    My C700 will often produce an erroneous purple fringe around strong
    highlights, especially when they're bordered by shadow. I think it's
    caused by blooming in the sensor rather than an optical defect. I
    don't see anything like that in your image, but it's really small.
    Have you actually shot this particular apple with the 20D? I would
    expect the 20D to produce better results by just about any measure, but
    if its spectral sensitivity is similar to your C2100's, it may record
    similar colors. I hope you'll try the experiment and post the results.
    If anything, it'll give me more ammunition in my campaign to upgrade my
    equipment. :)
    I wish I had your camera budget. :)

    Paul Allen
     
    Paul Allen, Dec 1, 2005
    #7
  8. Brett

    Paul Allen Guest

    No, it's not the lens. The sensor is seeing photons that are outside
    the visible spectrum but have the right energy to kick off electrons
    and get counted. It might be interesting to shoot the apple with and
    without the UV filter. Without knowing the response curves of the
    filter and of your sensor, it's hard to predict what will happen.

    Paul Allen
     
    Paul Allen, Dec 2, 2005
    #8
  9. Brett

    Brett Guest

    Paul, I did try this without the UV filter and still purple. I tried all
    the options of white balance available on the camera and still purple.
    Chalk it up to a limitation on this camera? Certainly, we don't want purple
    flanging around the apple.

    Brett
     
    Brett, Dec 4, 2005
    #9
  10. Brett

    Paul Allen Guest

    Well, yes, of course the purple was still there when you removed the UV
    filter. The purpose of the experiment was to see if the filter was
    doing anything or not.
    Yeah, adjusting the white balance probably wouldn't fix this.
    It's likely a limitation of many digital cameras. If you feed them
    photons that they can see but you can't, they're going to record them
    as numbers in RGB triples and you're going to be surprised.
    Now you're saying "around the apple". Which is it? A more-or-
    less uniform purple cast over the sunlit part of the apple? Or a
    purple fringe around the edge of the lit part of the apple where it
    borders shadow? They're two different problems, with two different
    causes.

    Paul Allen
     
    Paul Allen, Dec 4, 2005
    #10
  11. Brett

    Brett Guest

    Ok, I think we have identified the most probable suspect for the purple on
    the apple. Above I was referring to the same purple - it was the lit part.
    This is inside via artificial light.

    On another subject but not entirely, what is causing the weird bright pixels
    in this photo: http://www.flickr.com/photos/brettr/66672001/. You can see
    them especially in the water. I took some more night photos and see them
    there also. I tried ISO100, ISO200 with HQ JPEG and TIFF. The exposure is
    16s. Any ideas?

    Thanks,
    Brett
     
    Brett, Dec 5, 2005
    #11
  12. Brett

    Paul Allen Guest

    Looks like hot pixels in your sensor. My C700 has a few like that.
    They're always in the same places, and are most noticeable when the
    area around them is dark. Longer exposures make them worse. Changing
    ISO or image format will have zero effect. On images that matter, I
    clone them out with the GIMP.

    Paul Allen
     
    Paul Allen, Dec 5, 2005
    #12
  13. Brett

    Brett Guest

    Do higher end cameras have this problem? Does sensors writing to RAW make
    any difference?

    Thanks,
    Brett
     
    Brett, Dec 5, 2005
    #13
  14. Brett

    Paul Allen Guest

    I would hope not, but don't really know. I first noticed that I had
    hot pixels when my C700 was two years old. I just now dragged some
    July 4th fireworks shots from 2001 out of my database, and they don't
    show the hot pixels. It's apparently something my sensor developed
    over time. Disappointing for a camera that cost about $600 new.
    I don't think the compact point'n'shoots are designed to last very
    long. I hope the higher end models like the Fuji S9000 or the
    Panasonic FZ20 are more durable.
    Nope. The damage is already there when the camera reads the data
    out of the sensor. The good news is that it is predictable, so you
    can fix it to an extent with software.

    Paul Allen
     
    Paul Allen, Dec 6, 2005
    #14
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