purple fringe - due solely to Bayer filter?

Discussion in 'Digital Photography' started by digiboy, May 31, 2004.

  1. digiboy

    digiboy Guest

    OK, here's my theory (no doubt 10,000 flames inbound ;) ).

    We all know that the Bayer filter is short of data in the red and blue
    channels actually they have only half the data of the green channel.

    So imagine an image where an area of 100% white is right next to an
    area that is substantially less.

    The de-mosaic / interpolation process doesn't know where the dividing
    line is and so interpolates ie invents an intermediate value for each
    channel. Because the red / blue channels have 1/2 the data this is
    more obvious in red / blue ie you get a purple fringe.

    I have looked at loads of test images of on dpreview, stevesdigicams
    etc and all Bayer cams have the fringe.

    Please note that this is not a Bayer / Foveon post, just my thoughts
    on this one problem.


    digiboy, May 31, 2004
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  2. digiboy

    Searching_ut Guest

    Wow, I wish I had known this 30 years ago when I was blaming a cheap lens on
    my film camera for the problem. How does your theory fit in with fringing on
    Sigmas and film? As I recall, some have speculated that maybe the Sigma
    cameras suffer the problem worse than some of the bayer cameras because they
    capture the image more precisely.

    Well, you asked for thoughts, I personally think this theory needs a little

    For what it's worth

    Searching_ut, May 31, 2004
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  3. digiboy

    Paul Howland Guest

    Nothing to do with that. Chromatic aberration (aka. purple fringing) is
    due to the lens focusing red and blue light in slightly differently
    places. This is what so-called achromatic lenses in more expensive
    lenses are designed to solve. CA can also arise due to the microlenses
    in front of the sensor - both Bayer sensors and the SD10's Foveon sensor
    use these. Foveon and Bayer sensors are equally prone to this effect.
    Indeed, problems with chromatic aberration is a consistent (minor)
    criticism of the Sigma SD10 in some reviews. The SD9 does not suffer so
    much from this - other than due to the camera's lens - as it has no
    micro lenses over the sensor. But as a consequence, its low light
    performance is abysmal and it can go no faster than ISO400.
    Paul Howland, May 31, 2004
  4. digiboy

    Alan F Cross Guest

    I think you can very crudely determine whether your theory is correct.
    Shoot an image of a checkerboard. As the sensor filter is the same all
    the way across, you'd expect the same effect (on the same side of the
    light/dark transitions) at both ends, if it were the source of the
    problem. However, if the fringing is to the left on one side, and to the
    right on the other, you can reasonably assume that the problem is
    optical. It's either the lens itself, or the angle of incidence on the
    sensor itself.
    Alan F Cross, May 31, 2004

  5. The effect you are talking about - highlight-created purple fringes -
    seems to be Bayer specific and should not be confused with magenta and
    green, or yellow and blue, chromatic aberration fringes. The Sigma picks
    these up as badly as any camera, and they can be correct by the same
    chromatic aberration controls in things like Photoshop's CS Raw import
    plug in. The Sigma does not have the 'purple fringe' phenomenon at all,
    but then nor do most larger sensor DSLRs. It's mainly found in small
    sensors and especially in 2/3rds 8 megapixel sensors.

    Makers are still trying to eliminate it but the safest way is to use a
    small aperture. It seems to have something to do with lens aberrations
    combining with the way microlenses work on a Bayer CCD. At the least it
    may just be a non-chromatic aberration - coma - making itself seen as it
    used to on old photos.

    David Kilpatrick, May 31, 2004
  6. (digiboy) wrote in
    The normal purple fringing problem is radial symmetric and
    is therefore either due to lens color aberration or due to the
    incident angle the light hits the sensor. The microlenses are
    suspected to be one cause in the latter case.
    If this is true, then the fringe shall be no more than 1 pixel wide.
    It shall also be striped in blue/red along the edge. Now - all good
    interpolation algorithms I have seen smooths the chroma information.
    So - the edges should (in a perfect world) be the same color as the
    dark area in your example.
    Can you please point at some typical examples?

    Roland Karlsson, May 31, 2004
  7. Gee another Geroge Preddy alias.
    ~ Darrell Larose ~, May 31, 2004
  8. Please bear in mind there are two quite different types of chromatic

    Longitudinal or axial chromatic aberration (LCA) is the one you have
    described, i.e. the effect of a lens bringing light of different colour
    to a focus at different distances. It is improved by stopping down, and
    is constant across the field. This is not what is being observed here.

    The other, which is the one which normally causes colour fringing, is
    transverse or lateral chromatic aberration (TCA), which is a difference
    in magnification of the image from light of differing wavelengths. TCA
    is an off-axis aberration; it gets worse at greater angles of incidence,
    i.e. towards the edge of the field. It is not at all improved by
    stopping down. The fringes are normally blue toward the axis and red
    toward the edge of the image (though I suspect over-correction is
    possible, I doubt it occurs often). Interestingly, it gets worse for
    longer focal lengths, and is why fluorite or very expensive special
    glasses need to be used to make such lenses perform well.

    The effects you mention, if they arise from the taking lens, will almost
    certainly be TCA, which is visible in most lenses if you look close
    enough. Strangely enough, it is one of the easiest to correct using
    software. Bryan Caldwell, whose optical expertise is orders of magnitude
    ahead of mine, has explained this several times here before, and has a
    very interesting web site on how to do it.

    You should be able to check if TCA in the taking lens is causing the
    problem; if the fringes get worse at the edge, and re absent or very
    small at the centre, then it is very likely* that TCA is the cause. If
    it is constant across the frame it certainly is not.

    *I say "very likely" rather than "certain" as, of the two other causes
    discussed, sensor angle of incidence also increase towards the edge.

    Other tests: If the fringes also occur on a film body and get worse at
    the edge, it's certainly TCA.

    If the fringes get worse with a telephoto lens, it's almost certainly

    If the fringes get less with a long lens and worse with a wide, it's
    probably a sensor incidence angle/microlens issue and has nothing to do
    with the taking lens.

    If the fringes stay pretty well the same regardless of lens and position
    in the frame, it could well be a sensor artefact. How this could be
    proved will require expertise beyond anything I can offer.
    David Littlewood, May 31, 2004
  9. digiboy

    E. Magnuson Guest

    Sigh. Where do you get this?

    Look at this photo:

    Look at the man in the white t-shirt near the center of the frame with
    the sunglasses on his head. Look at the specular reflection of the sun
    on the sunglasses. See the purple fringe? (It's not the lens: the
    comments say that this was taken with the 50mm EX, arguably Sigma's
    best lens.)

    Early versions of the SD9 were notorious for this on many specular
    highlights. The SD10 seems to be better, but it still happens.
    It's also not uncommon on specular highlights on Kodak SLR/n photos.
    It's most likely a blooming artifact (charge leakage to adjacent cells.)
    The easiest way to reproduce it is to take a photo of a shiny black
    car (even better if it has lot's of chrome) in the sun.
    It does, it just takes a brighter light/dark boundary to see it.

    (BTW, if it's only a "small sensor" artifact, how can you say it's
    Bayer specific when we don't have any images from a small sensor X3
    E. Magnuson, May 31, 2004
  10. This isn't the same purple fringe as people are talking about. I am
    aware the Sigma can bleed over when shooting a light source - I've done
    it. Interestingly, the Sigma produced an identical effect to the
    halation exposure visible on Fuji Velvia from the same shot with the
    same lens, which leads me to think that the sensor may just be copying
    an effect also visible on film.

    If you've seen the purple fringes which people complain about from the
    Sony, Canon and Minolta 8 megapixel cameras; from the Minolta Z1/Z2 and
    its Kodak brothers; and one or to other cameras - they are directional,
    affecting one side of a highlight area typically, and not chromatic
    (i.e. they do not have any matching complementary coloured fringe on the
    opposite transition of light/dark in the same zone of the image).

    The Sigma doesn't produce this kind of fringe, and nor do most DSLRs.
    The new Kodak DCS Pro/n produces something similar but not identical. I
    have yet to try one with the firmware fix. In my opinion, the Pro/n
    suffered from an entirely different problem with chromatic shifts in the
    peripheral field emphasised on one side by a tendency to a blue
    chromatic aberration-like error.

    David Kilpatrick, May 31, 2004
  11. digiboy

    E. Magnuson Guest

    Whether it's the same artifact (or a related one) is a matter for
    speculation. It depends on if you attribute it to blooming or
    lens/microlens interaction or just lens. But nothing in your
    "directional purple" explanation suggests one way or another how the
    color filter array system contributes to the artifact. Indeed, if it
    were a mosaic induced problem, you might expect the color shifts to be
    different for different parts of the image because the filter array is
    not perfectly symmetric.

    Or it could be lens related. See http://www.pbase.com/image/24586310
    OK, then like I said, you cannot conclude anything about X3 vs. CFA
    from this evidence.
    E. Magnuson, May 31, 2004
  12. digiboy

    Tony Spadaro Guest

    Tony Spadaro, May 31, 2004
  13. I note the "full exif" data is empty...
    Darrell Larose, Jun 1, 2004
  14. It's not the CAMERA lens, but it could be explained away by the
    SUNGLASSES lens.

    A better place to see the chromatic aberrations is at the top of image.
    The chrome on the cars is doing a splendid job of making purple.

    In any case, purple fringing has been around long before Bayer or Foveon
    sensors. It's symptom of poor lens design.

    This image SHOULD be more problematic, yet it isn't:

    Brian C. Baird, Jun 1, 2004
  15. digiboy

    E. Magnuson Guest

    Like I said, that shot of Champa's used the Sigma 50mm EX macro @ f8
    AND it's near the center of the frame. It should not be the lens. It's neither
    directional nor the same color as in http://www.pbase.com/image/24586310.

    You can also find similar examples of blooming in the Sigma sample
    portrait in the highlights of the eyes (but you have to blow up the
    image quite a bit tosee them.)
    I'm not sure why you think this photo SHOULD be problematic. It has
    none of the characteristics where you most commonly find the problem:
    * no dark outline (of foliage) against a bright sky
    * no specular highlights against a dark background.
    E. Magnuson, Jun 1, 2004
  16. digiboy

    digiboy Guest

    well I was right to suppose I would get a load of flames :(

    Lets be clear. I have no axe to grind for or against Bayer. I have no
    axe to grind for or against Foveon.

    All I was saying is that if an image has a very high contrast
    transition occuring spatially in a small area, and you have to
    interpolate a channel to try to make up for another channel having
    more data, the interpolated channel will be incorrect, yes?

    Surely nothing too controversial. There are bound to be other effects
    at work.

    But all I can say is that if you get the high res pictures off
    DPReview and enlarge them in say photoshop then you will see whether
    you get a fringe.

    So I did.

    And this is what I found:

    The DC14N has fringing. The DC14N doesn't have microlenses
    The D100 has fringing. the D100 does have micro lenses
    The Sigma doesn't have fringing. the Sigma doesn't have microlenses

    OK I could go on. If microlenses give you a fringe, why does the DC14N
    have a fringe?

    As I have no axe to grind in any direction, the only logical
    conclusion I was able to come to was that the fringing is due to the
    method of imaging.
    digiboy, Jun 1, 2004
  17. digiboy

    digiboy Guest

    I had a look at this image, and zoomed right in using PS. One of the
    pixels you refer to measures as R251 G5 B234 so is basically magenta.

    This isn't really the cirumstance I am referrring to.

    If a Bayer had taken this image the mans white T shirt would have a
    fringe on his right sleave, like in this image from a D70
    Have a look at high contrast transition areas.

    As I said before I have no axe to grind.
    digiboy, Jun 1, 2004
  18. digiboy

    digiboy Guest

    your thoughts are fair enough. I was always lead to believe that
    chromatic aberation caused the different wavelengths of lights to
    focus at a different point. If so a lens with chromatic aberation
    might give a Foveon serious problems?

    fwiw I have never seen a coloured fringe on any of my films. I can't
    claim 30 years worth of films, but I can claim 20 years
    digiboy, Jun 1, 2004
  19. digiboy

    digiboy Guest

    yes, and its the fact that on my 2 Canons the fringe is always to the
    right that set me thinking. Sometimes the fringe is on the right and
    lower in the frame.

    But alot depends on the demosaic algorythm. If the sensor is short on
    data horizontally it'll be the same vertically as well. So this
    fringing is no surprise to me.

    I've no problem with lens problems interferring (so to speak) as well.
    digiboy, Jun 1, 2004
  20. digiboy

    digiboy Guest

    If microlenses are the problem why does the Kodak DSC14 get fringing?
    digiboy, Jun 1, 2004
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