Blue and red fringes on white objects toward edge of image

Discussion in 'Digital Photography' started by mailbox, Apr 28, 2007.

  1. mailbox

    mailbox Guest

    In a 3280x2454 RAW image produced by my
    Olympus SP-350, reduced by 4 in this
    JPEG version,
    http://cpacker.org/a1.jpg
    the white markers I placed in the
    scene as geometric control points on the
    left: http://cpacker.org/a2.jpg
    and right: http://cpacker.org/a3.jpg
    have blue fringes on the edges that
    face the center of the scene. This is
    chromatic aberration in the lens, right?
     
    mailbox, Apr 28, 2007
    #1
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  2. mailbox

    Rutger Guest

    Yes, it is.
    Easy to correct with photoshop (filters-distort-lenscorrection) and others.

    Rutger
     
    Rutger, Apr 28, 2007
    #2
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  3. mailbox

    Jim Guest

    Yes
    Jim
     
    Jim, Apr 28, 2007
    #3
  4. mailbox

    Marvin Guest

    The effect that is usually called purple fringing is not
    chromatic aberration. It is not likely to be from the lens.
    I've seen a lot of theories about what causes it, and mine
    is that it a software problem. But I'm not convinced of any
    of the explanations - including mine.
     
    Marvin, Apr 28, 2007
    #4
  5. mailbox

    mailbox Guest


    I've just found tutorials for both Photoshop
    and GIMP that connect the problem, when it
    happens with digital cameras, to overexposure.
    I need to determine whether it's that or the
    lens that's causing my problem. In any case,
    I don't like the solution provided by either
    software package, which is basically a
    seat-of-the-pants desaturation manipulation.
    I need something I can apply in bulk to
    each image to my series.

    First, though, I'm going to try dialing back
    my exposure level to eliminate overexposure
    as a possible cause. I've just learned how
    to use the in-camera histogram to adjust
    this and was _shocked_ to see how overexposed
    the sky was. I hadn't been paying attention
    to this because I was preoccupied with the
    geometric registration problem until now.
     
    mailbox, Apr 29, 2007
    #5
  6. mailbox

    Alan Browne Guest

    yep...

    In an uncorrected lens red will go towards the center of the image and
    blue will go outward.

    This is why the left white control shows red on the left (red creeping
    rightwards into the white) and vice-versa for the blue.

    This is happening everywhere that there are blue and red color
    components, of course, but the white triangles show it up.

    Cheers,
    Alan
     
    Alan Browne, Apr 29, 2007
    #6
  7. mailbox

    Alan Browne Guest

    The effect presented by the OP is in fact chromatic aberration. The red
    and blue are well separated and distinct.

    http://cpacker.org/a2.jpg is the left control.
    - The red is on the "outside" creeping inward into the white area.

    - The blue is on the "inside" creeping outward into the white area.

    http://en.wikipedia.org/wiki/Image:Lens6a.svg

    I've seen "purple fringing" in my camera in overexposed shots with dark
    grey branches against snow.

    Cheers,
    Alan
     
    Alan Browne, Apr 29, 2007
    #7
  8. mailbox

    Marvin Guest

    Purple fringing commonly occurs at a boundary with high
    contrast. Reducing the overexposure should help.
     
    Marvin, Apr 29, 2007
    #8
  9. mailbox

    Alan Browne Guest

    In fact, your "white" controls measure as 134, 148, 149 (R,G,B) which is
    why they look grey in your image. If you expose to make that white,
    then the red and blue aberations will increase. This is normal for the
    lens you have. Some s/w can correct the chromatic aberration (I've
    never done this myself).

    IOW, you are already "underexposed" if you wanted the controls to come
    out white. The sky looks right to me.

    Overexposure, in my camera, leads to distinct coloration (purple fringing).

    Cheers,
    Alan.
     
    Alan Browne, Apr 29, 2007
    #9
  10. mailbox

    M-M Guest


    Would you rather have an underexposed foreground or an overexposed sky?
    Pick only one...
     
    M-M, Apr 29, 2007
    #10
  11. mailbox

    mailbox Guest

    Yikes! Now that I know what to look for, I can see the effect on
    lots of tree branches. And I'm looking at an image taken in bright
    sunlight, where I was able to center the histogram and not overexpose
    anything. What a cheesey camera is this SP-350! I guess there'll
    be an SLR in my future...
     
    mailbox, May 1, 2007
    #11
  12. The effect is there too, for some lenses. You just have to learn to
    correct it.

    It can be red-blue "primary" or purple-green "secondary" in nature.
    There are plugins for Photoshop that can correct any form of it.
    The Canon raw converter for Photoshop has it built in. (Which of
    course is an admission from Canon that their lenses suffer from it!)

    Doug McDonald
     
    Doug McDonald, May 1, 2007
    #12
  13. mailbox

    Saguenay Guest

    This plugin works wonderfully :
    http://www.sd3.info/pf828/PFree/PFree0-1.html
    and it is free.
    Be carefull not to download it as a corrupt state, her I had to download it
    twice to make it work in Photo Impact.

    mb
     
    Saguenay, May 1, 2007
    #13
  14. mailbox

    Aaron Guest

    Lightroom also includes a couple of features for (nondestructively!)
    correcting these types of problems (chromatic aberration, vignetting,
    and some color calibration, not to mention automatic hot pixel repair
    and manual dust spot repair) and I have found it to work very well on
    all counts. If you shoot a lot of RAW, it's nice to have all of those
    features packed into a system that can also deal with unconverted RAW,
    apply development settings, and organize them.
     
    Aaron, May 10, 2007
    #14
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