Compensation for aspheric and uneven illumination of flat images.

Discussion in 'Photoshop Tutorials' started by liaM, Oct 8, 2007.

  1. liaM

    liaM Guest

    Are there photoshop tools or other programs, which compensate for
    the curving of lines which occur when taking photos of flat images?

    I've been using a copy stand to do this. Another problem which occurs,
    is the unevenness of the lighting. I imagine it would be possible to
    take a first picture of the surface of the copy stand platform using a
    white mat, and applying the resulting image to correct the unevenness.
    Is there a way to do this, too ?

    novice learning Photoshop 5
    liaM, Oct 8, 2007
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  2. liaM

    ronviers Guest

    The Lens Correction filter, Filter>Distort>Lens Correction, may do
    what you're looking for but I'm not sure when it was introduced. I
    only know that it is available in CS2.
    You may be able to correct the lighting with a gradient overlay but it
    would probably be better if you corrected that with the actual
    If you can post a link to a sample image I'm sure someone will give it
    a try.

    Good luck,
    ronviers, Oct 8, 2007
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  3. liaM

    pico Guest

    Do it right in the first place and you won't need Photoshop. We've been
    doing for a hundred years.

    Use a slightly long lens (longer than the diagonal of the format).
    If you still get distortion, get a better lens.

    Use even lighting.
    Be happy.
    pico, Oct 8, 2007
  4. liaM

    liaM Guest

    The CCD is a 1/1.8 inch interline transfer CCD
    (which I calculate having a 2 inch diagonal = 50mm)
    The lens zooms between 8mm - 24mm. (The camera is a Pentax A30)

    Now what ;) ?
    liaM, Oct 8, 2007
  5. liaM

    liaM Guest

    Thanks.. I guess I shall have to investigate ($$ shudder) CS2.
    Is CS2 easier to use than version 5 ? Does it have a batch mode ?
    liaM, Oct 8, 2007
  6. liaM

    hoffmann Guest

    Filter > Render > Lighting effects
    is a powerful tool for the correction of lighting.

    Best regards --Gernot Hoffmann
    hoffmann, Oct 8, 2007
  7. liaM

    ronviers Guest

    I don't know if it's any easier since I have only used CS2 - I would
    guess about the same but more. CS3 is out now so you might want to
    look at that too. There is probably a plug-in for version five that
    will do what you are looking for - hopefully someone will tell you
    about it.
    I am sorry to say that I don't know what batch mode even is.
    ronviers, Oct 8, 2007
  8. File->Automate->Batch perhaps?

    Andrew Morton, Oct 8, 2007
  9. liaM

    ronviers Guest

    I have noticed that but never looked into it. I have yet to do
    anything worth keeping much less a whole batch:)
    ronviers, Oct 8, 2007
  10. liaM

    Rob Guest

    Its still not ideal to use a curved field lens to copy flat surfaces.
    Rob, Oct 8, 2007
  11. liaM

    pico Guest

    Unless you have a 'full frame' digital camera it's more likely your
    nominal-normal lens is 12mm. Use the 24mm setting. If you still get
    pincushion or barrel distortion (vertical lines bend in or out), then you
    might want to borrow a single-focal length lens (incorrectly referred to as
    a 'prime' lens.)

    (Even so-called full-frame digitals aren't quite that size because a
    peripheral area of the sensor is used for support circuits.)

    So, rack it in to 24mm. Keep the focal plane parallel with the flat surface
    being copied. Use two lights, each at 45 degree angles and equal distance
    from the subject. That will get you there.
    pico, Oct 8, 2007
  12. liaM

    pico Guest

    :) All lenses are curved. He should do okay without such.
    pico, Oct 8, 2007
  13. liaM

    liaM Guest

    Mega Thanks, Pico !
    liaM, Oct 8, 2007
  14. liaM

    Rob Guest

    I'm talking about fields.
    Rob, Oct 8, 2007
  15. liaM

    Paul Mitchum Guest

    Take a better picture in the first place: Use the longest zoom setting
    that produces the least pincushion effect. Put the material on a stand.
    Put the camera on a tripod. Get some better lighting, even just work
    lights, and experiment with angles, though straight-on will work best
    for the lights. I'd say use a flash gun, if it's at all possible. Use
    the histogram to get a good exposure (using manual mode, or EV
    compensation if that's not available.)

    To get rid of the lens distortion: Hugin. It's free. Take an image of
    graph paper or a brick wall or some other flat surface with lot of
    straight lines. Tweak it in Hugin to get the lines straight again and
    undistorted. Now process the pictures you took using the same settings
    in Hugin.

    Paul Mitchum, Oct 9, 2007
  16. liaM

    pico Guest

    Just giving you grief. Fields it is!
    pico, Oct 9, 2007
  17. liaM

    pico Guest

    Can't you use a scanner for this? Or is the object too large?
    pico, Oct 9, 2007
  18. liaM

    liaM Guest

    Very interesting.. Hugin would seem ideal for copy stand photography
    of flat pages (with a vacuum table.. which I have rigged up) in front
    of a bay window facing north for the light..

    What would you think if, when using Hugin, I use a wide angle.. the
    advantage of which is depth of field tight focus?

    Hugin would correct the spherical distorsion and I'd get the best focus
    available from my camera. There must be a catch ;) !
    liaM, Oct 9, 2007
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