Can we 'twist' a digital picture to be square with the camera?

Discussion in 'Digital Photography' started by maryanne kehoe, Jun 12, 2006.

  1. What 'trick' am I missing to take a photo of my watercolor paintings?
    Can we 'twist' the digital photo to be square with the camera?

    I must be doing something wrong. When I edit my digital photographs of my
    watercolor paintings, I need to crop the photo but I can NEVER get the crop
    edges to be exactly square with the digital camera photograph of the

    Therefore I always find myself losing a few millimeters of painting so that
    the photograph is square. What am I doing wrong?

    Here is how I take the picture.
    a) I put my unmatted 30 inch by 22.5 inch watercolor on a white towel lying
    on the ground
    b) I stand over the painting, sometimes on a step stool, and position the
    Canon PowerShot A95 5MP camera over the center of the painting
    c) I snap the shot using flash & macro focus making sure I am as square as
    possible and taking an extra inch or so of border all around to facilitate

    Back inside the office, each and every time, when I crop the 2592 by 1944
    pixel digital results using Irvanview 3.98 on Windows XP, I end up with
    edges that are not square at first. I have to lose some of my precious
    painting just to get square edges when I crop.

    What am I doing wrong?
    What's the trick to get a photo that is head-on with the painting?
    Is there a way to twist the painting with software so I don't have to crop
    edges and lose painting data?
    maryanne kehoe, Jun 12, 2006
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  2. maryanne kehoe

    2 Guest

    It seems you have a viewfinder that is not accurate - shows more coverage
    than is there, and/or distortion is having its way. You should not need
    macro-mode for such pictures, either.

    Hang the painting on a wall. Shoot with a tripod. Leave the camera on the
    tripod and check the photos. Adjust until it is correct and leave it there
    for the pictures.
    2, Jun 12, 2006
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  3. maryanne kehoe

    Trev Guest

    Well you can rotate if its not SQ or maybe you have a ratio of width to
    height that's the problem As I cant see your image I dont know What size are
    you cropping the 2592*1944 image to.

    PSP has straightening and parallax correction tools. Does your picture
    have more white border at one side then the others.
    Trev, Jun 12, 2006
  4. What am I doing wrong?

    Your description suggests you're seeing barrel distortion, which is an
    effect of the lens. Have a look at this page from DPReview's review of
    your camera:

    Scroll down to the Macro Focus and Barrel and Pincushion Distortion
    sections - is that what you're seeing?

    To minimise the effect, start by switching macro mode off. I don't know
    why you're using that. Second, if possible get further away from your
    painting and zoom in on it a bit. This will be easier if you hang it on
    a wall. Your camera shows less distortion when it's not at its widest
    zoom setting. Failing that, you'll need to use an image processing tool
    to correct the distortion.
    Derek Fountain, Jun 12, 2006
  5. Either that, or the subject and sensor planes are not parallel.

    That is easy to get quite accurate by replacing the paintings by, or
    put in front of the painting, a mirror (tile),absolutely parallel to
    the painting's surface. If the camera lens is imaged dead center in
    the viewfinder (and assuming parallax shift is taken into account),
    every thing shouls square out except for lens distortions.


    Bart van der Wolf, Jun 12, 2006
  6. maryanne kehoe

    CB Guest

    You need to use tools to make sure that the picture and the camera are
    absolutely square, and that the camera is centered on the image. Handy tools
    are an easel for the painting, a tripod for the camera, a good level, a
    measuring tape, and a carpenter's square.
    If you get something like Photoshop or PhotoImpact OTOH, you can correct the
    misalignment in software.
    CB, Jun 12, 2006
  7. maryanne kehoe

    babalooixnay Guest

    In newer editions of photoshop and others you can "twist" the image but
    there will be some loss of image quality (possibly extremely minor but
    it will be there). If this is something you do a lot you might want to
    consider a copy stand although a copy stand large enough to accomodate
    your canvas and give you enough height to use a lens that will not
    cause distortion (I would aim for using a 40 - 60mm after taking crop
    factor into account) will not be cheap. A tripod and a wall will
    probably be better than the sytem you are using but you will still have
    geometry problems and repeatability will be difficult. That is where
    copy stands come in. Copy stands will also have facilities for various
    lighting set-ups.
    babalooixnay, Jun 12, 2006
  8. maryanne kehoe

    tomm42 Guest

    There are two problems you are running into:
    1) Alignment of the painting to the sensor plane
    2) Distortion from the lens
    What to do?
    There are several ways to align a painting/sensor. The mirror method
    has been posted, I find it inaccurate (does work for some). One thing
    needed is a tripod so you can fix the camera in one place, shooting by
    hand means every picture will have a different alignment. I place the
    painting on an easel, make sure the center and edges are parallel to
    the camera lens, use an angle finder (available for $6-$10 at a
    hardware store) Make sure the lens and painting are at the
    corresponding angles. This is more involved than what can be covered
    here and is described in a Kodak book on photographic reproduction.
    The lens problem comes from using a zoom lens, most zoom lenses have
    distortion throughout their range, most common are barrel and pincusion
    distortions. Depending on the quality of the lens these distortions are
    better controlled with the more expensive lenes are almost distortion
    free. The other distortion in copying flat objects is field curvature,
    this is maybe desirable in general picture taking, but annoying in
    reproduction work. Both of these problems are cured by using a true
    macro lens. this unfortunately is not an option with your A95.
    Best thing for you to do is get a decent tripod and work on getting the
    painting and sensor aligned. Do not use software to correct the
    alignment as that enters other distortions into the image.

    tomm42, Jun 12, 2006
  9. maryanne kehoe

    Dan Fox Guest

    The guy who shoots my slides first puts up a black drape on the wall larger
    than the work, then mounts the work in the center of the drape. The photos
    have a black border, look great, and there is no cropping involved.

    In my experience (before I gave up and had a professional do it) you can
    *never* get it so square that you won't lose a little of the work.
    Dan Fox, Jun 12, 2006
  10. For one thing, the fact that you're hand-holding the camera (especially
    while standing on a step stool) and trying to shoot straight down must make
    it extremely difficult to get the painting square with the camera. Isn't
    there some way you can put the watercolor against a vertical surface so you
    can use the camera on a tripod? This would make it far easier to get
    everything square.

    With that size subject I don't know why you're using the macro setting,
    though that probably isn't related to your problem. Also unrelated to that,
    but isn't flash causing you some problems as well?

    There is, though there may be a slight loss of image quality (it may not be
    noticeable, so might not matter to you at all -- especially with a
    watercolor which is usually not a highly detailed subject anyway). Paint
    Shop Pro, for example, has a Straighten tool which does exactly what you
    want, very easily. No doubt there are other kinds of software that will do
    this too.
    Neil Harrington, Jun 12, 2006
  11. maryanne kehoe

    Ronald Hands Guest

    As others have mentioned, it's probably barrel distortion that you
    are seeing. It is probably most apparent at the wide-angle setting of
    your camera.

    So I would suggest:

    1. Turn off your flash.

    2. Turn off macro mode.

    3. Hang the painting on a wall, preferably on a north wall outdoors so
    that you're shooting it in shade.

    4. Zoom the lens out to a medium position, probably the equivalent of
    70 or 80 mm in a 35mm camera. Put the camera on a tripod and line it up
    with the center of the watercolor.

    5. Bracket your exposures if you know how to do that. (You may have to
    switch the camera to P mode rather than Auto to use exposure
    compensation). Check the histogram which is available on your camera to
    make sure you are getting a full range of exposure response.

    You're fortunate in that your 30 x 22.5 watercolor is an exact match
    for the 4:3 sensor dimensions in your camera, so not much cropping
    should be required. I presume you are using the LCD display to frame
    your picture. The optical viewfinders in Canon Powershot models show
    only about 85 per cent of what you are actually shooting.

    My wife shot a bunch of pictures of her favorite Impressionist
    paintings in the d'Orsay museum in Paris, using a Canon A40. All of
    them show barrel distortion, since she was using wide-angle mode. She
    doesn't really care; she's just glad to have them in the album :)

    -- Ron
    Ronald Hands, Jun 12, 2006
  12. maryanne kehoe

    gpsman Guest

    I would:

    Hang the painting, if I didn't have a copy stand. Use a tripod for the
    camera. Align both painting and the camera's back as close as

    Use a medium zoom and mid-aperature settings, not macro.

    I'd experiment with lighting and white balance settings. It seems to
    me flash might wash out the colors in a watercolor and maybe cause some

    Use the timer to trip the shutter.

    - gpsman
    gpsman, Jun 12, 2006
  13. .. . .
    Definitely, though that doesn't seem to be what her complaint is. If it is
    barrel distortion that's the problem, that can usually be corrected in
    Neil Harrington, Jun 12, 2006
  14. maryanne kehoe

    2 Guest

    Correcting barrel or pincushion distortion in software is the last resort of
    the scoundrel. I don't know why folks put up with such a waste of time. Get
    good lenses and quit spending your life sitting on your butt at a computer
    'fixing' things that can be done right the first time.
    2, Jun 12, 2006
  15. maryanne kehoe

    Ed Ruf Guest

    Oh please. Now tell us exactly what P&S with some optical zoom is free
    from such distortion. Or even which higher end DLSR lens zooms are
    completely free of it. Not everyone is using perfect primes.

    Ed Ruf Lifetime AMA# 344007 ()
    Ed Ruf, Jun 12, 2006
  16. maryanne kehoe

    2 Guest

    You pay one way or the other. Make your choice. It's still just plain nutz
    to be spending hours fixing a picture that could have been made once,
    properly, in a few minutes.
    2, Jun 12, 2006
  17. maryanne kehoe

    Dan Guest

    PSP makes corners that are mathematically exact right angles. If your
    subject doesn't crop to right angles, then the problem is not with

    1. Lining up your camera and painting may need to be more precise.
    It's not easy to get those suckers exactly parallel. 2. Or (perish the
    thought!), your painting may be a lttle off-square. 3. Or, maybe your
    less-than-$5000 camera lens is producing a small distortion.

    In any case, you might not have to reshoot your subject. PSP9 (which
    is the version I'm familiar with) has a Perspective Correction tool
    that could be enough to satisfy your needs.
    Dan, Jun 12, 2006
  18. maryanne kehoe

    Alfred Molon Guest

    What a nonsense.

    With the PTLens filter in Photoshop fixing barrel or pincushion
    distortion takes a only few seconds. You can even set up an action to
    batch correct hundreds of photos.

    The only thing you pay is a slight loss in resolution due to the
    recalculation of the pixels, but in the era of megapixel cameras with
    tons of resolution this is a minor issue.
    Alfred Molon, Jun 12, 2006
  19. maryanne kehoe

    AZ Nomad Guest

    Croping to right angles is a trivial operation.
    I think you're trying to say something else. Perhaps a
    trapezoid->square transform?

    AZ Nomad, Jun 12, 2006
  20. maryanne kehoe

    Fred Hiltz Guest

    Yes. There are two kinds of distortion to be corrected. PSP does

    The first is pincushion or barrel distortion caused by the lens.
    Before any cropping or other distortion adjustments, correct it
    (from the Adjust > Photo Fix menu) to make the edges of the painting

    The second is pure geometry, a perspective shift caused by the
    camera's view point being just a little off center from the
    painting. Fetch the Perspective Correction tool. Move the corners of
    its rectangle to the four corners of the painting (or any other
    places that you know should outline a rectangle). Double click and
    magic happens.
    Fred Hiltz, Jun 12, 2006
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