Color correction

Discussion in 'Digital Photography' started by lsilvand, Feb 12, 2004.

  1. lsilvand

    lsilvand Guest

    Hi,

    I'm involved in a project for a university where I have to photograph
    metal samples after they have gone through a chemical reaction.
    I have thought of photographing the samples with a color target that I
    then could use to correct the colors in a graphics program. I wonder
    if someone could help me with what kind of color target and computer
    program I should use.
    I'm going to use a Canon A70. I don't know the light level, does it
    matter? I could borrow a light level meter. The smaller the color
    target is, the better.


    Regards,
    Linus
     
    lsilvand, Feb 12, 2004
    #1
    1. Advertisements

  2. lsilvand

    Ed Ruf Guest

    Probably would be helpful to get a gray card from a camera shop and
    manually set the white balance for consistency.
    ________________________________________________________
    Ed Ruf Lifetime AMA# 344007 ()
    http://members.cox.net/egruf
    See images taken with my CP-990 and 5700 at
    http://members.cox.net/egruf-digicam
     
    Ed Ruf, Feb 12, 2004
    #2
    1. Advertisements

  3. lsilvand

    Larry Guest

    A simple target (make at least 2) made from white card stock (non
    glossy), Painted with Red/Green/blue will do the trick. Leave at least
    1/4 of the target white (to check white balance). It works best if you
    do color stripe/white/color stripe/white/color stripe.

    A blank white card from the same stock can be used to set the white
    balance of the camera when needed.

    The second card is for putting near your display while correcting the
    images.

    This is a simplified form of the way NASA checks the color in the photos
    coming down from Mars.


    Larry
    Mystic
     
    Larry, Feb 12, 2004
    #3
  4. lsilvand

    Tom Thackrey Guest

    Use a photographic 18% gray card. From it you will be able to correct both
    white balance (i.e. color) and exposure. If your lighting and exposure are
    consist ant, you can shoot one frame of the gray card and use the
    measurements from it for correcting the other shots. Personally, I would put
    a small gray card in every shot. I presume you are using full-spectrum
    lighting.

    You don't need a big target if the lighting is uniform. You can shoot a
    large gray card to evaluate the uniformity of the lighting. Simply run the
    eyedropper tool (Photoshop) around the image while looking at the RGB
    values. There shouldn't be much deviation.

    I think most photo editors have the ability to sample the 18% gray and make
    the exposure and color corrections. I know Photoshop does.
     
    Tom Thackrey, Feb 12, 2004
    #4
  5. lsilvand

    lsilvand Guest

    What paint should I use? Are there any special paints that can be
    bought? I'm a total newbie at this.

    Regards,
    Linus
     
    lsilvand, Feb 13, 2004
    #5
  6. lsilvand

    lsilvand Guest

    I'm a total newbie at color correction and professional photography so
    forgive me if I ask guestions that have obvious answers. Is a 18% gray
    card all that it takes to do color correction for a photograph. No
    green, red, yellow, blue...?
    The main emphasis of the project isn't on the photography so I won't
    have a big budget for this. I've got access to a digital camera, a
    L*a*b color meter and Photoshop Elements 2.0. I've had a hard time
    finding a color target and when I've found them, they are usually way
    too big. I'm also looking for a computer program that I could use to
    correct the colors. Something like:pick 5 different spots on the
    color target and give the L*a*b, CMYK, RGB values to the program and
    it calculates the rest. Something simple and fast.

    Regards,
    Linus
     
    lsilvand, Feb 13, 2004
    #6
  7. lsilvand

    Larry Guest

    Since YOU are the generator of the target, and YOU have the second
    target available for comparison, the actual colors dont matter as much
    as a match between the two targets. Just be sure to use FLAT NON-GLOSSY
    colors so that glare and reflection wont be a factor in the target.

    You dont even need to use Red/Green/Blue. Any easily identifiable color
    triplet will work along with a white (or neutral gray) section. One of
    my friends uses YELLOW/ORANGE/GREEN (he shoots mostly land scapes and
    shoots at least one target per location/ lighting condition)on his
    targets, and another uses PINK/GREEN/BLUE (this guy shoots mostly
    portraiture indoors and uses a shade of pink he feels matches the
    highlights of Caucasion faces).

    I personally use spray cans of flat paint and paint whole cards with
    them, then slice them up and glue together the pieces. That way I can be
    sure to get good saturation of each color without too much trouble.
    Right now I'm using plain white Georgia Pacific "Card Stock".

    In actual practice I only use them occaisionally, as my shooting
    conditions are pretty much mundane (normally outdoors with only a few
    shots using "fill" flash)

    The trick is matching the colors you see on the crt with the colors you
    KNOW are on the target.

    All things being equal, if you can match the triplet closely on the crt,
    then all the other colors should be as good as you can get 'em with any
    particular camera.

    The other piece of advice I can give is DONT USE AN LCD MONITOR WHEN
    TRYING TO MATCH COLOR!!!!. Use a CRT, and calibrate it.

    Larry
    Mystic.
     
    Larry, Feb 13, 2004
    #7
  8. lsilvand

    Tom Thackrey Guest

    18% Gray is all you need. If you want to ensure the maximum tonal range you
    could include white and black. Gray reflects red, green and blue equally.
    Buy a photographic 18% Gray card it will be much easier than trying to make
    your own and they aren't very expensive. If your lighting is even, you don't
    need a very big target. Cut the card down to the size that works for you.

    I don't know Elements. Photoshop curves or levels has an eyedropper for
    white, black and 18% gray. Open the photo, open curves, select the gray
    eyedropper, click on the gray card in the image, done.

    If you want a small color swatch card, try the ' Kodak Color Separation
    Guide and Gray Scale-Small (Q-13)' B&H sells them for less than $20. You get
    two cards, a 20 step white / gray/ black card and a card with RGB CMYK plus
    white color patches.
     
    Tom Thackrey, Feb 13, 2004
    #8
  9. lsilvand

    lsilvand Guest

    Thanks for the information!

    Regards,
    Linus
     
    lsilvand, Feb 13, 2004
    #9
  10. It depends on what level of "color correction" you want to do. If all
    you want is to do "colour balancing", so that black and grey and white
    reproduce as neutral, a grey card will do most of what you want. It
    provides a reference for adjusting the relative gain of the three
    colours in the camera (using manual white balance) or later in
    Photoshop. Even better, get a grey wedge (a pattern with about 20
    different patches of grey each at a different brightness) and you can
    make sure that the camera remains neutral throughout the image exposure
    range.

    But if you want greys to be neutral *and* other colours to match as
    well, you might be getting into more complicated territory involving
    colour profiles of monitor and printer and camera, and colour space
    conversions.

    Figure out how to get greyscales right first, then shoot a colour
    target like a Macbeth colour checker, see how close it is, then decide
    if you need to do something more complex.

    Dave
     
    Dave Martindale, Feb 13, 2004
    #10
  11. lsilvand

    lsilvand Guest

    Thanks Tom and Dave!

    Regards,
    Linus
     
    lsilvand, Feb 14, 2004
    #11
  12. lsilvand

    Flycaster Guest

    All you really need is a small neutral grey target - available at any camera
    store. Balancing RGB values to neutral grey is a heck of lot easier than it
    is to colors. If you want to really get virtually all the color cast out,
    get a small target that has pure black, pure white *and* neutral grey - that
    way you can color balance at the spectal highlight and black point ends, as
    well as the mid-point.

    Any program that uses curves or levels will do the trick, provided it has a
    monitor calibration add-in. (otherwise, what you see on your screen won't
    match what others see on their calibrated monitors.)
     
    Flycaster, Feb 17, 2004
    #12
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.