Color shift...is it significant?

Discussion in 'Photoshop Tutorials' started by Tacit, Oct 19, 2004.

  1. Tacit

    Tacit Guest

    A printer said, after seeing the proof (or dummy, whatever), "now
    No, so far is not so good. There are many ways to convert an RGB color to a
    CMYK color. In order to give the printer something that is usable on press, you
    must know more information, and you must enter that information in Photoshop's
    CMYK Setup dialog box. If you do not, then you may end up with a CMYK image
    that looks great on your computer screen but absolutely can not be printed.

    The information you need to know includes:

    - Black generation
    - Total ink limit
    - Separation technique (GCR or UCR)

    Make the wrong choices, and your job can't be printed. If you have never opened
    the Photoshop CMYK Setup dialog before, or if you have but you're confused or
    intimidated by all the settings there, you should probably not be making CMYK
    separations yourself.

    Moving right along:
    A home consumer-grade inkjet printer is not a proofing device and *can not* be
    used to make test prints of a CMYK image. The printer uses CMYK ink, but the
    CMYK the printer uses is not the same CMYK as a printing press. The software
    drivers of a home consumer-grade inkjet printer can only work with RGB data.

    If you snd CMYK to a home inkjet printer, the driver converts the MYK to RGB,
    very badly--then converts the RGB back to the printer's own peculiar brand of
    CMYK.

    By the way, never, ever expect that something you run on a printing press is
    going to look like what you see on a home inkjet printer, in RGB or CMYK. It's
    not going to happen. The printed piece absolutely will not match your inkjet
    printout. It's not physically possible; the primary ink colors are different.
    Inkjet cyan ink is darker and bluer than the cyan ink used on a printing press.
    It means you're using a home consumer-grade inkjet printer. If you want to know
    how the job will look on press, buy a contract proof. A contract proof will
    match the press output, and will cost you anywhere from $25 to $50.
    Never color correct based on what you see on a consumer inkjet printer. Only
    color correct based on what you see on a true contract proof, such as a
    MatchPrint.
    If you care about color, it's very necessary. In the prepress industry, here is
    how it is normally done:

    1. Scan the images. Not on a consumer desktop scanner; for best results, use a
    drum scanner. These cost anywhere from $17,000 US to $340,000 US. That's why
    designers don't make their own scans; they pay a trade shop or service bureau
    to do it.

    2. Get a set of "random proofs", which are just contract proofs of all the
    scanned images. You'll probably pay about $50 for this.

    3. Color correct the images based on what you see on the randoms. If necessary,
    get some more randoms made to test your color corrections.

    4. Put the job together.

    5. Get a contract proof made of the finished brochure or book or whatever. Give
    the pressman the digital files and the finished contract proof. (Good printers
    will not accept a job without a contract proof, or will insist on making a
    contract proof before the job hits the press. It is called a "contract proof"
    because it represents the contract between you and the printer. the printer's
    job is to make sure the printed piece matches the proof. The press operator
    uses the proof to set up the press.)
    No, it's a consequence of the fact that consumer inkjet manufacturers use ink
    that's blue, not cyan, because home users tend to expect and want images that
    are, from a professional prepress standpoint, too blue. Photoshop has nothing
    to do with it.

    Now, some words on creating CMYK images in the first place:

    You can just use Image->Mode->CMYK, and you'll get a CMYK image.

    However, depending on how that image will be printed on press, and what kind of
    paper is being used, the results may not be printable.

    For best results, you MUST use File->Color Settings->CMYK Setup, and use the
    CMYK Setup dialog box to specify the separation parameters for your press and
    paper, EVERY time you create a CMYK separation.

    Also, many colors in RGB can not be reproduced in CMYK. CMYK has a different
    range of colors, or "gamut," than RGB.

    If your image contains out-of-gamut colors, these colors will be converted to
    their nearest approximation in CMYK. The result will be a color which is less
    saturated and somewhat flatter.

    Often, a little bit of color tweaking is necessary in the CMYK image. After
    separating an RGB image to CMYK, you may wish to use the Curves command
    (Image->Adjust->Curves) to increase contrast in the midtones slightly, as the
    separation often becomes flatter in the midtones.

    Specific colors can be tweaked with Image->Adjust->Selective Color. For
    example, if your blues have yellow in them, you can remove yellow from blues to
    make them more saturated and richer.

    When you color corrrect the image, you should, of course, have your Info
    palette open. Look at the numbers in the out-of-gamut colors; see if your
    primary colors have any contaminating color that can be reduced to increase
    saturation. For example, yellow in your blues or cyan in your reds can be
    reduced, if present, to make the colors richer.

    Vivid RGB blues often separate with too much magenta, making the colors appear
    more purple than blue. Using Selective Color to reduce magenta in blues will
    often solve this problem.

    The range and depth of color you can expect to get depends on the settings in
    your CMYK setup, which themselves depend on the kind of paper and press you are
    going to be printing on.

    As for setting up your CMYK separation:


    If you just go Image->Mode->CMYK without changing the values in your CMYK
    setup, the result will look okay on most sheetfed presses and coated (glossy)
    color stocks, but will not be acceptable for, say, newspaper printing.

    For best results, you should talk to your printer about what to do. Most
    importantly, make sure he gives you the values he needs for maximum ink and
    maximum black percentages; if you exceed these values, the image may smear on
    press.

    As starter points:

    For sheetfed presses printing on high-quality coated paper, you can usually use
    GCR, 100% black ink limit, 300% total ink limit, Light or Medium black
    generation.

    For very high quality lithographic output, your total ink limit can go as high
    as 310%.

    For web-fed presses on glossy paper, or sheetfed presses using high-quality
    uncoated paper at a 110-line halftone or above, use GCR, 100% black ink limit,
    280% total ink limit, Light or Medium black generation.

    Newsprint is a different story entirely. For newsprint, appropriate settings
    might be UCR, 240% to 260% total ink limit, 85% black ink limit. When you
    convert the image to CMYK, it will look washed-out and flat, but newsprint
    darkens *considerably* on press so this reduction in density is necessary.

    Hope that helps.
     
    Tacit, Oct 19, 2004
    #1
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  2. Tacit

    jjs Guest

    Your printer is incapable of being HIS printer. You cannot proof using your
    printer. It's a totally different thing (RGB, for one.)

    In this case, you might get a pre-press professional who can talk to the
    printer to discern _exactly_ what CMYK settings are neccessary. Those two
    professionals speak the language. In the meantime, study up. If you are very
    lucky you can get a pre-press pro to teach you the inside story. Good luck
    on that; my mate is a pre-press pro and to keep peace in the family we don't
    talk shop. :)
     
    jjs, Oct 19, 2004
    #2
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  3. Tacit

    Pixmaker Guest

    I'm a long-time photographer but a relative newbie at pre-press. For
    some time, I have been proofing (and sometimes displaying) pics
    printed on a simple HP inkjet (932C) with very nice results. In fact,
    I'm amazed at how well these images look. (For really nice stuff, I
    use an Epson 2200.)

    I use a calibrated Diamond Pro monitor, PS7 and work in the Adobe RGB
    color space.

    Recently, using Pagemaker, I created a brochure (11 x 17, singlefold)
    for a small company and printed it out on the HP. The pics were of
    aluminum fabricated parts on a white background (and some other,
    establishing shots.). To be certain that the side-by-side images would
    print about the same, I balanced the white background in PS by
    equalizing the RGB levels, using curves.

    The printed results were just fine...in fact surprisingly good.

    A printer said, after seeing the proof (or dummy, whatever), "now
    convert your images to CMYK, 20% dot gain, and give me the whole
    thing on a CD. Be sure to include the fonts."

    So far, so good.

    But I then tried to make some prints from the converted files (CMYK)
    (using the li'l HP) and all those white backgrounds developed a slight
    cyan tint as did the whites in some of the other pics.

    Questions:

    1. Is this result normal? Typical? Does it really MEAN anything?

    2. Is it necessary to adjust the color balance again? It seems to me
    that I'm now using a different process and what I see out of the
    printer may no longer bear a true (or even reasonable) resemblance to
    what may come off the press.

    3. How do I "test" the new CMYK files for proper color balance? Or, is
    it even necessary?

    4. I've read that cyan is the "weak" color in 4-color printing. Is
    what I'm seeing just PS's correction for a normal color adjustment?

    5. I understand that my li'l printer wants to receive RGB and that
    another conversion is probably taking place when I print from CMYK
    files. So, is what I'm seeing irrelevant?

    Any commentary will be gratefully received as I stumble along this
    path toward understanding.

    Pixmaker in FLL
    ==========================
    It's not the heat, it's the humidity!
    ==========================
    (...Think the humidity's bad?
    You should watch us vote!)
     
    Pixmaker, Oct 19, 2004
    #3
  4. Tacit

    Pixmaker Guest

    Hello, Tacit:

    YES! It helps. . . a lot! I greatly appreciate the time you spent
    outlining what for you must be elementary stuff. . .sort of like
    "Prepress 101."

    I am especially impressed with your ability to drive directly to the
    important points of my questions without any attempt to "dazzle me
    with footwork." What a refreshing approach! I suspect that it simply
    illustrates the difference between "them as can and <G> them as writes
    instruction books."

    Inasmuch as your brief explanation is worth more than I found in the
    four (count 'em, four) PS books I have purchased to aid my
    understanding of this subject, I heartily suggest you consider
    publishing, yourself!

    I'm a photographer, not a graphic designer or prepress expert, and,
    early on with PS, I bought a book to help learn this program. And
    then, I bought another...and another. It wasn't until I got Scott
    Kelby's book that I found someone who could say, "do this, do it again
    until you develop your skills, and you'll get this!"

    That's an example of how I view your explanation. Thank you.

    After too many years using everything from a Speed Graphic to an F4
    and years in a darkroom, I began to believe I was "gettin' kinda
    good." Now, with digital, I'm starting all over again and, in truth,
    I'm very impatient with the learning process. That's OK, though
    because I probably went through the same thing years ago...just forgot
    all that!

    I've talked with 7 different printing companies and have not found one
    that tells me much more than I mentioned in my first post. Is that
    because they really don't know? Or, perhaps there's more to be made
    from correcting mistakes from folks like me?

    Maybe, they're just tired of trying to explain a fairly complex
    process to people who aren't in their trade and have come to the
    conclusion that they're going to have to do it themselves eventually,
    so why waste the time trying to teach amateurs. I can understand
    that!

    Thanks!

    Pixmaker in FLL
    ==========================
    It's not the heat, it's the humidity!
    ==========================
    (...Think the humidity's bad?
    You should watch us vote!)
     
    Pixmaker, Oct 19, 2004
    #4
  5. Tacit

    Ed Clarke Guest

    There's a CMYK RIP for the Epson 1280. Can this be used as a pre-press proof
    printer? Actually, after reading your posting, it sounds like you have to get
    a proof from the actual printing press to be sure the colors come out right.
    Is that true?
     
    Ed Clarke, Oct 21, 2004
    #5
  6. Tacit

    Tacit Guest

    There's a CMYK RIP for the Epson 1280. Can this be used as a pre-press proof
    Not with Epson's RIP, no.

    There are RIPs which are designed to drive Epson printers and produce output
    that's color-accurate to a printing press, such as Black Magic and Star Proof.
    Expect to spend about $15,000 for the RIP software, not including the computer
    to run it on or the hardware to connect it to the Epson.
    No. You'll get an off-press proof, such as a MatchPrint or a 3M Waterproof.
    These contract proofs use the same film that would be used to make a printing
    plate, but they're not run on a press. The proof is then given to the press
    operator, who adjusts the ink flow on press to match the proof.
     
    Tacit, Oct 21, 2004
    #6
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