OT: how many ipd for a poster this size?

Discussion in 'Digital Photography' started by Asbjørn Bjørnstad, Nov 27, 2003.

  1. Biggest poster I've ever seen, I'm sure they don't use 300 dpi.

    http://heim.ifi.uio.no/~asbjxrn/poster.jpg

    The window-washers on the left gives an good indication of the size of
    this poster. It actually wraps around the corner, so you only see half
    of it on this picture, and while there is not really a picture on what
    you see here, there is a newspaper picture on the other side. Not very
    good quality, though.
    --
    -asbjxrn
     
    Asbjørn Bjørnstad, Nov 27, 2003
    #1
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  2. Asbjørn Bjørnstad

    Ray Murphy Guest

    ----------
    In article <>,
    (Asbjørn Bjørnstad) wrote:


    >
    >Biggest poster I've ever seen, I'm sure they don't use 300 dpi.
    >
    >http://heim.ifi.uio.no/~asbjxrn/poster.jpg
    >
    >The window-washers on the left gives an good indication of the size of
    >this poster. It actually wraps around the corner, so you only see half
    >of it on this picture, and while there is not really a picture on what
    >you see here, there is a newspaper picture on the other side. Not very
    >good quality, though.
    >--
    > -asbjxrn


    RM: There would be no need to use 300 ppi Tiffs to produce the
    computer artwork -- here's why:

    Huge posters - whether they are silk-screen printed or laser printed,
    are first made in the normal way for viewing at a normal reading
    distance. This would usually mean creating the (continuous tone)
    graphics with say 300 ppi Tiffs and printing at 150 Lines Per Inch
    (That's the half-tone dots of various sizes and shapes you see in
    professional printing).
    When the colour proof or "visual" is approved by the customer, a copy
    of the image is then enlarged to the required size and the halftone
    dots are made bigger, but we don't see them as huge dots because we
    are so far away.

    You can see this if you look at huge poster on the side of a bus. The
    screen ruling for the pictures may have originally been 150 lines per
    inch on the original proof, but the screen ruling may be only 15 lines
    per inch on the bus.

    Look at it this way - if you saw the original artwork for a bus poster
    on an A4 sheet folded in half lengthways, it would make a long strip
    105 mm x 297 mm, and it was enlarged 10 times, then the image would be
    1050 x 2970mm (1 x 3 metres). That would make the 150 line screen into
    a 15 line screen.

    Now in the case of screen printing (for bus posters) this coloured
    image is screened AGAIN in order for the 4 colour printing process to
    print the poster in CMYK (Cyan, Magenta, Yellow, Black). So the four
    passes through the screen-printing machine will reproduce the FOUR
    colours in the poster. The same would happen if it was done on an
    ordinary offset press if it was big enough. Theoretically this extra
    screening should not be necessary, but that's the way it's done.

    In the case of digital output (laser printing onto huge poster rolls)
    the same thing must happen. This means that the huge laser printer is
    provided with a huge computer document with the re-screened image on
    it. This is done by making the screen ruling coarser in a direct
    relationship to the enlargement involved.

    You could actually make your own example of this by making a tiny
    piece of coloured artwork with your inkjet say 20 x 20mm of say a red
    rose and afterwards simply enlarge your computer document of the rose
    image to 1000% (200 x 200 mm) and then change the number of pixels to
    30 per inch and print your job with a 15 Lines per inch screen. (You
    need 2 pixels for every halftone dot).

    This will give you VERY clear (in fact huge) halftone dots, but your
    laser printer or inkjet is working normally.
    After the job is done, just hang it on the wall and take several steps
    back and you will see no halftone dots, but the rose will look just
    the same as when it was only 20mm square.

    Ray
     
    Ray Murphy, Nov 27, 2003
    #2
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