Pixels per inch

Discussion in 'Scanners' started by N! Xau, Apr 27, 2005.

  1. N! Xau

    N! Xau Guest

    Hi,

    I want to scan some color pictures, size 4x6 inches, printed by a lab.
    My scanner can be set up to 9600 pixels per inch, but the default (and also
    automatic setting) is 200 ppi.

    What is your suggestion about it? It may seem a dumb question but I see the
    file size increase a lot increasing the ppi #. Also I get larger pictures,
    but at a certain point (I would say between 200ppi and 300ppi but my eye's
    not an expert one) I start seeing losses of quality in the pics.

    File sizes are:
    200ppi: 287 Kb
    300ppi: 698 Kb
    600ppi: 2,1 Mb

    Let's say I don't care how the file size is, at the moment. What could I do
    with a larger picture if the quality is proportionally lower?
    I mean, if I scan at 200ppi, then I resize the pic up to the dimension of
    the file I get at 300ppi, do I get the same quality? A lower one? A higher
    one?

    Please help me to clear the clouds.
    thanks
     
    N! Xau, Apr 27, 2005
    #1
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  2. N! Xau

    MCheu Guest

    Scan at the scanner's optical resolution. For most modern scanners,
    that's 300dpi or on some of the higher end ones 600dpi -- check the
    manual. Any higher than optical resolution is pointless because
    you're not getting any more information, the driver code is just
    interpolating the pixels in between.

    Doing that gives you the best image quality and speed. You can always
    resample it to a lower resolution if you need to.

    If you intend to print them, start with the scanner's optical
    resolution, and then resample the image down to the printer's native
    resolution (usually 150-300dpi). Most printer drivers will have
    options to do this for you, but they're slower, and some create weird
    patterns in the final print. Doing it in a photo editor program
    before printing is better.
     
    MCheu, Apr 28, 2005
    #2
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  3. N! Xau

    N! Xau Guest

    Scanner is HP 2175 all-in-one
    Here,
    http://h10025.www1.hp.com/ewfrf/wc/genericDocument?lc=en&cc=us&docname=bpu00872
    it says "Scans at 1200 x 2400 dpi optical resolution"

    Dpi differs from ppi, doesn't it? In the settings I have ppi, 200, 300 up to
    9600. %-)

    Yes I usually do it with Paint Shop Pro.

    thanks
     
    N! Xau, Apr 28, 2005
    #3
  4. Everything you ask above, and more, is answered at:
    http://www.scantips.com/

    In particular, read the page http://www.scantips.com/basics08.html
    which addresses your particular issue in detail, and explains why the
    experience you relate above is typical and expected.

    Note that Wayne misuses dpi for ppi throughout and expects the reader,
    however new to the topic, to be able to decipher the distinction. He
    does try to justify this confusion with some very poor logic early in
    his book, but apart from that most of the information he gives is a good
    start for anyone beginning an interest in the topic.
     
    Kennedy McEwen, Apr 28, 2005
    #4
  5. N! Xau

    Ron Guest

    Considering that the best Photographic Print Media maxes out at 16 LPMM,
    lines per millimeter extrapolate from there. 25.4 mm/inch X 16 = 406 LPInch.
    any thing over that is wasted. 300 or 600 will do just fine.
     
    Ron, Apr 28, 2005
    #5
  6. Citation please - you will see why this is necessary below.
    Unfortunately you have made a fairly common mistake of confusing lines
    per millimetre and line pairs (or cycles) per millimetre. This is
    common because the normal test pattern for continuous media is black
    lines on a white background and often only the black lines are counted,
    ignoring the equally important white lines between them. A black and
    white line pair is a complete cycle that is repeated throughout the test
    area, and you need a minimum of two pixels per cycle to reproduce that
    digitally. Consequently you need TWICE as many pixels as the lines per
    mm that is often quoted.

    Black and white print media can easily require more than 400ppi to get
    everything off the print and, even accounting for your 2:1 error, 800ppi
    can struggle.

    Wayne Fulton goes to great lengths on his site to explain that his "rule
    of thumb" of 300ppi is only really relevant to average colour print
    media and is not relevant to professionally produced colour or black and
    white prints. In particular, it is definitely far too low for black and
    white contact prints.
     
    Kennedy McEwen, Apr 29, 2005
    #6
  7. N! Xau

    N! Xau Guest


    Well, a bit in late but, thanks.
     
    N! Xau, May 11, 2005
    #7
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