Dots per inch question

Discussion in 'Digital Photography' started by Phil Stripling, Aug 17, 2004.

  1. I once knew the answer to this, but I forgot. I use a Nikon CoolPix, but I
    also have slides scanned to Kodak's photo CD. The show up in my image
    editing app as 72dpi, and we just got a printer that claims to print up to
    4800 x 2400 dpi. I can set the resolution in my image editing program, so
    I've just re-set the 72x72 as 300x300 and printed a scanned Photo CD image
    at 13 inches x 19 inches, and it's a stunner. (Uh, good stun, not bad

    The files in the Photo CD are .pcd, so I assume there's 4800 x 2400 dpi in
    there somewhere, right? I can set my image editor to open at that
    resolution and get it? My wife has a digital Rebel, and we'd like to get
    maximum results from that camera, as well.

    But what about the Nikon CoolPix images? Do I need one of those fractal
    interpolators or whatever they're called?

    (I realize whether anyone can tell the difference between 300 x300 and 4800
    x 2400 is a different issue. I'm just trying to get a handle on how to
    handle dots per inch. As a film shooter, this is not an issue I'm familiar
    Phil Stripling, Aug 17, 2004
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  2. (snip)


    You are making the same assumption and the same mistake here as many have
    before you... so don't feel that you are alone in struggling with the
    concepts. Pixels per inch (e.g. 72x72 or 300x300) are NOT the same thing as
    dots per inch at the printer (4800x2400.) In printing the number of dots
    required to make up one image pixel may range from none (for the colour
    white - or really that is representing parts of the printed document with an
    absence of any ink being laid down by the printer) to the other extreme of
    several dots of each of the available inks to represent a single pixel.

    When you ONLY make the change in resolution (72ppi to 300ppi) in the editor
    all you actually do is reduce the "apparent" native size of a 100% scaled
    image at the print stage.

    Thus an image of 1600x1800 @72ppi image will tell you (in the image
    properties in the editor) that it is ca 22x25 (printed) inches when scaled
    at 100%, BUT, the exact same 1600x1800 image @300ppi will tell you that it
    is 5.3x6 (printed) inches when scaled at 100% - AND there is no re-sampling
    done to get there. It is merely a scaling value for printing purposes.
    Further, you can also set your printer to print that image at various
    resolutions in dots per inch - whereupon the PRINTER DRIVER makes changes by
    interpolation to the image file between receiving it in the print queue and
    transmitting the data to the actual print heads - this is something you have
    only very limited control over and only by changing print quality parameters
    in the printer dialog between, say, draft quality and photo quality outputs
    and altering (usually) the selected paper type to accommodate the print
    quality (e.g. plain paper or photo paper). The printer, in turn, computes
    the way that each pixel is managed as far as the inks, and quantity of inks,
    laid down on the actual paper output.

    So far I have not talked about making changes, except for the printer
    quality settings, that affect image quality at the print stage... to do that
    would involve a re-sampling step such as in the image editor. Taking the
    original 72ppi image, for example, and command it to be 300ppi (ONLY reduces
    "apparent" print output size) BUT also command a change in image size -
    THEN - you will have made a change that is qualitative vs. the original
    image file and that change will have required resampling (probably, and
    almost certainly, creating data that was not part of the image in the first
    place) in the editor stage and before the printer gets it.

    Using the same image I used above: 1600x1800 image @300ppi the image
    properties will tell you that it is 5.3x6 (printed) inches when scaled at
    100% - I could command this to remain at 300ppi resolution but change the
    print size to, say, 10x12 (a multiplier of 4x total pixels and a dimension
    multiplier in each direction of 2x) and that image will then have apparent
    values of ca 3200x3600 @300ppi - the extra pixels are created out of some
    (usually selectable) resampling algorithm - but they are nevertheless
    machine created and incorporated into the original image. The native print
    size, scaled at 100% in the printer, is then as selected, 10x12inches
    @300pixels per inch.

    Journalist-North, Aug 17, 2004
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  3. Okay, so now I remember why I can't remember the answer.
    Yeah, that's why I can't remember, alright.
    Uh, thanks for the reminder. :->
    Phil Stripling, Aug 17, 2004
  4. Phil Stripling

    Swingman Guest

    You are making the same assumption and the same mistake
    That is a very excellent explanation of the issue, however I
    don't understand it in a practical way. To use your
    example, is it best to resample an image to 3200x3600 @
    300ppi if you wish to make a 10x12 print, or do the photo
    apps handle this in the background before they send the data
    to the printer?
    Swingman, Aug 18, 2004
  5. Phil Stripling

    Bob Williams Guest

    If your Photo Editor has Bicubic Sampling (like Photoshop), I'd let your
    Photo Editor do the resampling. You can never be sure how the printer
    driver upsamples the image you send it.
    Bob Williams
    Bob Williams, Aug 18, 2004
  6. (snip)

    So I have been told by others, and a view I tend to agree with, is that the
    image should be resized in an image editor for the best control. Even there,
    it is said to be something of a technical advantage to resize (where a
    significant change in size is made) in several steps rather than in one big
    jump. After the final resize there is also, if you are working in a proper
    edit application, the probability that you will want to apply sharpening and
    other tweaks before actually printing.

    You CAN re-scale in the printer (most) but with little or no control simply
    by telling the printer to print "full page" (or ANYTHING other than the
    native 100% scaled image size - whether that might be a larger or smaller
    value) and it will do that - such that you could take that example image of
    1600x1800 @300ppi (5.3x6inches) and just tell the printer to print it (scale
    it) to the full paper size. The printer may not actually resample beyond
    just managing the image to (as if it were) a lower PPI value to make it fit.
    Thus the 3,5x6 inch image you intended to print @300ppi will actually print
    more like @120 - 150 because it is being stretched to fit the paper size.
    The output quality will be affected if you do that.

    Here is a general rule of thumb for printing (on you own printer as well as
    sending images for commercial printing,) and that should always be done
    before printing. >>> Scale the image and image resolution to the full size
    you want it to print at in an editor before printing. <<< The printer is
    then only being asked to do one thing - manage the ink as it is applied to
    the paper but not manage the ink as well as also trying, at the same time,
    to resample the image to make it fit (either adding additional data or
    removing data that it can not use at the print settings you have selected =


    Scanner gurus will also tell you that when scanning the same rule of thumb
    applies - that is, to ALWAYS select the desired output size (inches/pixels)
    and resolution (PPI) of the END USE output and then scan the original medium
    to produce that file. Thus, scanning for only web use you can select a
    resolution of 72ppi and at a physical size (scaled in pixels) to fit a
    computer screen; but for printing you can (and should) select something
    higher (150, 200, 300ppi resolutions) and at the native image size (100%
    scaled in inches) it is to ultimately be printed. For our example image - to
    make a scan for a nominal computer screen (see note) that will be 3.5x6
    INCHES on screen, and assuming that many / most monitors will display at
    72ppi then the 100% scaled image need only be 252pixels wide x 432pixels
    long (instead of the 3.5x6 inches printing example of 1600x3200) and with
    the obvious saving in image file size that results.

    Note: As a computer screen can vary in two ways independent of the image
    itself: physical dimension (14", 15", 17" ect) and by screen resolution (a
    selectable value) the same image will not display at the same size on a
    screen set to display 600x800pixels as on one set to 768x1024pixels... or...
    on screens with different physical sizes from the one you work on, but even
    where the screen pixel dimensions may be the same. Screen display should
    always be thought of in pixels rather than inches. It is not a complex
    problem but one that requires some thought and consideration in constructing
    a web page.


    Journalist-North, Aug 18, 2004
  7. Phil Stripling

    swingman Guest

    Yes, that IS better - thanks!
    swingman, Aug 19, 2004
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