When is DPI really DPI

Discussion in 'Digital Photography' started by scotty96LSC, May 10, 2008.

  1. scotty96LSC macrumors 65816

    scotty96LSC

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    Charlotte, NC
    #1
    Since moving to the digital world from the 35mm world, there has been some debate with myself and others -- when is 300 DPI really 300 DPI.
    If you take a 72 dpi, RGB jpeg and convert to a 300 dpi CMYK tif image, is it really 300 dpi? I've heard that it is not really, but the details are sketchy.
    Any thoughts would be appreciated.
     
  2. Stratification macrumors regular

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    #2
    DPI only comes into play when you're printing something. So all you've accomplished in going from 72 dpi to 300 dpi is making the printed size smaller.
     
  3. compuwar macrumors 601

    compuwar

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    #3
    You're all over the board here- RGB to CMYK is going from an additive to a subtractive color space with transmitted vs. reflective light. RGB also has a wider gamut. So the color space conversion is very likely to lose information.

    JPEG to TIFF is going from an 8-bit lossy compressed format to a lossless format that's capable of a lot more bit depth, as well as holding true color or indexed color- so the only reason to not just go to JPEG is to not lose more information on the recompression. Your JPEG and TIFF are measured in pixels per inch (ppi) though, not dots per inch (dpi.)

    In any case, dpi is only for printing, otherwise it's ppi, and "really 300 dpi" is Clintonesqly dependent on "what you really mean really means." Sure, if you uprez, you'll get a 300ppi image that will print at 300dpi smaller than the original 72ppi image will print at 72dpi, but those extra ppi and dpi will be interpolated, which isn't the same thing as taking a 200ppi or 300ppi image and doing the same thing in terms of detail rendered in the final print.
     
  4. cmcbridejr macrumors 6502a

    cmcbridejr

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    #4
    Even in printing, dpi is just a marketing number.

    There are so many factors that effect the true quality of dpi - toner, fuser, etc.
     
  5. Westside guy macrumors 601

    Westside guy

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    #6
    I disagree with this (well, to some degree anyway). At work we've run into this issue before when we want to use an image in a print publication, but the only version a person has available is the one they've got on the web.

    For example: A image on the web works fine even if the overall dimensions are reasonably small: say 300 pixels by 200 pixels. On the typical modern LCD monitor, that image is going to display at about 3 inches by 2 inches. But if you want to put that image into a print document, and want it to measure 3 inches by 2 inches in that print document, that means the resolution is only 100 dots per inch - and a person's eyes can notice this. You can use Photoshop to up-rez it somewhat, but it doesn't end up working as well as starting with a photo that actually has the greater level of detail recorded (that is, a version of the photo that was taken at 600x400 or 900x600 or even higher).
     
  6. cmcbridejr macrumors 6502a

    cmcbridejr

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    #7
    Disagree all you want. I work in the print business and no professional printer asks, "oh, what dpi can it output?". It is not the deciding factor, its only a marketing number.

    We have not gone past 1200 dpi printing in years, simply because the human eye can't tell a difference with more.

    There are high end production machines that do 600 dpi better than the others that do 1,200 dpi, again because of toner and fusing technologies.

    However, we are discussing both input (camera) and output (printing) so do not get confused.
     
  7. scotty96LSC thread starter macrumors 65816

    scotty96LSC

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    #8
    Let me clarify.
    We build catalogs at work. Many of our clients only have jpeg images since much of the world thinks web first and print second.
    These catalogs, 1,000 pages or more, we build are for print. We always take a jpeg and convert it to 300 dpi (cmyk or grayscale) tif image using photoshop. In some cases my printer tells me some of these converted images are coming out at 150 dpi. Others are fine.
    Looking for a formula or explanation why a 300 dpi image is not really a 300 dpi image.
     
  8. Westside guy macrumors 601

    Westside guy

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    #9
    You're quite obviously dancing around what I actually said, though. If DPI is "just a marketing number", explain how in your mind there's no difference between printing an image at 100dpi versus printing an image at 300dpi.
     
  9. Digital Skunk macrumors 604

    Digital Skunk

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    #10
    The only time i ran into that problem was when our (college) paper didn't package an image correctly, didn't convert it correctly, or the image was transferred in a horrible God forsaken program like Microsoft Word.

    The only thing I can say is to check how you are converting them in Photoshop.
     
  10. decksnap macrumors 68040

    decksnap

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    Apr 11, 2003
    #11
    When resizing an image to 300 dpi, you want to make sure you are keeping the same amount of pixels, not resampling it. So, increasing the dpi but keeping the pixel count constant simply decreases the dimensions of the image. This is what you want to do when converting to 300 dpi. It gives you the actual dimension your image will look good at when printed.

    So, in photoshop, under 'image size', uncheck 'resample image' and punch in your new higher dpi. The pixels stay the same, and the dimensions shrink.

    It is possible they are getting scaled over 100% in your layout program, which would cause a problem. A 300 dpi image scaled to 200% in Quark will only print at half the quality.
     
  11. ChrisA macrumors G4

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    Redondo Beach, California
    #12
    The "DPI" saved in the file means nothing. Only when the file is displayed in print or on a screen do you actually have soe dots that you can measure
     
  12. cmcbridejr macrumors 6502a

    cmcbridejr

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    #13
    "Up to a point, printers with higher DPI produce clearer and more detailed output. A printer does not necessarily have a single DPI measurement; it is dependent on print mode, which is usually influenced by driver settings. The range of DPI supported by a printer is most dependent on the print head technology it uses. A laser printer applies toner through a controlled electrostatic charge, and may be in the range of 600 to 1800 DPI."

    http://en.wikipedia.org/wiki/Dots_per_inch

    This wikipedia link may clarify what dpi is.

    I understand that you are trying to justify size with quality, but that relates more to ppi, which comes from an input, such as a camera or scanner.

    Obviously, the higher the ppi during input, then the better the image quality will be when blown up to a larger size.

    However, dpi is related to output, such as printing toner on paper. I don't know of any professionals outputting on anything less than 1,200 dpi.

    Technologies in printing have come to a point where we aren't really moving past 1,200 dpi, as most human eyes can't notice above that value. Instead, we are now focused on creating more precise lasers that output within that dpi value and use better technologies that can disperse toner more smoothly within that given range.

    So, crap in is crap out. You must have high ppi in order to get quality dpi.

    As far as the "marketing" numbers of dpi that I am referring to, there are so many other variables that come into play when judging quality.

    For example, take a look at a 1,200 dpi print image off a Canon C7000VP versus a 1,200 dpi print image off a Canon iR C5185. They simply do not compare, and this is due to technological advancements in toner, fusing, and many other variables.

    I know this is slightly off topic from what the OP initially asked, but other forum posters did refer to printing.

    Please understand that there is a difference in terminology between dpi and ppi (or spi).
     

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