PNG format and DPI

Discussion in 'Photoshop' started by Jon, Aug 11, 2007.

  1. Jon

    Jon Guest

    If I scan an image at 600 DPI and save it as a Tiff the resolution (as
    displayed in Bridge) is, as I would expect it to be: 600 DPI.
    However, if I save it as a PNG the resolution drops to 72 DPI!
    Is there a way to change this?
    Jon, Aug 11, 2007
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  2. "Save As" -=or=- "Save for Web" ?
    Sir F. A. Rien, Aug 11, 2007
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  3. Jon

    Jon Guest

    'Save as'
    Jon, Aug 11, 2007
  4. Jon

    Joel Guest

    "DPI" doesn't mean much or anything in graphic world. Newer Photoshop now
    use "PPI" but it too doesn't mean much by itself.

    - You can change the "PPI" using Resize Image, and the more you increase the
    "PPI" the more WxH will decrease.

    Or the whole file is based on WxHxR (or W x H x PPI). Many old digital
    cameras have the format set to "72 PPI" and they didn't effect the image
    Joel, Aug 11, 2007
  5. Jon

    Gary Guest

    The comment about older digital cameras is very accurate, Joel, and helpful.
    We've all seen images that come in at 72 dpi but are 28 inches wide and 18
    inches high. That's not a lot different then a 5x4 at 300 dpi/ppi. But
    sizing images to the print/output size at a dpi/ppi (the same, really when
    considering this issue) is twice the screen on the press. Digital presses
    are changing this, certainly, but sheet fed and web presses still run 150
    line screens for the fast majority of the work they do, so 300 dpi/ppi works
    -- as long as the width and height are close to perfect relative to the
    printed size.

    Good point and thanks for your comments.

    Gary in tampa
    Gary, Aug 11, 2007
  6. I'm not using Bridge and don't see any dpi reset unless I "Save for Web".

    Perhaps one of the 'monitors' from Adobe can jump in and help.
    Sir F. A. Rien, Aug 11, 2007
  7. Jon

    Joel Guest

    Bridge? you mean ARC or LightRoom? They have option to save to "MAX" and
    it will save to whatever *original* MAXIMUM.

    Anf back to the good old days, many digital cameras set to "72 DPI" and
    they didn't lose any quality of whatever max PIXEL they had.
    Joel, Aug 12, 2007
  8. Jon

    Jon Guest

    Thanks, but I'm still not clear on what's happening here.
    OK, so I scan a 7" x 5" photo straight into PS at 600dpi.
    Before I save it I check Image Size which tells me that I have the
    Pixel dimensions: 4200 x 3000
    Document size: 7" x 5"
    Resolution: 600DPI

    Saved it as a TIFF Bridge shows it as:
    Pixel dimensions: 4200 x 3000
    Document size: 7" x 5"
    Resolution: 600DPI

    Saved as a PNG Bridge shows it as:
    Pixel dimensions: 4200 x 3000
    Document size: 7" x 5"
    Resolution: 72DPI

    If I open both photos in PS and check Image Size they both state:
    Pixel dimensions: 4200 x 3000
    Document size: 7" x 5"
    Resolution: 600DPI

    So what's happening in Bridge? Why is it displaying a different DPI?

    If I scan at 72DPI the image dimensions come out at 504 x 360 pixels and
    in Bridge this shows 72DPI. In fact, whatever DPI I scan at, when saved
    as a PNG, Bridge shows 72DPI (even if I scan at 50DPI) whereas Photoshop
    (Image Size) always shows the actual DPI.

    With JPGs there is no discrepancy between Bridge and PS.

    ARC or Lightroom? Irrelevant. I'm talking about a discrepancy between
    Photoshop and Bridge.
    Jon, Aug 12, 2007
  9. Jon

    tacit Guest

    This is impossible; you can not have a 7" wide, 4200-pixel image at 72
    pixels per inch. The math doesn't work. You have found a bug in Bridge;
    it is displaying resolution information incorrectly, that's all.
    tacit, Aug 13, 2007
  10. Jon

    Owen Ransen Guest

    The titleof your post

    PNG format and DPI

    may explain it all. PNG does not have, in itself
    a DPI setting. PNG does not know the size of the
    final image when printed. It knows how many pixels
    there are, but that is all.

    See also:

    Easy to use graphics effects:
    Owen Ransen, Aug 13, 2007
  11. Jon

    Jon Guest

    Thanks for the link. It is not the first time I have read about DPI/PPI
    and to be honest I'm still confused.

    When I scan a 7" x 5" image into Photoshop I set my scanner software to:
    Resolution: 600 DPI
    When the image appears in Photoshop I check 'Image Size' and that tells
    me the following:

    Pixel Dimensions:
    Width: 4200
    Height: 3000

    Document Size:
    Width: 7"
    Height: 5"
    Resolution: 600 Pixels/inch

    I then save the image as a PNG and Photoshop Image Size states exactly
    as above.

    I then view the PNG image in Bridge.
    This tells me the following:
    4200 x 3000 @ 72DPI

    OK, so if DPI is purely to do with the Printer regardless of whether I
    print the image or not then why is there a discrepancy between PS and
    And what if the printer I send the images to hasn't the capability to
    print at 600DPI?
    Well, I can't print the image directly from Bridge anyhow so presumably
    the 72DPI IT shows is meaningless and possibly a PS/PNG bug.

    However, what throws the whole DPI/PPI/Resolution thing into a cocked
    hat is that the Resolution/DPI I set in the scanner software determines
    the Pixel dimensions of the resultant image that shows in Photoshop ie.
    setting the Scanner software to Resolution 300DPI for a 5" x 7" photo
    gives me an Image Size/Pixel Dimension in Photoshop of Width 2100 -
    Height 1500.
    And this determines the Print size I can achieve when I send the image
    for printing to an online photo printing outfit.

    So if there's anyone out there who can explain what all this is about, I
    would be most grateful.

    Furthermore, while they're at it, could they also explain the purpose of
    Photoshop's 'Image size' Auto button which confuses the issue further by
    introducing Lines/inch. I notice that within this 'Auto' option, if I
    select 'Best' then it reduces the Pixel Dimensions of the image to:
    Width: 1862
    Height: 1330
    Resolution: 266 Pixels/inch

    Jon, Aug 13, 2007
  12. Jon

    Owen Ransen Guest

    Or maybe there were so many "expert designers" who could only
    think in DPI that they put in a number there to make them
    feel safe. I do web pages and they STILL ask me what DPI
    I want the images in.

    That sounds right.
    Well, it determines probably the *maximum* print size which will
    not lose detail. Print larger (say twice the size) and you may
    begin to see pixels. Print smaller (say half the size) and the image
    may appear sharper but you will lose some detail.
    *I* can't!

    Easy to use graphics effects:
    Owen Ransen, Aug 13, 2007
  13. Jon

    Anon Guest

    First let me say I do not understand the reason why many of the 'Auto'
    options were included.

    Dots per inch (dpi) is a size measurement for input or output devices.
    The actual size of the digital image is read as pixels.

    Dpi does not really relate to the onscreen image because the onscreen image
    can be viewed at different sizes by zooming, and it can be otherwise
    manipulated by software.
    However, dpi is important when scanning and printing. When you scan at
    600dpi you are telling the scanner to look 600 times for every inch it
    travels in width, and height. When you scan at 72dpi you are telling it to
    look only 72 times for every inch, resulting in a lower resolution scan.

    The dpi of the printed image is usually determined by the software from
    which it is printed. Some software determines/adjusts the dpi by the
    requested size of the print, say 4x6, or "to fit within margins". Some
    software imports images at a set resolution, Outlook Express and IE, for
    instance, only display and print at 72 dpi. Most printers/graphics layout
    software imports at a set resolution like 300ppi, but allows you to resize
    it in the layout. Most graphic apps, like PS, will print at the saved
    resolution, (i.e. 600dpi).

    72dpi is the closest resolution to the 100% size onscreen, so it is the
    default resolution in many web authoring apps. PNG as there name implies,
    (portable network graphics), is a format closely ascociated with the
    internet so many apps will use 72dpi as a default value, for that file
    format. GIFs are treated similarly. I am not familiar with Bridge, but try
    printing the image from there to see how it is printed.

    4200pixels at 600dpi is 7"
    3000pixels at 600dpi is 5"
    Image will be sharp.

    4200pixels at 72dpi is 58.333"
    3000pixels at 72dpi is 41.667"
    Image will be slightly pixelated.

    Hope this has helped.
    Anon, Aug 13, 2007
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