Apple Preview Image Resize

Discussion in 'Digital Photography' started by leerkeller, Jun 7, 2013.

  1. leerkeller, Jun 7, 2013
    Last edited: Jun 7, 2013

    leerkeller macrumors member

    Joined:
    Nov 1, 2011
    Location:
    Baltimore, MD
    #1
    I have a bit of a concern with the way Preview handles images. I am using it to change the dpi setting in the EXIF header from 72 dpi to 300 dpi. The problem that I am having is that I believe in completing that process I am degrading the quality of the images.

    When I click adjust size and leave the image dimensions and all other setting unchanged I still have the option of clicking OK. I just am not clear on what OK does, other than that it reduces file size. Clicking OK causes the file size of the image to be reduced by as much as 3/4ths despite the dimensions remaining the same. This leads me to believe that some image degradation through compression must be occurring.

    Ideally what I want to be able to do is change the DPI in the EXIF tag and not change the file quality. From what I understand changing the DPI should have no effect on file size, so something else must be going on.

    In the screenshot below you can see what I mean. I simply click on adjust size in the tool menu and the adjust size window pops up. The resulting file size states that the file will be reduced from 11.6 MB to 2.4 MB without changing anything. I just would like to be able to change that setting of 72 pixels/inch to 300 pixels/inch and have nothing else change. Any suggestions and can anyone tell me what is done to the file when I click OK?

    [​IMG]
     

    Attached Files:

  2. leerkeller thread starter macrumors member

    Joined:
    Nov 1, 2011
    Location:
    Baltimore, MD
    #2
    One more screen grab below that may make my question more comprehensible. In the adjust size window I have changed the dpi from 72 to 300 dpi, and the resulting file size calculation doesn't change, but it still has been reduced through some process unrelated to the dpi change. How can I use the adjust image feature to simply change the DPI and not reduce the file size (from 11.6MB to 2.4MB in the screengrab) and image quality that is related[​IMG] to file size.
     

    Attached Files:

  3. swordio777 macrumors 6502

    Joined:
    Apr 3, 2013
    Location:
    Scotland, UK
    #3
    Hi leerkeller,

    I'll cut straight to the chase, I have no idea why preview does this. But you're correct - if the file size is changing that much then the conversion is definitely compressing/throwing away data.

    Personally, I definitely wouldn't recommend using preview to make this change. But I must ask why you're doing it at all? The DPI / PPI maintained in the EXIF info has absolutely nothing at all to do with anything. The fact that it's included at all only really serves to confuse.

    The size of the pixels on your screen are obviously fixed, so there's really no such thing as "Pixels Per Inch" (PPI). The only way to change the image's size on screen is to reduce the number of actual pixels in the image or view it at a percentage of its actual size.

    "Dots per Inch" (DPI), on the other hand, is very real, but it only affects the output device (a printer). Historically, this figure would be set by the printer operator to ensure the image was printed at the correct size, however all new software lets you simply select the output size you want and does this calculation for you (completely disregarding any PPI figure in the EXIF).

    Personally, I'd recommend just leaving all your images in whatever PPI your camera shoots in - it honestly makes absolutely no difference. But if you are determined to change the PPI in the exif then I'm sure you'll be able to do it in the free image editing software "GIMP" without changing the file in any other way.

    Hope that helps.
     
  4. ijohn.8.80 macrumors 65816

    ijohn.8.80

    Joined:
    Jul 7, 2012
    Location:
    Adelaide, Oztwaylya.
    #4
    Go have a read of this page which explains what's going on really simply and easily.

    In a nutshell, using 3 pictures at 300x400 pixels, but, at different ppi ratios, it shows:

    Preview, it appears, is smart enough to take this into account!

    Basically, on the web, you only need 72ppi. I'm lazy in that regard and couldn't be bothered creating another copy of my images at a lower ppi that I then need to catalogue and store as well as the originals, so my images are posted at 300ppi. :eek:
     
  5. leerkeller thread starter macrumors member

    Joined:
    Nov 1, 2011
    Location:
    Baltimore, MD
    #5
    Oi Vey.....the DPI/PPI thing. There was a thread that went on forever and ever about this back in 2009. People kept coming up with metaphors of increasing complexity, finally ending with the post I am pasting below....at least that was what it ended with the last time I checked.

     
  6. swordio777, Jun 9, 2013
    Last edited: Jun 9, 2013

    swordio777 macrumors 6502

    Joined:
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    Location:
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    #6
    Hi Leerkeller,

    Apologies if you didn't find my previous post useful. I was trying to keep things as simple as possible in case you were new to this, but it looks like you've done quite a bit of investigation into this in the past.

    Your post above is basically saying much the same as I did before: essentially, the PPI/DPI tag does not matter.

    I'll try to avoid all the metaphors. The key paragraph from that whole post is this:

    "It really is nothing new that a fine magazine wants a 300 ppi/dpi image at the actual size of the print in the magazine. This number is driven by the line screen of the printing. Printers *think* in inches; ie the peice of paper the actual reader is going in his/her hands. This is reality; ie a physical product is going to be on a newstand. Having at least 300 pixels per inch allows a great line screen. Giving them MORE allows cropping."

    What this paragraph is saying is not that the image's exif tag needs to say 300dpi, but that your digital photograph must have 300 pixels of width / height for each inch you want to print. For example, a 6 megapixel image is 2000 pixels tall x 3000 pixels wide (rounding down from 3008 for ease of calculation).

    If you want to print this image 10 inches wide at magazine quality then you can do so, because there are enough pixels in the image to allow for 300 pixels per printed inch (10" x 300 DPI = 3000 pixels). This is true even if the photograph's tag says that the DPI is only 72.

    However if you wanted to print that same image at 15" wide then your print shop may tell you it can't be done. This is because there are not enough pixels in the image to allow 300 pixels for every printed inch. Regardless of whether the EXIF tag says the image is 1 DPI, 72 DPI, 300 DPI, or 5000 DPI, it simply doesn't change the fact that there are not enough pixels in the image to print at that large size. The image would need to be 4500 pixels wide to print it 15" wide at 300DPI (15 x 300 = 4500).

    When a print shop asks for an image that's 300 DPI, this is what they're asking for. They're not asking for an image that has an exif tag which states the image is 300 DPI.

    If you want to make prints that are 6" x 4" then your image only needs to be 1800 pixels wide (6 x 300) and 1200 pixels tall (4 x 300). That's a 2.2 megapixel image. The larger you want to print, the more pixels you need if you want to retain your 300DPI print quality.

    From the image in your original post I can see that the photograph in question is 4000 x 3000 pixels. That means you'll be able to print this image at any size up to 13" x 10" while still retaining full magazine print quality of 300 DPI.

    If you want to print larger than this, but there are not enough pixels in your image to print at the size you want, then you must reduce the print quality to compensate.

    For example, you cannot print a 6 megapixel image (3000 x 2000 pixels) at 15 inches wide and still retain 300DPI print quality. However you can print it that wide if you reduce the print quality to 200DPI, assuming your print shop can do this (15" x 200 DPI = 3000 pixels). I hope that clarifies what I was saying previously.

    As I mentioned before, if you really do need to change the exif tag to please a client then you'll be able to do this in most photo manipulation software (GIMP, Photoshop, Lightroom, Aperture, Elements, etc). If you choose to do this, make sure you DO NOT "resample" the image. You only want to change the tag, not the actual image dimensions.

    Best regards.
     

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