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Help in understanding differences in PP file sizes

Discussion in 'Image Processing' started by roundball, Jun 9, 2015.

  1. roundball

    roundball TalkEmount Hall of Famer

    Oct 8, 2013
    USA
    I've noticed something that surprised me, don't understand it, hoping someone here might be able to explain it...has to do with a dramatic change in the file size after only a single post production tweak, like 'Sharpening'.

    An example I just tested:
    JPEG photo file size SOOC to my PC is 4,449 KB.
    I make a work copy and import it to Lightroom.
    I simply ran the 'sharpening' slider to its midpoint (75 units).
    Then exported / saved that work copy back onto my PC.
    Its file size was cut drastically from 4,449 KB down to 1,889 KB.

    PS:
    Same thing happens when I go through the exact same steps using Photobucket's editior...run sharpening slider halfway (50 units in that case) and the resultant file size is hugely reduced to an even lower size...from 4,449 KB way down to a mere 349 KB.

    In the case of Lightroom, I don't readily notice a physical difference in the size of the displayed image...I do notice a reduced physical size of the image saved back from Photobucket.

    Soooo...any "post processing reduced file size explanations for dummies" out there??
    Two fold question really:
    "Why does this file size reduction occur"?
    And "how can this happen yet actually result in improved images"?
     
  2. WoodWorks

    WoodWorks Super Moderator

    Dec 12, 2012
    Ashland, OR, USA
    David
    I suspect it's because you're saving a jpeg. Every time you save a jpeg, it gets compressed and you end up with less information in the file. My guess is that you're also (perhaps inadvertently) decreasing the pixel dimensions of the file. Have you compared the pixel dimensions (not the file size) of the before and after files?

    My guess is that the improvement you're seeing isn't really an improvement. It just looks improved on your screen. A downsized and sharpened image is certainly going to look better on your monitor, just as a huge print will look better from a distance rather than if you're standing twelve inches in front of it. But if you ever decide to print that downsized image, its degradation will become immediately apparent.
     
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  3. WNG

    WNG TalkEmount All-Pro

    Aug 12, 2014
    Arrid Zone-A, USA
    Will
    A wiki search for JPEG compression will probably clear up this issue for you better than trying to explain it in a nutshell.

    Compression is simply a method to reduce the amount of actual data before our senses deem the degradation significant. Sophisticated algorithms are employed to assess where data could be tossed and substituted with metadata to reconstruct the original digitized information, ie. video, photo, audio.

    One can selectively choose how aggressive these algorithms do this. (usually labeled encoding)
    'Quality' can be 100% and yield a jpg as large as a RAW or TIFF file.

    There is likely a Lightroom option to select the 'quality' and encoding type for JPEG.
    Photobucket is likely using a lot of compression to save storage and bandwidth.

    IMHO, the SOOC jpegs from the Sony is rather too compressed. It surprised me how small the files are for a 24Mpixel image. And artifacts are visible in challenging light scenes. They provide a choice of FINE and STANDARD....more user options would be better.
     
    • Agree Agree x 1
  4. roundball

    roundball TalkEmount Hall of Famer

    Oct 8, 2013
    USA
    In each test the file was only saved 1 time, and the pixel count of the LR tweaked file was the same as the original. Was different from Photobucket.

    Original file = 4,449 KB and pixel count of 6000 x 3726
    Sharpened in Lightroom, file = 1,887 KB, pixel count was still the same at 6000 x 3726

    Sharpened in Photobucket, file = 349 KB, pixel count of 800 x 497
     
    Last edited: Jun 9, 2015
  5. WNG

    WNG TalkEmount All-Pro

    Aug 12, 2014
    Arrid Zone-A, USA
    Will
    Pixel count is not directly related to file size in compressed jpegs.

    If pixel count was constant, the same image can be a number of file sizes.
    For example, take the same subject and same exposure, have your jpg setting in camera set to fine and then standard, you'll get a smaller file for the latter.

    Process a RAW file, save it to jpeg format.....in Lightroom, you will find 'Quality' options, encoding options (Huffman, standard, progressive, etc).
    They will yield differing file sizes.

    Easiest analogy, is to take a photo with the lens cap on. check to see how small that jpg file size is. Magnitudes smaller. You won't have that for the RAW file (to such an extent).
    In the JPEG file, it records the first pixel is 'black' == R:0 G:0 B:0, and the rest of them are the same, so no data saved for the remaining pixels, just the instruction to treat the rest like pixel #1.
     
    Last edited: Jun 9, 2015
  6. roundball

    roundball TalkEmount Hall of Famer

    Oct 8, 2013
    USA
    Just ran sort of an interesting test.

    Had an original JPEG photo with a file size of 1,675 KB and 2126 x 3191 pixels.
    Saved it to a different name file “test 1”
    Then saved test 1 to a different name file of “test 2”
    Then saved test 2 to a different name file of “test 3”

    The file size and pixel count never changed during the progressive savings.
    The photo that was saved 3 times had no changes in its appearance, file size, or pixel count.
     
  7. WoodWorks

    WoodWorks Super Moderator

    Dec 12, 2012
    Ashland, OR, USA
    David
    Yes, and if you set your photo editing software to save jpegs at their maximum quality, you can even make the jpeg file size increase. See this example, where I exported a jpeg from a raw file in Lightroom, and then opened and saved it in Photoshop three times at maximum quality:

    Screen Shot 2015-06-09 at 6.52.17 PM.

    As you can see, the file got bigger each time I saved it. Where do you think that extra data came from? It came from Photoshop, during the compression process, extrapolating data that wasn't there in the original file. And each time, even though the file size got bigger, the image was being degraded. If I kept doing that to an absurd extreme, I'd eventually have an image of complete mush that was >100 GB in size.
     
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  8. roundball

    roundball TalkEmount Hall of Famer

    Oct 8, 2013
    USA
    Actually in that most recent little compression test of mine above, saving / re saving a file a number of times to a different file names each time, was just to see if any aggregate effects of the "save compression" would occur to the file size and pixel count without any alterations being made to the file, and there were no changes in file size or pixel count.
    So Lightroom and Photobcket editors making changes to a file...(therefore I assume photo editors in general)...are what causes the change in file size & pixel count...not just routine saving/re-saving a JPEG file 4-5 times on a PC.
     
  9. WNG

    WNG TalkEmount All-Pro

    Aug 12, 2014
    Arrid Zone-A, USA
    Will
    I'm beginning to see that there possibly is a misunderstanding between 'save' and copy/cut/rename operations.

    When you asked the initial question, it was in regards to what has taken place in Lightroom or Photobucket. Therefore you are saving your changes.
    Pixel count remains the same unless the operator changes the image size.
    How your work is saved is user defined. That's what David and I were explaining.

    Semantics perhaps, but in your test of saving to another name, isn't the same operation. That's a file manager operation of copying, moving, renaming an original file and make a near duplicate with a new name. The digital content saw no changes and therefore nothing gets added or removed. File size remains the same. Of course it's not identical to the original, the metadata of the new file is different, ie. file name, date/time of creation, etc. But the contents are the same.
     
    • Informative Informative x 2
  10. roundball

    roundball TalkEmount Hall of Famer

    Oct 8, 2013
    USA
    Agree...in discussing this subject on and off over the the recent past, there seemed to be a common phrase like "anytime a JPEG Is saved it gets compressed / degraded"...without any qualifier that was limited to saving from a photo editor and I began to wonder...although I'd never noticed any such issues with simply saving JPEGs over the years, I ran that little test to reaffirm to myself first hand if a simple JPEG rename & save operation indeed made no such changes...
     
  11. chrid

    chrid Super Noob

    807
    May 5, 2014
    australia
    Chris
    I'm going to sneak into your house one day roundball and change your camera setting to raw hehe :hiding:
     
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