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heatherenke
Participating Frequently
December 13, 2018
Question

Image quality

  • December 13, 2018
  • 2 replies
  • 828 views

I've been searching for an hour online and can't find the answer to this.

I've got an image that is 24x36 and is 102MB (wood texture background). I need to reduce it to 20MB or slightly less in order to upload it onto an online platform and then print (it's a site where my customers can personalize, then download their own designs):

-I opened it up in PS, clicked 'Save As' and saved it as 'High' for quality which brought it down to 18MB, so that is good and I can now upload it, BUT I'm confused because I opened up 'Image Size' and it says it's 300ppi in both the original photo and now the newly saved photo. I thought that by saving the photo as lower quality (even though high) would change the amount of pixels in a given area. This image is going to be printed and I am completely lost as to why I can't find any difference showing in the 'Image Size' popup box in the newly saved photo compared to the original.

My final question is will this image lose a significant amount of quality when printed and should I just not use it? If needed, I can upload the images.

    This topic has been closed for replies.

    2 replies

    Bob_Hallam
    Bob_Hallam
    Legend
    December 13, 2018

    It seems your saving a jpeg file.  Jpeg is an image compression method to reduce file size by reducing the information included in the image.  This will, of course, reduce image quality.   It doesn't change the image resolution or size as you found out.

    Here's some more info on jpeg compression  https://en.wikipedia.org/wiki/JPEG

    When making a jpeg file always be aware that you are limiting the image quality at that time and in many cases removing the option to make any significant color changes to the file because of the image data reduction and the higher potential for banding and artifacts showing.   

    To answer your last question, you can run a quick test.   Set your history palette prefs to allow non-linear history

    then open the orig image and begin to save jpeg files at various amounts of compression.  Open those files and view each channel to see if the artifacts are showing.    Jpeg compression is image detail dependent so what happens on one image will not happen on another image unless the detail, color, and contrast are the same

    ICC programmer and developer, Photographer, artist and color management expert, Print standards and process expert.
    H
    heatherenkeAuthor
    Participating Frequently
    December 13, 2018

    I'll take a look at the provided link and try as suggested. Thanks for your response.

    T
    Test Screen Name
    Legend
    December 13, 2018

    Here's another way of looking at it. You've got three factors that affect file size

    1. How big is a pixel? A pixel is often held in 3 bytes, if it's "8 bit" RGB. You need 4 bytes for CMYK. 16-bit colour, if you use it, would use 6 or 8 bytes for a pixel.

    2. How big is the image? To get this we work out the number of pixels. You've given us that your image is 300ppi, 24x36 inches. How many pixels is that? ( 300 x 24 ) x (300 x 36 ). That's 77,760,000 pixels. If your image is 8 bit RGB, with 3 bytes per pixel, it's 3 x 77,760,000 bytes = 233,280,000 bytes.  That's 222 megabytes!

    3. How is it compressed? Images are always compressed, and this is where we can make it a lot smaller.

    JPEG is magic, but it's dangerous magic. It makes images a lot smaller BY THROWING STUFF AWAY. What it throws away is not parts of the picture, but the exact detail. In the picture. By slightly changing colours, in a way the eye might not even see, it can reduce an image to 25% of it's size, perhaps. By allowing more changes you might get it to 2% of it's size. Such magic numbers have a big cost: the image will probably be a bit fuzzy. The JPEG quality setting controls this. Choose low quality and your file will be tiny, but your customers probably up in arms.

    You need yourself to do careful tests on how small you can get the file and still have it good quality, because it depends on your originals. Wood grain may be a particularly difficult thing to compress without visible damage. Or not. You need to check.

    c.pfaffenbichler
    Community Expert
    c.pfaffenbichlerCommunity Expert
    Community Expert
    December 13, 2018

    -I opened it up in PS, clicked 'Save As' and saved it as 'High' for quality

    Save as what?

    If you did not change the image size (either via Image > Image Size or in the export dialog) then naturally the image’s size should not be changed.

    Please post a side-by side screenshot of the same detail in the original and the jpg taken at View > 100%.

    H
    heatherenkeAuthor
    Participating Frequently
    December 13, 2018

    Here's the original image (100MB - 300ppi, 24x36 inches)

    Saved as a JPG at lower quality (still high), which brought the file size to 19MB:

    Closed, then opened up both JPG files and cannot see any difference in quality:

    Can this still be printed at 24x36 inches without making too much difference? It's got to be about 20MB in order for me to use and it's still showing 300ppi, so I assume it will be okay. Am I correct?

    c.pfaffenbichler
    Community Expert
    c.pfaffenbichlerCommunity Expert
    Community Expert
    December 13, 2018

    I had specifically asked for screenshots taken at View > 100%, but as you posted jpgs they would have been pretty much useless anyway for assessing the original jpg damage,

    Please take png screenshots and post those.

    Once again:

    If you have not changed the pixel dimensions (no matter how) then they can naturally be expected to remain unchanged.

    jpg quality settings affect the compression, not the number of pixels.

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