3 Replies Latest reply on Apr 5, 2014 9:45 AM by choiceimport

    why is image compression information different on method used

    choiceimport

      The image size starting out is 72dpi resolution @ 1280x800px dimension.

      The final target output settings when using "Save For Web" in Photoshop should be:

       

      72dpi resolution @ 1280x800px dimension

      JPEG Quality:75 @ 246.7 Kilobytes.

       

      1. If I re-import the image that is supposed to be at 246.7 Kilobytes in Photoshop and click image size,

      it is telling me that the image is 2.93 Megabytes.

       

      2. When using Apple's "Preview" app and click the inspector it is telling me that the image is 253 KB (Kilobytes).

       

      3. When using Apple's "Quick look" feature, selecting the image then hit space bar, it is telling me that the image is 247 KB

      (rounded off) which is closer to the original final target output settings of 246.7 Kilobytes.

       

      4. Lastly, when I click Command i to get the image info, it is telling me it is 253 bytes or (254 KB on disk).

       

      Why is all the image info that is available so different and which method should I trust or use?

        • 1. Re: why is image compression information different on method used
          Curt Y Level 7

          Photoshop does really not have a true image size.  It gives you document. size, but that is the space PS reserves to edit image.  If you want the image size it is height x width in pixels.  Otherwise open and look at in Bridge or some other browser like you have been doing.

           

          Hope this helps.

          1 person found this helpful
          • 2. Re: why is image compression information different on method used
            gary_sc Adobe Community Professional

            Hi Choiceimport,

             

             

            Don't sweat the small stuff. The subtle differences between Preview, Quicklook, and Command-i are due to a variety of things such as how many sectors that program may be looking at to get all the data. Some programs over-reach the sector reading. One area where you'll really see this is if you saved your image and it's 253 kb but when you put that in an email client it might be (and I'm making this number up) 370 kb.

             

             

            The big issue is why is it so big in PS. To understand this, you have to remember that JPEG images are compressed in a Lossy format. There are two kinds of compression here, the lossy aspect (in that the format actually looses information (ergo lossy), so that the greater the compression, the more of the data is removed and tossed (forever, never to be returned), and the general compression algorithms of the format. Thus, the file has a 247 kb storage size (or thereabouts) but when expanded so it can be seen is 2.93 MB. This is how much PS has to deal with in regards to ram to work with the image but is not its storage size.

             

             

            And FWIW, I like to specifically call out "storage size" when referring to how big the document is on the hard drive as opposed to the size of the image which (to me) refers to the dimensions of the image (either MPXL or X pixels wide by Y pixels tall).