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EGibbon
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Optimize JPG image size reduction by reduced compression quality vs. reduced pixels?

Feb 28, 2013 10:00 AM

Tags: #image_quality

I have many images of slides scanned at high res (4800 DPI, maximum pixels 5214x3592).   Although I will be saving these as lossless TIFs, I also wish to make JPGs from them that I wish to be just less than 5 MB in file size.  Aside from cropping, I know I can achieve such a reduction of JPG file size by a combination of saving to lower quality JPG compression or reducing image size.  My question is, what is theoretically or practically better, achieving this mostly by reducing image total pixels or by reducing  JPG compression quality.  Thank you

 
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  • Currently Being Moderated
    Feb 28, 2013 10:30 AM   in reply to EGibbon

    "aside from cropping -------- lowering quality compression or reducing image size.  By reducing image size are you talking about resolution in ppi?  To do this you would have to have resample image checked.  Also how much compression an image takes depends somewhat on the image detail.

     

    I would just try it both ways with test images and look at image on screen to see what is sharper.  If this is not clear take the image size down to 1 meg with same techniques and see which style wins. 

     

    Report back on findings.

     
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  • Currently Being Moderated
    Feb 28, 2013 8:20 PM   in reply to EGibbon

    Without testing or having read anything technical to "prove" my point,

     

    but from many .jpg files I have produced....

     

    I would say "it depends" on the image....and the final use.

     

    if there are few colors involved, such as a large portion having a consistent blue sky,  then compression may be done more easily. 

    If the image overall has many distinct colors, then fewer number of pixels may make a smaller file size more easily.....

     

    however you may also want to consider the final end use....such as print quality and size needed vs. display only on a monitor screen at 72 to 96 ppi.

     

    Like I said, "it depends"  on more info and one answer may not fit all needs.

     
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  • Currently Being Moderated
    Mar 1, 2013 8:07 AM   in reply to EGibbon

    There is a quirk in jpeg quality setting you should be aware of.  Never use the setting 7.  I goes into some weird funk so the qualilty is really worse than setting 6.  If you look at the image size when you hit 7 you will see it make a huge dip.  If you google this you will see a more technical explaination.  http://www.petapixel.com/2011/08/26/a-higher-quality-setting-in-photos hop-sometimes-reduces-jpeg-quality/

     
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  • Noel Carboni
    23,514 posts
    Dec 23, 2006
    Currently Being Moderated
    Mar 2, 2013 1:26 AM   in reply to EGibbon

    Something else to consider...

     

    Noise / grain in the image can greatly increase compressed file size.  Maybe there is some image prep you'll be doing on the JPEGS and if so you might consider doIng some noise reduction As part of that.

     

    -Noel

     
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  • Currently Being Moderated
    Mar 4, 2013 10:06 PM   in reply to EGibbon

    It seems to me that by downsizing you will be irretrievably eliminating detail throughout the image, while by opting for more compression you will be eliminating detail only in portions of the image, while keeping the overall image.  Obviously, there needs to be some balance.  You wouldn't want to maintain a 10 gigapixel image at full size with compression set to 1 to keep the file size down.  I would keep the files as large as possible at a compression setting of 8, which doesn't hugely affect the quality.

     
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  • Currently Being Moderated
    Mar 6, 2013 6:36 PM   in reply to EGibbon

    The quality at Adobe JPG 10 level is pretty good, and keeping the images at 4k or more on the largest side will give you few, if any, noticeable artifacts.  The artifacts occur most noticeably in areas of high contrast, so look around the edges of signs, letters, ridgelines, etc. to see if you are getting degradation.  I suspect that at level 10 compression it won't be noticeable even when you are looking for it in most images.

     
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  • Currently Being Moderated
    Mar 6, 2013 11:33 PM   in reply to EGibbon

    You can find a detailed technical explanation of jpg compression on the web, but it won't help you. Just play with it until it looks right.

     
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  • Currently Being Moderated
    Mar 7, 2013 9:39 AM   in reply to Lundberg02

    If it were me, and my goal was a file size of approx. 5MB

    and the .jpg from Ps at quality of 10 resulted in a file as much as 5.5MB (exceeds goal by 10%)

    then I would not resize or do anything further to reduce quality and spend added time for very little value; 

    You can see file size with a 1 sec. glance and avoid added work to make slightly smaller files.

     

    I'm guessing that some files may be less than 5MB and if so, collecting all, they will probably average 5MB anyway.

     
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  • Currently Being Moderated
    Mar 7, 2013 9:53 AM   in reply to Doug.S

    If you use save for web it can strip out the metadata to reduce file size.

     
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