3 Replies Latest reply on Aug 28, 2015 12:38 AM by ManiacJoe

    Predetermined Image Export Sizes in LR CC

    PhotoJohnWS Level 1

      This is a question about exporting an image (edited from RAW) to JPG and doing my best to pre-determine the effective download size by manipulating the dimensions or through other methods. 

      This is not a question about printing.   


      I do understand the difference between shooting a white wall and shooting a colorful scene, regarding image size.   I also know that editing heavily in Lightroom and or Photoshop will affect the final size of the image too.     


      But what I am having trouble with is managing the output size, after cropping.    And speaking of Cropping - I may export a single file in three different crop sizes so using the same dimension for the "long side" is not an effective tool.    


      For one online location I am limited to 1500 Kb in total size and less than 16,000,000 pixels or 4,000 x 4,000.    For other online locations, I will require different limitations.     So what I am trying to do, is just get "Close" to the approximate pre-determined maximum file size and keep my guessing and 're-downloading" to a minimum.   


      Here is what I do know .....


      Using one image, (any image actually ... these numbers are approximate) ......The unrestricted 300 PPI Resolution JPG downloaded may be 8,000 Kb, so there is a lot of data to work with!   By using the "Limit File Size" feature, I can enter "1500 Kb or less"; leave the "dimensions blank" and I may get a file that is 600 Kb or less.     So, thinking like a semi-intelligent person, (which may be the problem), I might jump the "Limit File Size" up to 2500 KB, but then that may give me a 1700 Kb file, which is too large!  The next step might be to lower the "Limit File Size" to 2200 KB and I might still end up with a 750 Kb size.   There seems to be no rhyme or reason in using the "Limit File Size" feature!   

      Using a finished 8x10 cropped file, I might suggest the long side to be 1,000 Kb and leave the "Limit File Size" category and the second length blank.   This often falls within a reasonable range of 1200 Kb.  But that is never guaranteed and of course it changes when I use different crop sizes and changes drastically if I have edited heavily in Lightroom or PhotoShop.     

      INFO:   I usually default to 120 PPI for the resolution, but I can change that if necessary.... I know that 72 - 96 would be fine.   And maybe that is my problem?   Should I be picking 72 or 96 consistently for online use??      

      Any suggestions?     


      The bottom line is, I am looking for some consistency ..... or better yet, a math formula??


      Thanks in advance!!

        • 1. Re: Predetermined Image Export Sizes in LR CC
          ManiacJoe Adobe Community Professional

          In the Develop module, you use the crop function to change the shape and contents of the image frame. While LR can show you the pixel size of the resulting crop, this info may or may not be helpful to you. (I like to see it.)


          When you export to a file, you have controls over the various sizings:


          You can control the pixel dimensions by using the resize options. If you pick 4000x4000 pixels, LR will scale your image down to fit into that square while maintaining the correct shape (no stretching). If your image is smaller than your limitation, you can keep it that size (the common option) by selecting "don't enlarge".


          The size of the file is based on the pixel dimensions and the amount of JPG compression used. Here you can pick the compression level, or you can pick the desired kilobytes size where LR will adjust the compression to create a file no bigger than your limit.


          In your example of 4000x4000 pixels and 1500 kb, I would just input those numbers. For the other site that wants different limits, just input those limits and export again.


          Note: the resolution number is meaningless unless you are trying to describe the image dimensions in inches or cm when you should be using pixels in the first place.

          1 person found this helpful
          • 2. Re: Predetermined Image Export Sizes in LR CC
            PhotoJohnWS Level 1

            Joe..... Thank you.   


            I read your message a day or so after you sent it, but I have been forgetting to search for the "pixel size" referencing of the crop itself.   I just looked, and I have not found that anywhere.  ............  any hints? 


            Most everything else you provided, I was already aware of, but also, since your message, I have been doing some sample exporting with a controlled set of images, and I think I have a better understanding of the process.   I am still having trouble keeping the results consistent, but my tests are showing that the "data" that makes up the size of the image is impacted by "what I am cropping in and what gets dropped out".    Common sense, I suppose. 





            • 3. Re: Predetermined Image Export Sizes in LR CC
              PhotoJohnWS Level 1

              Here is the example of what I am dealing with.   I guess I don't have a huge problem with it, .... except that I can't understand why it happens and I don't like having good control over my export sizes.  


              The first item on the list is the Full Size image (which is a cropped 5x7) and you can see the exported image size is 1783kb   

              As you look down the list, the numbers that follow "MM" show various "Limit File Sizes" for the exact same export.  No other size restrictions were picked and you can see how the numbers start to change and "not" change.  


              Notice between 1750 and 1700 the file size drops 724kb and then that file size does not change again until the "Limit File Size" changes to 999 where it then drops another plus-30%.


              This is the crux of my problem... I would like to keep a file size as close to the 1500 kb mark as I can, but it appears that the "Limit File Size" has no real control of the final outcome!





              • 4. Re: Predetermined Image Export Sizes in LR CC
                ManiacJoe Adobe Community Professional

                The file size is not a good indicator of JPG quality.


                The "limit file size" is just another way to set the compression setting. You pick the max size you want, and LR figures out what compression setting gets you something "just" under that number.


                One thing to note here is that while LR has a "quality" slider that goes from 1-100, in reality there are only about 12 actual compression levels. This means that your file size limits are a lot less granular than you think.


                Some good reading:

                Jeffrey Friedl's Blog » An Analysis of Lightroom JPEG Export Quality Settings


                When I export a file, based on the intended use, I pick the pixel dimensions and the compression setting. Based on this I will have a vague idea of the megabytes of the file size due to experience, but as long as it is in the reasonable expectations the actual MB don't matter. Now, obviously some sites do place a limit on the pixel sizes and/or megabytes of images they host or link to. In those cases, I do adjust my export settings accordingly.