Limit file size on export...what is it doing?

[Vcize]

I have a stitched panorama that is 12,000 x 5000 pixels.  When I export it as a jpg without any resizing it comes out to 70mb.  If I click the "limit file size" button to limit the file size to 25mb and of course it does that.  When I open the two files side by side and pixel peep I can barely see a difference in the quality of the smaller file, if at all, despite it being 30% the file size.

I guess I should just enjoy and maybe not question it, but I'm worried if a big print is ordered from that smaller file if some differences might show up.  What exactly am I losing in compressing the file that much, what is Lightroom doing to lower the file size that much without lowering the resolution?

[Joe Foe Toe]

View both images at 1:1 in Lightroom and you'll have a "true view" of any image differences. Compression is a weird beast - some images with large compression are "unnoticeable", while some images with a lot of fine definition show the compression clearly. This is a great article: Jeffrey Friedl's Blog » An Analysis of Lightroom JPEG Export Quality Settings   If you try saving the Lightroom at a given %, you can compare the exported filesizes to get an idea of what level LR is using for your 25mb compression. I just tried an image at LR 100% vs LR 70% and the 70% was bout a third the size with no sign of image change at 1:1, so my guess is you're fine.

[Tony_See]

Can I ask why you want to reduce the file size by 2/3rd's?

I'd be concerned also about the fear of a file being ordered with that much pixel information missing . . .

However, the other factor of course is the print size you'd be aiming for with that file: larger prints require optimal viewing distances and therefore less file res so you might be OK

[dj_paige]

I'd be concerned also about the fear of a file being ordered with that much pixel information missing . . .

Read the link given by joefry99​, this is all explained, many JPG files can be reduced greatly in size without noticeable loss of quality.

[Vcize]

Tony, I want to upload it to Fine Art America, which caps its file uploads at 25mb.

[Bob Somrak]

The real solution to this problem is to find a print lab that doesn't impose arbitrary restrictions on your file sizes.  I don't know anything about Fine Art America but my print lab doesn't impose such restrictions.  They will inform the customer if the submitted file will cause print issues if it is not up to their quality standards though. 

[Vcize]

Bob,

Yes of course, and my typical prints are done here with a local lab where I can deliver them 800mb tif files without them batting an eye.  However I would also like to make my work available on fine art america since their SEO prescence is so strong, but they limit uploads to 25mb.

[Bob Somrak]

Thanks for the info.  I logged onto the Fine Art America site and I now see why you are doing this

another way to see what is different in a c

low compressed file and the same pixel dimention highly compressed file is to load both files In Photoshop as layers and change the Blend mode of the top one to DIFFERENCE to see how many pixels are changed.

[Todd Shaner]

Make sure your images when viewed at 1:1 Zoom are not over-shapened and/or have insufficient noise reduction. This can inflate the file size by a factor of 2x or more! In general the vast majority of images will show no quality loss when using a LR JPEG Export Quality setting of 70 or higher.

BTW there are really only 12 LR Quality settings:

  * PS Quality level 7 should be avoided:

http://petapixel.com/2011/08/26/a-higher-quality-setting-in-photoshop-sometimes-reduces-jpeg-quality...

More information on JEPG quality settings with examples:

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

[ItWasNotMe]

Every image will be different in how much 'detail' is lost by creating a jpg file

even when you use the same settings. Thats the nature of 'lossy'compression

Consider these examples:

1. Am image that is completely uniform in every pixel; you can massively compress that without any loss of detail

2. Now, an image that has a uniform gradient across the image (like the sky in the Friedl example); any significant compression is going to cause banding but you can get away with quite a lot.

3. Finally an image that is similar to white noise and all adjacent pixels are high contrast to their neighbours and there is no pattern in the image; detail will be lost at any compression.

At its simplest, any 'real-world' image is some combination of items like this, so you will loose detail in some areas but not others. How much depends on degree of compression and the algorithm thats been implemented; where depends on the image but is likely to be in any large areas with lots of complexity.

[ItWasNotMe]

If you want to see which pixels have changed and are happy with command line tools you could try ImageMagick.

You'd need to look at the compare function.

see https://www.imagemagick.org/script/compare.php