The simple answer would appear to be that ACR and Photoshop appear to use different JPEG algorithms, and that “equivalent” settings do not equate to the same file size, nor may the settings always be “equivalent”.
I just tested one random image at JPEG level 12, saving as baseline JPEG from Photoshop (ACR does not offer different format encoding options).
FileSize = 9.5 MB
269 lines of ExifTool metadata
FileSize = 10 MB
289 lines of ExifTool metadata
I then used ExifTool to remove all metadata, however there is still an approx. 500kb difference in file size, even taking metadata out of the equation.
So at level 12, there is only a slight file size difference. When I have time I’ll try level 10 to see if there is a more pronounced file size difference as in your example.
Interesting, as you say there is still unaccounted for data.
It's curious as well as the images I am trying this with still have quite a significant difference even if I "max" out the settings in PS. I still notice that they are almost double the file size of the Camera RAW output.
For example one I have just tried, direct from RAW, with no alterations or adjustments, just straight copy to .JPEG gives me an Camera RAW output of 6.62 MB (oddly also with richer colours than the PS version in this attempt) and the PS version is 10.2 MB.
Really curious as to what is going on here, especially now that I have noticed colour degradation in the PS export.
Any additional testing and advice is greatly appreciated, and thank you for your input on this so far.
The difference is “accounted for” if the two algorithms are not identical. Removing metadata levels the field, all that is left is the binary image data and the base data that a given file format requires for it’s file format.
Generally, if a file is double the size, then it is often 16bpc vs. 8bpc – however this should not be the case for JPEG files.
Are you perhaps applying output sharpening when saving the JPEG from ACR?
I’ll look into this a bit deeper, time permitting.
This is the thing, maybe I am missing some form of additional layer of modification at output level.
But looking at settings, everything seems to be normal. The difference remains high no matter which levels I set the quality output at.
For example, see the screenshots below. F = Camera RAW output, psf = PS output
What is, as I say, equally odd is the difference in colour, seen here below in the two examples:
Image on the left is Camera RAW, image on the right is PS output.
Below are the export settings for each, first Camera RAW and second PS options
Really not sure what I am doing wrong to have the difference, but the colour properties on the smaller file are more appealing, yet I don't want to be losing data.
Again, thank you for thinking this through.
Thank you for the detailed reply… To some degree, it does help explain the differences that you are seeing!
ACR only has one method to create a JPEG.
Photoshop has three common methods to create a JPEG:
Save As... (Save as Copy)
Export > Save for Web (Legacy)
Not all methods are created equal (by design).
When saving from ACR, you are using the “medium-wide gamut” Adobe RGB colour space and saving this colour profile into the image.
When using the Export as JPEG option from Photoshop, you may not be saving the Adobe ICC profile into the file, which would account for why colour managed software treats the two images with different levels of visual saturation.
Also note that the Export JPEG option uses a % based quality method, not a 1-12 level quality method.
Export As is intended for creating web/monitor based images.
I would compare Save As to the ACR Save Image option as the two are closer to each other.
It would appear you've identified the rub. Doing it as suggested makes the file sizes a lot closer and retains the saturation levels.
There is still about 800kb to 1mb difference, but no doubt this is to do with the settings that I am using to save the files and ways I am doing things. I clearly have a LOT more to learn with photoshop!
I will continue to learn and play but I think we can call this one resolved for now.
Thank you for all of the support and going through it line by line with me, i fear that i was getting stuck in a loop on trying to find the solution to this.
Thanks again, and all the best!