LR JPEG compression vs. Photoshop JPEG compression

Question

I haven't found any documentation of the meaning of the 0 - 100% JPEG compression value in LR's (v1 or v2) Export File window. And the default value of 100% is overkill and results in huge files. At least I'm familiar with the Photoshop's 0-12 JPEG quality scale with associated quality names: Low, Medium, High, and Maximum.
Via trial and error, I have found that LR has the same 13 quality levels as Photoshop and gives the same results, they are just mapped on a 0 - 100% scale. This also means that changing a few percent may not make any change at all, since a quality change only happens about every 7 percent.
For those who might find it useful, here is a table of the mappings:
The first column is the Photoshop compression number and name; the second column in the range of Lightroom percentages that will give the same results.
0-Low 0-7%
1-Low 8-15%
2-Low 16-23%
3-Low 24-30%
4-Low 31-38%
5-Med 39-46%
6-Med 47-53%
7-Med 54-61%
8-High 62-69%
9-High 70-76%
10-Max 77-84%
11-Max 85-91%
12-Max 92-100%

Answer 1

I looked at this again using PS's 'Baseline Standard' JPEG format option instead of 'Baseline Optimized. LR does not provide the format options Standard, Optimized, and Progressive, but appears to use 'Baseline Standard.' The equivalent compression level LR file size is within 16KB of PS's file size, which is probably due to slight differences in in the file metadata.
This pretty much confirms LR and PS use the same 'Baseline Standard' JPEG compression algorithms. The PS level 7 reduced quality is also seen at LR's level 54-61 JPEG Quality setting. Jeffrey Friedel mentions this in his analysis of LR's JPEG Quality settings and a reply from Brian Tao:
http://regex.info/blog/lightroom-goodies/jpeg-quality
Jeffrey Friedel's comment:
One thing I find interesting (but don't understand) is that in the first example, the difference in file size between the  47〜53  quality and  54〜61  quality is considerable (49k to 66k bytes), while in the second example, the the same two levels of quality produces essentially the same file size. There seems to be some kind of switch in compression algorithm once Lightroom is at a quality setting of 54 or above that puts the emphasis on encoding the easily-discernible smooth gradients of the sunset example, and if they are lacking in the image, as with the reed-window-shade example, the attempt at extra quality fails, and the file size does not increase. That's my guess, but it's just a guess.
Brian Tao's Reply:
This is due to the downsampling (basically, a reduction in resolution) of one or more of the image channels before passing it to the actual compression routine.  Human vision is much more sensitive to changes in luminance (brightness) than chrominance (colour).  JPEG takes advantage of this by reducing the amount of colour information stored in the image in order to achieve higher compression ratios.  Because it is colour and not brightness that is sacrificed, this is called “chroma subsampling”.  Look up that term in Wikipedia for a far better and more detailed description than I can provide here.
In a nutshell, Adobe products will use either a 4:4:4 subsampling (which is no subsampling at all, and thus full resolution) or 4:2:0 subsampling (both red and blue channels are reduced to one-quarter resolution before compression).  There is no switch to specify the amount of subsampling to use.  In Photoshop, the change from 4:2:0 to 4:4:4 happens between quality 6 and 7.  In Photoshop’s Save For Web, it happens between quality 50 and 51.  In Lightroom, you already noticed that something unexpected happens between 47-53 quality and 54-61 quality.  Guess what levels those correspond to in Photoshop?  6 and 7… exactly as expected.
You can very easily demonstrate this by creating a worst-case scenario of JPEG chroma subsampling.  Create a small image in Photoshop with a pure blue (RGB = 0,0,255) background.  Now type in some pure red text (RGB = 255,0,0).  For maximum effect, turn off anti-aliasing, so each pixel is either full on red or full on blue. Zoom in to 500% or so for a clear view of the pixels.  Now save the image as a JPEG.  With the JPEG quality dialog visible, you will see a real-time preview of the effects of JPEG compression.  Start at 12, and work your way down to 0, one step at a time.  Watch what happens when you go from 7 to 6.  You can do the same with Save For Web and with Lightroom to confirm where they switch from 4:4:4 to 4:2:0.
The file size discrepancy is more noticeable in the sunset shot because most of the information (relatively speaking) is needed to encode the gradual change in chrominance values.  There is virtually no luminance detail to worry about, except around the silhouette of the bird.  But in the photo of the reed window shades, the fine detail and texture and lack of colour result in practically no difference going from 4:4:4 and 4:2:0.
Because of this hidden (and inaccessble) switch, I have been recommending that to be safe, one should never go below quality 7 in Photoshop, or 51 in Save For Web.  In Lightroom, this corresponds to quality 54.
Hope this helps.