On my Mac, Lightroom writes to the ACR cache even if Photoshop isn't invoked and no export from Lightroom is done. Going to Develop is enough to write to the ACR cache.
I've just been investigating this for an unrelated reason and 'think' there are some interesting observations.
First, LR for me is my main RAW converter with C1pro for the finer work. I don't use ACR as a rule.
I've always kept my LR indexes and previews on a drive separate from my system/programs drive. Recently on my desktop workstation I put them on their own drive for the luxury of speed with excellent results. I built 1:1 previews for my entire catalog and moving between any folder/collection and zooming in on any image is now instant..
I've also always had the .xmp side car files enabled in LR.
So.. I have the default ACR cache on my system drive in the default location. And a LR3 cache on a separate drive.
I'm using LR3 and CS5 with the latest ACR 6.1 update.
My observations:
If I open an image previously imported into LR in ACR.. it uses the LR cache. I didn't realize this when I made my previous post.. my apologies.
If I open a 'virgin' image, say off a flash card or external drive or any image not already imported by LR.. into ACR.. it uses the default ACR cache.
Thinking about it.. it would make sense they've both have separate caches. This would be the same even if using the default LR3 and CS5 cache locations because they are different.
What isn't intuitive.. a nice way of saying it doesn't make 'immediate' sense.. is that the first .xmp side car file assigned then determines which cache it uses. Thinking about it more in-depth it does make sense.. making the .xmp smart enough to know which .xmp capable control program you're using to view that image and the cache locations would probably be difficult.
Secondary observations:
And I'm still not sure about this.. but it's where I'm at, at this point in time. I recently upgraded my monitors to a pair that offered sRGB emulation. Before this my system was set up to import in Adobe98 and more recently Prophoto. All my archive images were processed for the old monitors and these two different color spaces. With the new monitors in sRGB Emulation mode.. they all looked way off. Also, even if I reset that image and started over.. it took a lot of work to get the colors right. With the old monitors in the old color space these adjustments were slight if any and easy.
Keep in mind I've upgraded from CS3, CS4, CS5, LR1, LR2, LR3 on the same system build.
My observations.. as I rarely use ACR.. and by rarely I'd say every few months at the most.. I opened it tonight, set ACR to 16bit, sRGB, and under camera controls to Camera Standard vs. Camera Adone. Saved it. Opened up LR and now all my previous images are much closer than before, only a slight adjustment is needed to bring them into line with the new sRGB monitors.. and if I reset them.. they're as easy to adjust as they were with the previous monitors.
This tells me LR3's engine.. is using or looking at the ACR engine for its default settings. Or at least 1 or more of the 3 settings I adjusted.
Or maybe I'm wrong and seeing things. This has always been the Bermuda Triangle of color management settings.. never explained, rarely documented, and never talked about..
Of course if I'd wiped my build and started over from scratch.. like I often recommend others to do when having issues.. everything would have probably lined up as I installed each program providing I'd installed SVII and selected my color modes prior to installing applications. Still, I like learning how this stuff works.. or at least that nice confused feeling you get when working with Adobe color management settings..
