# Camera raw cache



## fotojensen (Apr 2, 2009)

I'm wondering exactly what Camera Raw Cache does, and do I need to keep these files?
I thought that the previews was the files that made Lightroom fast. Do i need  the previews in the previews folder and the cache files in the camera raw folder? I now that previews in the previews folder make 1:1 faster, but what exactly are the cache files doing?


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## Victoria Bampton (Apr 2, 2009)

Hi Kai

The camera raw cache holds the previews for Develop module.  Develop module usually reads the raw data, but it's quicker to load from that cache, whereas the other previews are used by Library module.  You can clear it out any time you like, and it'll rebuild as it needs it.


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## fotojensen (Apr 3, 2009)

Thank you Victoria, i find it strange that Lightroom need two separate preview files, one for the library and one for the develop. If i want Lightroom to work fast i must keep both? But when i am in the develop module i thought Lightrom had to build up a preview anyway, from the raw file. But that must be where the casche file comes in.
One other thing, i told Lightroom to build previews for all my files yesterday, and it build [COLOR=#''''''] thousand[/COLOR] of cache files, and i hope thousends of preview files, is that how it works?
I just want to now whats going on


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## Brad Snyder (Apr 3, 2009)

Build previews only applies to the (Library) preview cache. The camera raw cache operates to some undisclosed internal algorithm.

Despite a lot of questions, Adobe hasn't revealed a great deal of information about the camera raw cache, so I'm assuming there's some trade secret/proprietary things involved there. But, I'm just guessing.


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## pauldunlop13 (Apr 3, 2009)

Does anyone have their preview folder on an external drive?
Is it possible?


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## Victoria Bampton (Apr 3, 2009)

Brad Snyder;4'899 said:
			
		

> Despite a lot of questions, Adobe hasn't revealed a great deal of information about the camera raw cache, so I'm assuming there's some trade secret/proprietary things involved there. But, I'm just guessing.



I'd say the same.  Funny how the file sizes are identical for images off the same camera, even though the raw file size varies.  There's something techie going on!


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## Denis Pagé (Apr 3, 2009)

My take on it is that the ACR cache store the pixel info which is the same pixel count from image to image. It is the _pixel's sandbox_...


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## Victoria Bampton (Apr 3, 2009)

pauldunlop13;4'9'7 said:
			
		

> Does anyone have their preview folder on an external drive?
> Is it possible?



Technically it's possible, at least on the Mac version using Symbolic Links, and I guess on the Windows version doing something similar, but there's no great advantage to doing so.

You can move the ACR cache to another drive from the preferences dialog - that may be worthwhile, depending on the speed and drive space available.

And welcome to the forum!


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## fotojensen (Apr 3, 2009)

Thanks for your answers, but i still  wonder why Lightroom build cache files each time i render 1:1 Previews. I also would like to now why every file is exactly the same size exactly 86'7 kb from my 1DS MKIII. And is there something to win on performence if i keep the files insted of delete them?


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## Victoria Bampton (Apr 3, 2009)

It's preloading them into the ACR cache on the basis that you're likely to view them in Develop.  It does the same when it renders new Standard sized previews too - which is great, because I've been trying to figure out how to pre-load the ACR cache.

Performance - in Library module, there's no advantage to the ACR cache (unless possibly if you use Quick Develop panel) so you could purge that cache.  In Develop module there's a huge performance benefit to having images in that cache, which is the result I've been aiming for.


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## fotojensen (Apr 3, 2009)

Thank you Victoria, i think i will keep my cache files for a while, i have 9' ''' images that i have to look into. For best performance it sounds like a good idea to keep those files.

It is good to have this forum to get better knowledge about Lightroom, thanks!


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## Victoria Bampton (Apr 3, 2009)

The ACR cache is a rolling cache - it max's out at the setting you set in preferences, and as it runs out of space, it throws out the oldest and adds in the latest.


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