With Lightroom 2.7 and 3 Beta 2 both receiving a limit boost, now is a good time to talk about the Camera Raw Cache. So what is the the Camera Raw Cache anyway? Well in Lightroom, Develop uses a different preview system to Library. The Library Previews are created either on import via the Preview options, or as required (e.g. when you zoom into 1:1). In Develop, Lightroom goes back to the original file and creates the previews from scratch and applies whatever settings specified by the user. As you move to the next file and back, Lightroom again regenerates this preview. While this sounds tedious, you need it for accurate rendering, so that the changes you make in Develop will be what gets exported as your final version for further use.
Surely there must be some parts of this process that are the same each time? Well in fact there are. The early part of the process includes decoding, decompressing, linearizing, hot pixel filtering, and demosaicing an image. The results of this are the same each time, so for this reason, it’s perfectly legitimate to use a cached version of this as the basis for a preview. To help speed up the workflow in Develop, this partially processed version is stored in the Camera Raw cache.
How does this work in practicality? Well, the first time you open an image in Develop, it must do a full render. The partially processed version is then stored in the Camera Raw cache, for the next use, so when you return to the image, the creation of preview takes about half the time. The next question I would ask is, How can this benefit me? Well the default Camera Raw cache setting is only 1Gb. That’s equivalent to about 200 5DMkII Tiff files. The engineering team feels that it’s a good balance between space and speed. The maximum used to be 50Gb, but is now 200Gb, so obviously power users deemed a need for a much larger space. If you have spare space, I’d recommend increasing the cache size. Try 20Gb for a start, and if you are going back and forward over a lot of images in Develop, go even higher. Of course, it’s not just Lightroom that makes use of this cache, as the name implies, Camera Raw uses this also, so previews are shared between the 2 applications.
To change it open Preferences (Lightroom>Preferences on Mac, Edit>Preferences on PC) and click the File Handling tab. You should see a dialog like above at the bottom of the Preference dialog. You can see there’s 2 options. The 2nd one is what we’ve been talking about. If you have a spare internal drive, you can use the first preference to set the location of the cache. This way the catalog and cache (and possibly the images, depending on how you work) are on different drives, so Lightroom is not accessing the catalog and the cache from the same read head on one drive. It’s also a good idea to keep the cache off of your startup drive, if you’re using a large one. 200Gb is still a large chunk of space on a 1.5TB drive. The final option is the ‘Purge Cache’ button, which clears out the cache.
So what happens when the cache fills up? Easy, Lightroom just removes the oldest images from the cache, to make way for new ones.
Obviously you don’t need to use 200Gb, but I feel that even a modest increase from the default should make Develop zippier for most users. I know a few people on Twitter have thanked me for the recommendation.
Thanks to Eric Chan for information relating to what the cache stores exactly.