This is a really arcane question, but I'm guessing that at least one person here knows the answer. I've searched in vain for an explanation from Adobe. As I understand it, LR maps raw data to a proprietary variant of ProPhoto called Melissa. The image stays in that color space until one exports it, moves it into Photoshop or another external editor, prints it, or whatever. ACR has a menu that allows converting to other color spaces, including Lab. LR lacks this. So, here are the disagreements: 1. Does that ACR menu have any bearing on the internal color working space, or does it use the same working space as LR, only mapping to another space, such as lab, when moving the image out of the application into photoshop? 2. If you want to work in Lab in Photoshop, choosing the Lab option in ACR would open the image as a base layer in Lab in Photoshop. Coming from LR, one would have to open the image in Photoshop in whatever working space one selects, let's assume ProPhoto to keep things simple, and then convert the base layer or a copy to Lab. Would that make any practical difference? Thanks for any help with this.