If you have either Photoshop or Photoshop Elements (the consumer version of Photoshop) installed on your computer, Lightroom chooses that as your default external editor, but you get some options for how the files are sent over to Photoshop (or Elements). Hold down the Command (PC: Ctrl) key and press the comma (,) key to go to Lightroom’s Preferences, and click the External Editors tab (see Figure 3).
The top of the dialog box shows options for choosing the file format of photos sent to Photoshop, along with your choice of color space and bit depth. I use TIFF, which also lets you choose a compression method so the files aren’t so large.
In the dialog box, notice that for the best results Adobe recommends ProPhoto RGB as your color space, at a bit depth of 16 bits. If you shoot in RAW, that’s good advice, because thus far no color profile has been embedded in your photo. However, if you’re working with JPEGs or TIFFs from your camera, the camera already embedded a color profile such as sRGB or Adobe RGB (1998) in your photo. In that case, choose that same color space for the Color Space option to keep everything consistent.