Behind the scenes, Lightroom desktop creates Smart Previews of photos marked for sync and uploads them to the Creative Cloud servers. Smart Previews retain much of the editability and detail of the source images (even raw files) but occupy much less storage space. In Lightroom mobile, the app downloads low-resolution previews for display in its Grid layout, and when an image is opened it pulls down the higher-resolution Smart Preview file (enabling you to zoom in to check details if needed).
Make a change to a photo on the iPad, and that change should appear in Lightroom desktop within seconds, removing the need to export or import images. Edits you make to the photo synchronize back to Creative Cloud and Lightroom desktop when you close the image—in fact, only a small XML file describing the edits is transmitted, which is why updates appear in the desktop and mobile applications quickly.
Adobe’s Lightroom application made its way to iOS overnight with the launch of Lightroom mobile for iPad and Macworld’s Jeff Carlson has a detailed preview of the app. Adobe looks like they have done a really nice job with bringing the app to the iPad thanks to their implementation of a clever sync system (as explained above) and extensive compatibility between this mobile version and the desktop version of Lightroom.
As with most of Adobe’s offerings in recent years, Lightroom mobile is bundled as a part of their Creative Cloud subscription service, so whilst the app is free to download, you will need to sign up for one of their subscription plans to use Lightroom mobile.