Understanding Lightroom's treatment of photos is always a challenge for people new to the program. The concept that a photo is "IN" Lightroom, or needs to be gotten "OUT" of Lightroom, is quite common. And the relationship between the thumbnails in the catalog and the actual, real images (be they JPG, TIFF or Raw) is equally confusing. Here's how I like to look at it:
Think of your images as books in a library. Each book is on a shelf somewhere (folders of your hard drive). The books and shelves may be organized in some fashion that may or may not make it easy to locate an individual book.
The catalog is the little cabinet in the front of the library that is full of cards describing each book – who is the author, where is it located in the library, what keywords it has, and so forth. When you look at a card in the catalog, you see a thumbnail of the image, and have a pointer to exactly where in the library that image is located.
A collection is a list of some of the cards in the catalog that meet certain criteria. In a regular collection, you pick and choose which cards to include in a new list (for example, my favorite images). In a smart collection, you pick the criteria to match, and the computer does the work of searching through the catalog for cards that match (all images with keyword "vacation" that were taken in 2011). With Smart Collections, the list is generated automatically.
Catalogs and collections do NOT contain images. They contain the cards that tell about the image – its location, information about the image, even a thumbnail of what the image looks like.
This is why moving images around outside of Lightroom is bad. You move the image on you hard drive, and the catalog of cards is still indicating that the image is located where it used to be. Try to find it there, and you get a big question mark.
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