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By entering xnoword search into the search box, you will get hits in images which do not have metadata. When searching for xnoword in a specific field, you will get hits for images where that field is empty.
Note: For indexed archives you also need to make sure that the fields that you want to search as empty are “indexed as empty” inside the “Fields” tab on index properties. For all changes a rebuild is required.
Caveat lector: I used to work with support at FotoWare. This blog is my own space, I am not paid to write it. I write about FotoWare and FotoWare products because I believe in them.