I'm not sure I follow your answer. I know how to search for a specific tag, and how to limit my searches to a limited number of fields (which can then be reduced in the search box for a particular search). What I am not clear on, is how do I search for NULL tags? My apologies if this is obvious.
You pointed me in the right direction. A search on Tags using 'NOT " "' (NOT and a single space between quotes) will bring up untagged images. However, if you try to save this as a dynamic album, the search criteria is lost and needs to be reentered when the dynamic album is being created.
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If you want to search for something which does not contain 'X' then you would do a search on !X (or NOT X). Search has full boolean logic