The advanced search GUI will not allow you to search for a single focal length, it enforces a range value search. For example if I enter a single value, eg. 23, it will search for all images >= 23mm. It will not allow searching for exactly 23mm. The minimum range allowed by the search GUI is 1mm, so you can only search for images between 23 to 24mm. By modifying the XML of a saved search I have confirmed the search back-end does allow searching for a single focal length, eg. exactly 23mm, so this is likely just an issue with the GUI.
Git commit 147606c36135e03786341f444ad0da470e61201b by Maik Qualmann. Committed on 21/11/2017 at 20:41. Pushed by mqualmann into branch 'master'. fix minimum range value for the search fields FIXED-IN: 5.8.0 M +2 -1 NEWS M +3 -3 utilities/searchwindow/searchfields.cpp https://commits.kde.org/digikam/147606c36135e03786341f444ad0da470e61201b