When I am looking at, say, the contents of a folder in dolphin, hitting "CTRL - I" brings up the filter field, where I can narrow the results shown in the folder.

Is there a shortcut I can use to bring the focus back to the folder from the filter field?

I think the only thing you can do is to press Tab or Shift+Tab multiple times or close the Filter Bar with Esc. See also the related bug report https://bugs.kde.org/show_bug.cgi?id=183394.

Thanks freininghaus, I'll stick with TABbing my way around for the time being.