excluding relative paths in duplicity

The [duplicity documentation](http://duplicity.nongnu.org/vers8/duplicity.1.html) doesn't appear to fully document its behaviour when relative paths (or bare filenames) are passed to the --exclude option. If I pass the option --exclude foo, for example, will this cause each file or directory named foo in the entire heirarchy under source_directory to be excluded, or will it only exclude a file or directory with that name in source_directory itself? If the latter is the case, is there a way to exclude source_directory/foo without having to type the full path to source_directory for each such option (other than by using a shell variable)?