Systemd vs logging collector

In Arch Linux, Postgres is started by systemd. If you configure logging_collector in postgresql.conf this has no
effect.All of the logging output is send to the journal, where you can read it with “journalctl -u postgresql”. 

The configuration for systemd contains a StandardError option. I’ve tried various settings but none result in a regular
Postgreslog file. The Postgres settings for systemd can be found here: /usr/lib/systemd/system/postgresql.service 

Is there a way to let Postgres do its own logging when it is started by systemd?

Kind regards,
Andomar

P.S. Trying to figure this out in order to answer this question:
http://dba.stackexchange.com/questions/105384/postgresql-doesnt-log/