Ran the test, AF settings set to off. Apparently the sx420 doesn´t have a native AF lock other than holding the shutter halfway. First test, all enabled, crashed the camera (log shows data not written completely). Second test, only set_mf(), successful. Third, set_focus() failed, shut down cam and wrote data only halfway. Fourth and last, set_aflock(), worked (didn´t crash at least, couldn´t really read what the screen said)