In yesterday's post, I should have noted that there are problems when using inexpensive auxiliary optically triggered flashes with Canon main flashes in ETTL flash mode.
With optically triggered auxiliary flashes, it is important to know that Canon main flashes emit a "pre-flash" as part of the Canon ETTL system. Most auxiliary flashes are highly likely to be triggered by the pre-flash; this includes the in-camera flash of the G9. Therefore, it is essential to set the G9 for manual flash in order to properly trigger the auxiliary flash. My Quantaray MS-1 falls victim to the Canon pre-flash in every mode except for Manual.
Yesterday's post has been edited to include the above note.