Hello Patrick:
The optocoupler Siemens LH1056 that I use is open collector technology, it is like a solid state relay or and it has no polarity, and that is an advantage. It also supports loads of 100mA, compared to the 50mA that supports the HCPL-817 that you indicate, according to the data sheet.
I have not had any problems, I feed the diode with a resistance of 330 ohms and directly the output controls my Canon G11 and the EOS 40D. Works fine.
I attached 2 links for better information:

schematic optocoupler]https://drive.google.com/file/d/0BxtwalY8tpnAbDNGNDhPSnAtM0U/view?usp=sharing]schematic optocoupler

http://pdf.datasheetcatalog.com/datashee...LH1056.pdf


I wish you good luck.
   Jesus

Hi Jesus,
my local dealer didn't had the LH1056 on stock, so I gave the 817 another shot and it works fine. Don't have clue what the issue was, probably wireing.

Thanks for your help anyway.
With best wishes.
Patrick

It is always the same story: try, correct and learn, until finally ... everything works! Congratulation.