Thanks everyone for your help. Apparently the culprit was a faulty PC sync flash cable. Wasn't easy to pinpoint for a couple reasons:
- it would work when flash was connected to the breadboard without the opto
- when I tried that circuit ( Optocoupler ) I used a standard yellow LED with a 330 Ohm resistor and the LED would actually blink but it was so dim I couldn't even see it. The fact that I have a pretty bright working light over everything only made the blinking impossible to see.
I do however still have a question that has come up from this. Why is it that the LED in this circuit Optocoupler is sooooo dim when connected with a 9V battery and an optocoupler 4N35, 330 Ohm resistor, yet without the opto it is very bright. Does the opto's transistor side "use up" so much current that there's very little left for the LED? Or is it something else completely?
Just trying to understand what is really happening in this kind of circuit.
Thanks for all your help again everyone! Much appreciated!