Trigger old camera flash
Camera flashes, especially old ones, may have significant voltage accross their switch terminals when the switch is open.  Let it charge up and measure between the two lines with a voltmeter.  It could be surprisingly high, easily higher than the average opto-isolator.

Next you have to consider the current requirements when the switch is closed.  Again, that can vary considerably between flashes.  Modern "electronic" flashes have a low open circuit voltage and require low current to close the connection.  That is because this is a signal into other circuitry that actually initiates the flash.  Old flashes were much simpler, and the camera was assumed to have a mechanical switch closure to trigger them.  This switch more directly triggered the flash, so could be nearly 200V open and a sudden burst of current when closed.  Both these specs would be well beyond most opto-isolators.

I recommend that instead of using a opto, have the microcontroller activate a relay.  Wire the flash trigger leads to the normally open relay contacts and it should work fine for all types of flashes.