DSLR Hotshoe wiring to Arduino
A simple optocoupler would be the best bet, to protect both sides. The led side would be connected to the trigger and ground of the hot-shoe, while the other side is connected to the arduino.



The Resistor on the arduino side is a weak pullup to the arduino power supply. You could skip it if you enable the internal pullup instead, in which case you do not wire the vcc/resistor part.

In this case, when the hotshoe is triggered, the optocoupler enables it's transistor, and the arduino would register a change from logic High, to logic Low.