Here is a simple Optoisolator Circuit (also known as an Optocoupler) that I use for my Nikon D750 camera, for an external remote Digital Shutter Trigger on my Camera’s Power Slider.
It is highly recommended that you isolate your Camera’s Trigger Control Circuit from any external voltage, even an Arduino!!! The only way to efficiently and effectively do this is via an Optical Isolation Circuit!!!
R1 and R2 are Current Limiting Resistors in this circuit and are designed for the 4N25 Phototransistor Optocoupler, driven from a Digital Logic CMOS Buffer IC, powered by a 12v Power Supply (I use a 12v Batt. for remote shoots to power my camera slider and the logic that controls the slider).
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Usually you want separate controls for trigger and focus. I made a similar rig for firing 32 Canon DSLRs simultaneously. If you want the photos to be guaranteed at the same time, then you must apply the focus line (even when using manual focus) wait for a short period, say around a second, and then trigger. Asserting the focus line puts the camera into an ‘alert’ state, where it is expecting the trigger, and in this condition there is a fixed latency. Just in case that may help someone else.