Well, if the camera shoe and the flash are both ISO compliant, you can use the flash of a system on the shoe of another system, and the flash will be triggered at the same time as the exposure. But that's the only function you'll have. No TTL, no high-speed sync (FP), no flash menu control, no flash exposure compensation, no wake up from sleep mode, no 2nd curtain, no zoom lens corresponding to the focal length of the lens – anything that requires communication between the flash and the camera body other than the "fire!" the signal will not be communicated.
If you look at the camera's hoof and the flash's foot, you'll understand why.
Almost all camera manufacturers meet the ISO standard for flash instant (from 1985 to 2012, this is not the case of Sony, but with the old iISO Minolta hoof, but since the NEX6, they have switched to the multi-interface shoe conforming to the ISO standard). The ISO standards for flash feet and running shoes for cameras give the physical dimensions of the foot and the shoe and specify that the rails on the shoe are ground and that the contact in the center of the "square" of the foot / shoe is synchronized. signal.
Everything else is owner. For example, Canon and Nikon have their contacts / pins unsynchronized placed in different areas. Therefore, if the flash is properly installed on the shoe, there is no electrical contact or communication.
However. Four-thirds, micro four-thirds, Canon's Fuji X and Hotshoe contacts all use the same placement (but not necessarily the same number of pins). Therefore, if you mix marks between these three marks, it is possible that a cross-talk will occur. And since each brand uses a completely different signaling protocol, there may be errors and possible damage (although the risk is low). Therefore, if you are very paranoid, it may be advisable to stick the contacts or remove the pins. I pulled the TTL pins (completely reversible) on a Nikon SB-26 for use on my Canon shoes.
On the rise, however, this similar investment as well This means that you can also use Canon TTL cables for MFT and Fuji (for example, you can connect an MFT flash to an MFT camera and a Fuji flash to a Fuji camera with Canon TTL cable and enjoy all the functions of TTL synchronization and high speed.) but this only works with passive cables that only transmit the signal.
It does not work with TTL-compatible radio triggers, which can manipulate signals in one form or another. In addition, the latest versions of the MFT and Fuji X hotshoes now feed accessory flashes with additional touch that the Canon hotshoe does not have or does not use that way. Canon TTL cables will not work with these flashes (such as the Fuji EF-X8) because they have no other power source.
Although Nikon and Pentax have separate pin arrangements, strangely, the wake-up signal is identical, whether it is with respect to the placement of the contacts or the electronic signal, so that something like a Nikon Yongnuo RF-603II can actually activate Pentax flashes, TTL / HSS cross brand with Nikon equipment on Pentax.
Sync voltages, if you are using flash models of the digital era, should not be a problem. The sync voltage of all digital flashes tends to be <10V, and most Canon / Nikon night shoes have a 250V limit, while non-mirror devices are expected to have limits around 20V. The 6V limit you often see in Web references is for the first generation of Canon DSLRs. If you have a Canon camera model manufactured after the original dRebel (300D) model, the limit is 250V.
If you really need a cross-brand TTL / HSS compatibility with conventional flash, you can consider the Cactus X-TTL trigger system.