I have flash brand X and would like to know whether it will work with camera brand Y. What do I need to consider to determine whether the combination will work?
Other questions of relevance:
I have flash brand X and would like to know whether it will work with camera brand Y. What do I need to consider to determine whether the combination will work?
Other questions of relevance:
Well, if the camera hotshoe and the flash are both ISO-compliant, you can use one system's flash on another's hotshoe, and the flash will fire in sync with the exposure being made. But that's the only function you will have. No TTL, no high-speed sync (FP), no menu commanding of the flash, no flash exposure compensation, no wake-up from sleep, possibly no 2nd curtain, no matching the zoom to the lens's focal length—anything that requires communication between the flash and the camera body other than the "fire!" signal won't be communicated.
If you look at the camera's hotshoe and the flash's foot, you'll see why.
Nearly every camera manufacturer follows the ISO standard for flash hotshoes (from 1985–2012 Sony didn't, using the old Minolta iISO hotshoe, but as of the NEX6, they've moved to the ISO-compliant (sorta) multi-interface hotshoe). The ISO standards for flash feet and camera hotshoes give the physical dimensions of the foot and 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 the sync signal.
(Footnote for Canon T7/SL3/T100 users, see this fstoppers article about the missing sync connector on those models, and the Canon EOS 4000D Hot Shoe for Sync (with Studio Flashes) Q&A, but be aware there are also reports that newer copies of the T7 have it reinstated. For Canon R50 users, see this Q&A).
Everything else is proprietary. For example, Canon and Nikon have their non-sync contacts/pins placed in different areas, so if the flash is seated correctly on the hotshoe, there is no electrical contact and no communication.
However. The four-thirds, micro four-thirds, Fuji X, and Canon hotshoe contacts all use identical placement (although not necessarily the same number of pins), so if you mix brands among these three, there is the possibility of cross-talk. And since every brand uses a completely different signaling protocol, there could be some errors and possible damage (although the risk is small), so if you're very paranoid, taping over contacts or removing pins might be advisable. I pulled the TTL pins (completely reversible) on a Nikon SB-26 to use it on my Canon hotshoes.
On the upside, however, this similar placement also means you can use Canon TTL cables for MFT and Fuji as well (i.e., you can connect an MFT flash to an MFT camera and a Fuji flash to a Fuji camera with a Canon TTL cable and have full function with TTL and high-speed sync); but this only works with passive cables simply doing signal pass-through.
It does not work with TTL-capable radio triggers which may do some form of signal manipulation. And, newer versions of the MFT and Fuji X hotshoes now supply power to accessory flashes with an additional contact the Canon hotshoe doesn't have or use in that way, so Canon TTL cables will not work with those flashes (such as the Fuji EF-X8), as they have no other power source.
While Nikon and Pentax have separate pin arrangements, oddly, the wake-up signal is identical both contact-placement and electronic signal-wise so something like a for-Nikon Yongnuo RF-603II can actually wake up Pentax flashes, but again, there's no TTL/HSS cross-brand with Nikon gear on Pentax.
There is one speedlight line out there that can be used with TTL/HSS communication on more than one brand of flash hotshoe, and that is Jinbei's HD2 models, rebranded as the Westcott FJ80 in the US, and the Rollei HS Freeze 1s in Europe. The same is true of the Jinbei TR-Q7 transmitter. These devices use a special "universal TTL" foot that can cover all the pin/contact patterns (even Pentax's):
And the flash can internally switch between Canon, Nikon, Sony, Fuji, and MFT communication protocols. But Sony MI hotshoe users need to use an (included) physical adapter for the foot.
Sync voltages, if you are using digital-era flash models, should not be an issue. All the digital-era flashes tend to have sync voltages of <10V, and most Canon/Nikon hotshoes have a limit of 250V, while mirrorless cameras are guesstimated to have limits around 20V. The 6V limit you often seen in web references are for the first generation of Canon dSLRs. If you have a Canon camera model that was made after the original dRebel (300D), the limit is 250V.
If you really need cross-brand TTL/HSS compatibility with legacy flashes, you may want to consider the Cactus's X-TTL triggering system, which also has a foot similar to the one on the Jinbei HD2 and TR-Q7. But as of 2021, they've gotten out of the photography gear business and no longer offer support.
Well, if it has a hot shoe, and the flash fits in the hot shoe, then it's compatible, sort of. Normally it will fit, but there were some cameras (Sony is one) that did their own shoe thing, preventing all others from fitting. I suppose that was to sell their own flash brand.
Many third party flash brands (like say Yongnuo) make "dedicated" models, designed expressly to fit say Nikon or Canon camera systems. Or some are Manual mode only, working with any brand camera (with a standard hot shoe).
But compatible has different levels of meaning. There are several different configurations of the smaller pins in the shoe
At best, it means Manual flash mode should work regardless of brands.
TTL mode of one brand surely will Not work on a different camera brand (due to hot shoe communication differences from the smaller shoe pins).
And even in Manual flash mode, the frills of the flash reading the communication pins of a different brand shoe probably never work (so it may not zoom, or show f/stop, etc). This could be true of old vs new of the same brand, if there were communication changes.
But yes, the flash should fire when triggered in Manual flash mode.
One other concern is some old flashes used a very high sync voltage (meaning like 200 volts DC or more, including both old hot shoe flashes and some old studio lights). That can damage some digital cameras not designed for it. Many DSLR can accept up to 250 volts DC, but some cannot, and they advised to NOT use flashes measuring more than 6 volts, which is more typical today (the ISO spec is 40 volts max).
You can measure this voltage yourself to be sure, measuring from the foots large center pin to the metal foot or shoe contact, with flash turned on and not connected (and NOT aimed into your face, and NOT face down on anything that could be scorched, because it could fire when being measured. Shorting that pin is how the flash is triggered.)
The first thing you need to do is define what you mean when you say "work."
The point is, there are way too many variable to make this a general question. The subtleties of each possible combination compared to what the user wants to be able to do make this a much more complex matrix than asking, "Will lens with mount X work on a camera with mountY?"
There are several independent brands of flashes that are compatible with various brands of cameras. When you purchase these flashes, you must specify that you want the one that is compatible with your brand of DSLR.
Each brand of DSLR has a hotshoe with one large circular contact in the center surrounded by multiple smaller round contacts. The large contact is the one that sends the signal to the flash telling it to fire. The other contacts are used for through the lens metering (TTL). The arrangement of these contacts are unique to each brand of camera. The exception to this rule is with cameras that do not utilize some form of TTL (e.g. A-TTL, E-TTL). These cameras will still have a hotshoe but it will only have one large single contact. Some of the Sony Alpha series of cameras are an example of cameras that only have the one large contact.
Every brand of DSLR will use the same pattern of contacts on their hotshoes. This makes it very easy to tell if a 3rd-party flash is compatible with it or not.
Just as long as the pattern of contacts are the same on the flash and hotshoe, the flash is compatible with the camera. However, this is not to say that the flash will be able to utilize all of the flash technology built into the camera.
You can use flashes with a single pin on cameras with multi-pin hotshoes. The only caveat here is that some flashes may use a different voltage which could damage a camera. This is not common among the major brands of flashes such as Metz and Sunpak.
Does it have a hotshoe contact? Then yes it's compatible.
The number of flash features which will be made unavailable or made manual increases with temporal or corporate distance from the manufacture of your camera.
P.S. be careful connecting very old flashes without hot shoe contacts to modern cameras. Some of those old flashes pass 110vAC through their sync cables!
Update: Many incompatibilities due to trigger voltage (I.E. older flashes with very high voltages) can be overcome with a "universal hotshoe" Such a unit often also includes a sync-port. Most cameras with 6v trigger include overvoltage protection and I've seen cameras with blown out overvolt circuits which sacrificed the hotshoe to save the rest of the camera. Probably better to just buy the $20 part and not take the chance.
Bear in mind that there are two points of potential incompatibility. First is mechanical incompatibility. This is easy. If it fits and everything lines up (contacts touch where they should and don't where the shouldn't) then you are good to go. The second is electrical compatibility. This is trickier since there is no visual check you can make (unless trigger voltage is printed on the flash) and it would be quite a task to make a list of every flash and which cameras it is compatible with. so find a manual, do your homework and, when in doubt, use an overvoltage protector.