This note is a summary of all the incompatibilities I am aware of between DSLRS, hot-shoe flash units, and various optical and radio devices used to trigger these flashes off-camera.
I have came across a number of such incompatibilities myself, and I also see users report them on various photography boards, so I thought it handy to collect them all in one place.
Note that I only list incompatibilities for units that one would expect to be compatible. If you are looking for re-assurance that a Canon ST-E2 Wireless Transmitter is not compatible with a Nikon SB-910 Speedlight, this is not the place to look.
Most Canon EX-series Speedlites do not work well with most battery-free slave triggers from Fotodiox, Hama, Kaiser, Seagull and Wein. The flash will fire, but only once. The flash need to be power-cycled before it will fire again. The reason for is that the complex electronic circuits in these newer flashes prevents the Silicon Controlled Rectifier (SCR) used in these slave receivers from resetting. The Speedlite 580EX II, however, will work with such slave triggers if you connect it to the slave trigger through the built-in pc-sync port (it is just as uncooperative with these optical slaves as any EX-series flash if you connect through the hot-shoe). When buying an optical slave to use with a Canon Speedlite, you should check that is compatible first.
FlashZebra supply some optical slave triggers they claim to work with most Canon Speedlites. Another battery-free slave trigger that is marketed as compatible with Canon EX-series Speedlites is the Seagull SYK-5.
Some newer Canon Speedlites, in particular 430EX, 580EX and 580EX II, fire at random with older versions of the Gadget Infinity/Cactus radio triggers that use the 344 MHz and 433 MHz bands, and have significantly reduced reliability with some other radio triggers, including PocketWizards. This is because these Speedlite models emits strong RF noise across a broad spectrum of frequencies that interferes with the radio triggers. To resolve this, you can fit an external barrier that shields the RF noise, or modify the circuits inside the flash to filter the noise. See this note for details. Some Speedlites, for example the 550EX, 430EX II, 420EX, and 270EX, do not have this problem.
In addition to this, there are a number of users that report that after using their Canon Speedlite 580EX II with PocketWizard triggers for some time, it no longer outputs controlled amounts of light (i.e. it always fire at full power). Analysis of the failure conducted by LPA Design (the designers of PocketWizard) indicates that the Canon Speedlite 580EX II has a design flaw where intense use of the HSS mode may damage the IGBT transistor inside the flash, causing the failure. Their report is here.
The Canon DSLRs EOS 10D and EOS 300D (aka. Digital Rebel / Kiss Digital) does not work with the Cactus V5 transceiver. Connecting the transceiver to one of these Canon bodies produces ERR99. According to the always reliable Chuck Westfall (in an interview with TheDigitalJournalist), this error code indicates a wide range of malfunctions, including problems “caused by the use of non-Canon accessories such as lenses, memory cards, battery packs, electronic flash units, etc.”
The Speedlight Nikon SB-400 will not trigger when placed in a plain (two-contact) hot shoe as used by plain optical or radio triggers. It can only be triggered when connected to the hot-shoe directly, or with a dedicated off-camera shoe cord such as the Nikon SC-28.
Newer Nikon Speedlights such as SB-600 and SB-900 are not compatible with the “wake-up”-signal from the Nikon version of the YongNuo RF-602. To use these Speedlights with the YongNuo RF-602, you must turn off power-save mode. The Nikon SB-800 and all the Nikon legacy (pre i-TTL) Speedlights are fully compatible, including responding to the “wake-up”-signal.
Nikon's legacy SU-4 optical trigger draws it power from the flash, but apparently not from the centre pin, as it does not power up when it is connected to a generic third party flash. It also fails to power up when connected to some third party units with a Nikon compatible hot-foot.
The Samsung NX MILCs are reported to behave erratically (or not work at all) with non-Samsung hot-shoe flashes and radio triggers. Most reports are about the NX10 model, but, depending upon firmware version, there also seems to be problems with the other models (e.g. NX5, NX11, NX100, NX200) in the series. According to this thread in DPreview's Samsung Talk forum Samsung removes support for using third party units in the hot-shoe through firmware “upgrades”.
The Nissin Di-466, Nissin Di-622 (Canon version, and Nikon version produced before February 2010), Sunpak PF30X, Sunpak PZ42x, and Sigma EF-500/530/610 DG ST (Canon version) will not trigger unless most of their pins receive sensible TTL signals. This means that these units will not trigger when placed in a plain (two-contact) hot shoe as used by plain optical and radio triggers, and pc-to-hot shoe adaptors, and most of them will also fail with non-TTL triggers that have multi-contact hot-shoes (such as the YongNuo RF-603). These dedicated flashes need to have sensible signals on the additional (system-specific) pins found on dedicated flash units to trigger.