Why can't I use a Canon speedlight off camera as a slave to a Nikon's optical trigger?

How does the speedlight know that Canon is not sending the trigger flash?

Is there some kind of radio signal from my Canon camera telling my Canon speedlight to go off? Then why can't it work without the flash? Because there's no radio trigger, you have to pay extra for that, and it only works for the 600ex rt.

So how does it know?

And how the heck does it read the TTL information?

side note: Girlfriend uses Nikon (has commander mode). Yongnuo speedlight works as a slave for both our cameras (doesn't have TTL).

I don't know STOP ASKING WTF

You can't expect to sync Nikon Flash (in the Commander Mode) and expect it to communicate with a Canon product.

You can set your 600EX as a slave and let the Nikon flash trigger it, but you will have to manually set the power output of your Canon flash.

Specific off camera flash signals from Nikons CLS system (and Canons proprietary system) use IR signals via the trigger flash to do more than just trigger - they set things like the power output the flash is to fire at. These signals are unique to that particular brand.

Certain third party manufacturers have since decoded these transmissions to make cheaper versions.

But, if you are using non TTL compatible units, then you HAVE to use them in purely manual mode, and use optical triggering only, with the trigger flash in manual, not TTL or 'commander' mode.

Haha.

The Speedlite EX RT uses a radio signal to transmit the TTL data and fire remote flashes. RT = Radio Transmission
All other Canon speedlites use infra red transmission either from a dedicated transmitter (ST-E2), from an EX 5… Series gun or from the onboard flash of a remote equipped camera (eg 700D, 70D).

You can't use optical triggers because Canons work by sending out a tiny pre-flash to determine exposure. That pre-flash is sufficient to trigger any optical slave units that then don't have time to recharge before the main flash fires milliseconds later. That causes gross under-exposure. If you reduce the sensitivity of the optical slave trigger to ignore the pre-flash then the output of your slave guns is not taken into account by the TTL pre-flash calculation so when they fire on the main flash you get gross over-exposure.

The only way I have found around that is to use the flash exposure lock. Press the FEL button. All optical flashes will fire. Before taking the shot allow time fo ryour slave guns to recharge, but don't wait so long that the FEL times out!

Or - just buy Canon Speedlites! 550EX and 420EX are available used very cheap. I have both and they are very good.