a master/slave question

excuse me for my ignorance, as im new to this flash world
i got three questions

when u set a flash unit to slave, is it triggered by any light of any other flash or is it triggered by master flashes only ?

IF a slave flash can be triggered by any light of any other flash, y would i have to buy an expensive master flash ? cant i just buy 2 regular flashes, trigger one of them, n it will trigger the second ?!

when u set a flash unit to slave, is it triggered by master units of the same brand or any master unit whatever its kind ?!

TTBOMK, all flashes set to 'slave' mode will trigger off of any sudden increase in ambient light. Thus, if you have units set to 'slave' and you are with other photographers utilizing flash, your slave will very likely trigger from their units. There may be some 'smart slave' flashes available, but I'm not aware of any such unit.

If you are likely to be in a situation where there are other photographers utilizing flash, you would be better off utilizing radio-controlled flash. Even then, there is the possibility of interfering signals - but, not like the normal slave scenario.

You can even trigger a 'slave' unit from your built-in pop-up flash.

GJim wrote in post #12089180
TTBOMK, all flashes set to 'slave' mode will trigger off of any sudden increase in ambient light. Thus, if you have units set to 'slave' and you are with other photographers utilizing flash, your slave will very likely trigger from their units. There may be some 'smart slave' flashes available, but I'm not aware of any such unit.

If you are likely to be in a situation where there are other photographers utilizing flash, you would be better off utilizing radio-controlled flash. Even then, there is the possibility of interfering signals - but, not like the normal slave scenario.

You can even trigger a 'slave' unit from your built-in pop-up flash.

Everything you said applies to a simple optical slave flash only. Canon slave flashes don't work this way.

Canon slave flashes will not be triggered by a simple flash of light. They will only respond to a coded signal sent by a Canon master flash. IF you had four different photographers in the same room, you have the option of selecting 1 of 4 channels so that there is no mis-fires. The big advantage of this kind of system is that you retain ETTL.

Same goes for Nikon master/slave systems, EXCEPT that some Nikon flashes as have a option to use a built in simple optical slave flash mode. Same goes for the Sigma "Super" series of flashes.

Mike:

Thanks for the info. So, there must be some sort of 'pre-flash' light signal sent to the slave units by the master?

GJim wrote in post #12089478
Mike:

Thanks for the info. So, there must be some sort of 'pre-flash' light signal sent to the slave units by the master?

Correct. Unless you are using a Canon ST-E2 to trigger the slave (infrared).