The S1 & S2 optical slave feature built into many speedlites can be really useful and, at times, can sometimes be a life saver if your radio triggers let you down. S1 and S2 optical slave modes are effectively a third type of wireless communication based on an optical flash of light. We’ve already discussed Canon’s optical EX pulse wireless communication and also radio wireless communication but S1 and S2 wireless is much simpler and easier to use.
Speedlites that have S1 & S2 optical slave modes have an integrated light sensor which triggers the flash when it detects another flash from a nearby speedlite or even a camera’s pop-up flash if it has one. Put the speedlite in S1 mode and every time it sees a flash go off nearby it’ll fire a flash itself. Obviously it needs line-of-sight to work. If the flash is blocked by a wall or the slave flash is completely hidden within a soft box then the light is blocked and it won’t fire. Similarly if you are outside on a sunny day this system isn’t very reliable as the flash from the source speedlite / pop-up flash can be drowned out by the bright sunshine.
S1 mode fires as soon as it detects an external flash firing. This works well when there is just a single external flash i.e the external flash (or source flash) is set to manual power mode and there’s just one quick pulse of light. However when the source flash is set to E-TTL II the source flash sends out a weak pre-flash first to allow the camera to calculate the required flash exposure and then it fires out a second flash. The second flash is the “real” flash i.e the one used to illuminate the subject. If we want the slave flash to fire in sync with this second “real” flash then we need to tell the slave flash to ignore the pre-flash and fire only when it sees the second flash. S2 mode does exactly this. S2 mode just fires when it sees the second flash.
S1 optical slave mode – fires on the first flash it sees. Used when the source flash is in manual power setting.
S2 optical slave mode – fires on the second flash it sees and ignores the pre-flash. Used when the source flash is in E-TTL II mode.