The Canon ST-E2 is Canon's dedicated wireless flash transmitter, designed to be mounted on a camera's hot-shoe, and can trigger one or more Canon flashes wirelessly in E-TTL mode.

When mounted on a camera's hot-shoe, the Canon 580EX and 580EX II flashes can also be used as a master device for triggering and controlling multiple remote Canon flashes in E-TTL mode.

However, there are a number of differences in functionality between using a 580EX / 580EX II as a master, and using the ST-E2 as a master device, with advantages and disadvantages for both options.

The tables below summarise the differences between using a Canon ST-E2 or a Canon 580EX / 580EX II as a master flash trigger, with differences highlighted using a different background colour.
Details were sourced from Canon's technical specifications and user manuals, as well as the results of my own testing with the ST-E2 and 580EX II.

	Canon 580EX / 580EX II	Canon ST-E2
wireless transmission coverage	+/- 40 degrees horizontal +/- 30 degrees vertical	+/- 40 degrees horizontal +/- 30 degrees vertical
wireless range	12-15 metres indoors 8-10 metres outdoors	12-15 metres indoors 8-10 metres outdoors
AF assist range	0.6-10 metres at the centre 0.6-5 metres along periphery	0.6-10 metres at the centre 0.6-5 metres along periphery
batteries	4 x AA	1 x 2CR5 lithium
battery life	approx 1500 transmissions (master unit firing disabled)	approx 1500 transmissions
size (W x H x D) (1)	580EX: 76 x 134 x 114 mm 580EX II: 76 x 137 x 117 mm	62 x 51 x 80 mm
weight (excluding batteries)	580EX: 375g 580EX II: 405g	100g
weight (including batteries) (2)	580EX: 482g 580EX II: 512g	140g
wireless transmission to remote flashes	visible light	near infrared light
cost	580EX: no longer available 580EX II: USD$445	USD$220

	Canon 580EX / 580EX II	Canon ST-E2
number of flash groups (3)	three (A, B and C)	two (A and B)
focus assist light	yes	yes
remotely trigger flashes set to ETTL mode	yes	yes
remotely trigger flashes set to manual mode	yes	yes
control output of remote flashes   set to ETTL mode	yes	yes
control output of remote flashes set to manual mode	yes	no, needs to be set manually on each flash
flash ratio control for A:B ratio	8:1 to 1:8 in half-step increments	8:1 to 1:8 in half-step increments
high-speed sync	yes	yes
second-curtain sync	no	no
stroboscopic flash	yes	no
flash exposure compensation	yes, configurable via 580EX controls or via camera	yes, configurable via camera
FE lock	yes	yes
flash exposure bracketing	yes	no
wireless test (4)	yes, via pilot button	yes, via pilot button
modeling flash	yes, via camera's depth-of-field preview button	yes, via camera's depth-of-field preview button

Which Device Wins?

In my view, there's not necessarily a clear winner. The most suitable solution (ie, ST-E2 or 580EX) depends on the type of photography that you do.

If you want to be able to remotely trigger one or more remote flashes in ETTL mode, and don't need a flash on the camera itself, then the ST-E2 is a better solution, as it is cheaper, and significantly smaller and lighter than a 580EX flash.

different sizes: Canon ST-E2 Wireless Flash Transmitter and Canon 580EX II Flash
(both mounted on Canon flash table stands)

However, the ST-E2 has a few limitations. When in ratio mode, the ST-E2 can only trigger flashes in groups A and B, and will not trigger group C. In comparison, the 580EX in ratio mode can trigger group C with a configurable flash exposure compensation.

If you want the flexibility of also having an on-camera flash in some situations, then a 580EX on your hot-shoe is a more appropriate alternative.

I Use Both

I personally use both a 580EX II as a master trigger, and a Canon ST-E2, at different times, depending on the specific requirements for lighting at the time.
If I want to trigger one or more flashes off-camera (with ETTL), I use the ST-E2 because it's much smaller and lighter. However, if I want an on-camera flash, and want to trigger one or more off-camera flashes (with ETTL), then I use a 580EX on my camera as a master device.

Communicating Via Light

Both the 580EX and ST-E2 use light for communicating with slave flashes, and hence need line-of-sight or reflective surfaces to bounce the signals around. Despite this, I have found both the 580EX and ST-E2 to work suprisingly well even when there was no direct line-of-sight between the master and slave devices.
For more flexibility, a Canon TTL flash cable (such as the Canon OC-E3) can be used to connect the master device (580EX / 580EX II or ST-E2) to your camera's hot-shoe. This allows you to move the master device away from your camera, as well as allowing you to point it in a different direction.

No Second-Curtain Sync

Note that neither the ST-E2 nor a 580EX / 580EX II in master mode will support second-curtain sync triggering of flashes, which is an annoying omission.



What about you?
Do you use a 580EX / 580EX II flash in master mode to trigger remote flashes?
Or do you use a Canon ST-E2?
Which one do you prefer? Why?