From The Digital Picture
"One of the primary differences between the 430EX and the 580EX II is the amount of light each can provide. The 430EX has a Guide Number of 141' (43 m) while the 580EX II has a Guide Number of 190' (58 m) (both at 105mm). The 430EX is not under powered for many uses, but the 580EX II's significant power advantage can become important when using a flash modifier such as an umbrella, softbox or diffuser - or bouncing light off a high ceiling. In addition to being more powerful, the 580EX II allows 1/1 (full power) to 1/128 power in 1/3-stop increments while the 430EX allows power settings from 1/1 to 1/64 in more granular 1-stop increments.
The other primary difference is that the 580EX II can be used as a master to trigger remote Canon wireless slave flashes while the 430EX can only act as a slave flash. Any number of Canon 430EX
, 420EX
, 580EX II, 580EX
and 550EX
Speedlites can be combined as slave units but only the Canon 580EX II, 580EX and 550EX Speedlites can act as slave masters. In addition, the Canon Macro Ring Lite MR-14EX, Macro Twin Lite MT-24EX
, or Speedlite Transmitter ST-E2
can be utilized as wireless slave flash masters. Many people start out with a Canon Speedlite 430EX Flash (or started out with 420EX) and later move up to a 580EX or 580EX II. The 430EX can then be used as a remote flash.
Those using Canon bodies with 45 focus points (1-Series bodies) should take note that the Canon Speedlite 430EX Flash's AF assist only covers up to 9 focus points. You can still utilize the 430EX's focus assist, but need to know that all focus points are not covered as in the 580EX II.
The fact that the 580EX II can be powered from an external power source will be important to photographers expecting heavy flash use. The 430EX does not have this capability, nor does it have the PC terminal.
While the Canon Speedlite 430EX Flash head moves up/down from 0 to 90°, left from 0 to 180°, and right from 0 to 90°, the 580EX II allows an additional -7° down and a full 0-180° rotation right. While both flashes feature a pull-out/flip down wide angle diffuser to cover lenses out to 14mm (FOVCF equivalent), the 580EX II includes a white plastic pull-out catchlight reflector panel in addition. The Canon Speedlite 430EX Flash features 6 custom flash function settings, the 580EX II features 14. Instead of a dial for adjusting settings, the 430EX utilizes rounded parenthesis-shaped buttons. I can understand making buttons safe from accidental changes, but these buttons are narrow and require an excessively deep press to make changes. The dial is much easier to use.
If any of the 580EX II's advantages are important to you, buy the 580EX II. Otherwise, the 430EX is a very nice flash as well. It is about 15% lighter and more than 25% smaller in overall volume. Perhaps most important is that the Canon Speedlite 430EX Flash
is less expensive. "