A higher model number means a higher guide number (i.e. more light output). At maximum power, the 580EX puts out about one more stop of light than the 430EX. The 580EX has a few more electronic bells and whistles, like the ability to fire other flash units via an infrared remote signal. If you don’t need the extra power or the extra bells and whistles, the 430EX will be just fine.

You can also buy discontinued EX models like the 550EX, the predecessor to the 580EX, or the 420EX. Look for used flash units at KEH.com and other used photo equipment dealers on my LINKS page.

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