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I use 2 600 EX-RT units.  I was about to get a ST-E3-RT when I thought that maybe a 430EX III-RT would do the same and give me an extra RT flash.  Any thoughts??  They a close to the same price.  They will be used with a 5D MK III. 
Can the Canon Ring Flash be used as the master with the 600 EX-RT as a slave?
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