Off Camera Flash for NikonD800
This pretty much said it all except with the caveat that you said "inside" a softbox. If you're actually putting it inside a softbox such as a Westcott Apollo -- as opposed to at the back end of a softbox (where your SB600 is exposed) you can't use CLS since there would be no consistent way for the sensor on the SB600 to "see" the trigger signal.
'Nice pen, bet you write good stories with it.'
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|Yellow Warbler by LeeS|
from A Big Year - birds
|Waiting for the Parade by tcoker1103|
from - La Vida Loca - (Black and White Street Photography+ A Border)