soft box and flash power?

[Adrian Fischer]

Greetings All,

Im looking to get a couple 150cm octaganol softboxes (maybe bigger). The umbrella style ones that pop up and mount to a bowens connector. I do a bit of on site shooting and my current rectangular ones which are put together ones just take too long to put up. (its all relative....). I do portrait work and use 300 what studio flashes. My question is; is there a correlation between softbox / umbrella size and the power of your flash? I would think at some point there would be but Im not trying to flash the inside of a barn. Can anyone point me to a reference on this or give me some personal experiences?

Thanks

[maccaroneski]

Adrian, I would have thought that a review (or even the manufacturer/retailer) of the particular piece of kit would be able to tell you how many stops of light a particular modifier will eat, but yes there is I would imagine a correlation between size and power output, but it would probably be even more specific to the particular item (factors such as double baffling will increase the amount of light eaten etc).

[I @ M]

I think the only way you are going to be able to reach any meaningful conclusion is to try the new softbox.
I feel that there are too many variables between them, the reflectivity of the interiors of each, the thickness of the diffusion material at the front end of the different softboxes and the area of each.

I haven't seen any formula or rules about size / power ratios but that doesn't mean to say they don't exist.