Im not sure if the title of this post described what it is I’m trying to do, if it doesn’t I apologize. I currently use a Godox AD600BM as my key light and a cheap Speedlight as a fill or hair light attached to an S-Type Bowen’s Mount so that I can use it with my strip softbox with grid. How can I figure the power output of my Speedlight compared to my Strobe? I’d like to be able to easily set my power levels to 8:1 (Key:Fill) or another similar ratio but since they do not have the same 1/1 output I am getting lost in the conversion. I recently bought a Sekonic 308-X and am aware of how to use it and have been practicing with it, but whenever it comes time to use it I start to get confused.
Guide Number ought to give you a reasonably accurate basis for comparison, as long as you keep the speedlight angle of coverage (beam spread) in mind in comparison to the studio flash's angle of coverage. See this page and the associated site for some great explanatory articles. Compare Power Rating of Camera Flashes with Guide Numbers The Godox is listed as having an ISO 100 Guide Number of 285, in Feet (87 in meters). So, at 10 feet distant, the Godox at Full Power ought to deliver f/28.5 with an ISO 100 digital or film speed in use.
Right, just meter it. Obvious best way. Guide Number isn't going to work with a softbox anyway. See this about metering for lighting ratio: Camera Speedlights vs Studio Lights
