Need to figure flash outputs for Speedlight to Strobe Light ratio.

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.

 

Or just meter it. I use one of these.

 

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

 

Yep, as adamhiram said use a meter. There is no reliable way to determine exposure when modifiers are used from guide numbers.

I don’t know why since the advent of digital people don’t use hand held meters. Chimping is only viewing the jpeg not the Raw data. If you take care of them they last for years and years, I replaced my Minolta Flashmeter III about 5 years ago, it lasted over 33 years. Replaced it with a Sekonic L-478DR-PX.

 

Obviously, there are three people who are not reading the OP....if the OP HAD a flash meter, he could easily do the comparison. I mean, how obvious is it that the OP lacks a flash meter?
The OP asked about "power output"--not the actual, final f/stop.

Here you go OP...spend $219 and your problems are solved.

Sekonic L-308X-U Flashmate Light Meter 401-305 B&H Photo Video

Figure 1 stop lost to the softbox, in any case. Pretty close to the same light efficiency lost on a softbox and a strip box. Once you have the power output determined, it's a simple matter to shoot the shot and see if the main-to-fill ratio is how you want it to be. if one is too powerful, reduce its power output level, or move it farther from the subject. If one light is too dim, raise the power output on that light, or move it closer.

You can use the histogram to determine how the exposure is.

The power output of a flash, and the exposure level needed, are two different things.

 

The OP has a flash meter.

"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."​

His problem is that he does not know how to use it to determine the light ratios.

@CDA, as with most anything new, you need to be methodical, patient and practice.

First, determine what lighting ratio you want. Then determine what the f-stop difference is for that ratio. That is your target.

• 8:1 is 3 stops difference.

Set up an empty set, with the lights (in the modifiers) 10 feet from the subject (a dummy subject for this exercise).

• Note1: You MUST make the measurements with the light modifier on the flash. The modifier you use will affect the light from the flash and change the light level on the subject. So if you change to a different light modifier, you have to do this exercise all over again, for that modifier.

• Note2: The 10 feet distance is just a convenient starting point, that happens to be part of the guide number formula. So just use whatever distance works for you.

First we do this for each light separately:

• Measure the KEY light only.
  • Adjust the power level or distance, so that your meter reads f/11
  • You may have to raise or lower the ISO level on the meter so that you can get the meter to read at f/11.
    • You have to set the ISO on the camera to the same level as you have the meter set to.
  • Note: f/11 is not carved in stone, it can be higher or lower, but it NEEDS to be high enough to allow measuring 3 stops lower for the fill light.

• Measure the FILL light only.

The difference between the lights is your exposure ratio, in stops.

Then do as Derrel said, adjust the power level and/or distance from flash to subject to get the ratio to be what you want it to be.
To make it simple, just adjust the power level or distance of the FILL light, so the meter reads f/4 (3 stop down from f/11).

If you used a different f-stop target for the KEY light (ie f/5.6), your FILL target should be 3 stops lower (f/2). This is why you have to have the key light high enough, to be able to measure 3 stops lower for the fill light.​

NEXT. Use BOTH lights together.
Depending on the subject and light angles, the two lights could/will spill over and mix, so you would get less than the 3 stops as determined in the individual light test above.

• Aim the meter at first the key light then the fill light, determine what the 2-light mixed ratio is.
• Then adjust the power level or distance of the FILL light to get back to your 3 stop difference.

Note: After you adjust the power/distance, you will have to measuring each flash again, to determine the ratio. This is because of the spill and mixing.

Now draw a diagram of the lighting setup, record what light modifier is used on each flash, the power level of each flash, the distances of each flash to the subject, the angle of the flashes from the lens axis, and the height of each light above the floor.
If you do the diagram correctly, you can recreate the setup starting from a bare room, and get your 8:1 lighting ratio.

Once you get comfortable with this then you can progress further, such as adjusting/moving both flashes, example if you need a higher or lower level of key light on the subject, or different light ratios.

• 1st adjust the power level of the key flash to get the f-stop exposure you want.
  
• Then adjust the power/distance of the fill flash to get the fill level you want. In your case 3 stops lower than the key light.
• Remember that there is interaction between the key and fill flashes due to spill, so you have to meter and adjust both flashes.

gud luk

 

The Sekonic 308-X only has a non-moveable incident dome so when reading ratios make sure to either disable all other lights or shield the dome so no other light influences the reading.

 

Are you sure of this??
Every Sekonic 308 meter I've seen had a sliding dome for either incidence or reflective readings and the dome could be replaced with an optional "Lumidisk " which is designed specifically for measuring lighting ratios.

 

Yes, the L308 dome slides to the side, to the position used for reflected readings. But yes, for setting ratio, each light should be metered individually, with only that one on. Then when ratio is set, turn both main and fill on to meter for camera exposure.

Here is a description of how to meter the portrait lights:
Camera Speedlights vs Studio Lights

 

You can meter, or just take a picture at each power level and compare:

Comparing the power and recycle times of Flashpoint flashes

 

Sorry what I meant to say was the meter does not by default have the ability to do ratios with a flat lumisphere, like my L-498DR, one must buy it as an option which IMO is ridiculous. Every lighting scenario involves ratios, knowing them with a turn of a collar is invaluable.