Flash/incident light meter questions - off camera flash triggering, power settings?

[daggah]

OK, so I watched a video by Zack Arias talking about the relationship between aperture and flash power, and it makes plenty of sense. However, I have some questions. Currently, any of my off camera flash work is handled completely through manual flashes triggered with a CowboyStudio NPT-04 radio transmitter. If I pick up a Sekonic light meter (the L-308s is the one I was looking at on Amazon), I can just plug in a NPT-04 transmitter via PC sync to trigger flashes with it, right?

And then it basically becomes a spot-meter, I'm assuming? So, I'd just set up my lights, dial my desired exposure settings in my camera for depth-of-field and ambient exposure, then place the spot-meter by where I want to meter for and use the results to adjust flash power, right? Hypothetical setup:

ISO 100
F/2.8 for desired depth-of-field
Shutter speed set for whatever for desired ambient/background exposure
One light set at 1/16 power

Say the light meter reads F/8...but since I don't wan't to increase DoF, that means I'd in turn adjust my flash power by 3 stops, right? I.e., instead of 1/16, I'd go to 1/128? Am I understanding it correctly?

 

[KmH]

If the trigger transmitter has a fire button on it you don't have to plug it into the L-308. Just fire the flash unit with the trigger.
The L-308s would have to be in flash mode and held where you want to measure the light.

IIRC the L-308 doesn't have a feature that can fire flash units.

Yes, f/2.8 @ 1/128 flash power = F/8 @1/16 flash power.

 

[pgriz]

IF the ambient is sufficiently low that your shutter speed for your settings falls at or below the sync speed, the process you described would work. However, if your ambient is putting your shutter speed above the sync speed, you'll get banding (due to the shutter being only partly open during the flash exposure).


In the first scenario, you bring your flash-meter to the subject you are photographing, and set off the flash (so it is NOT in spot-meter mode, but incident light mode, with the dome pointed at the source - the flash). As you described, assuming your hypothetical flash gives you f/8, you need to dial down your flash power by 3 stops, which you correctly calculated as going from 1/16 to 1/128.

In the second scenario, let's say the ambient light level is around that of a heavy overcast (EV12). Ambient exposure would be ISO 100, f/2.8, 1/500sec. Let's also assume your sync speed is 1/125 sec. If you are using manual flash triggering (ie, no E-TTL functionality), then any flash setting would still create banding, because when the flash goes off, the shutter will be only partly open. So you will need to either increase your f/stop to f/5.6 to reduce the shutter speed to 1/125 sec., or you need to add a 2-stop ND filter to bring the ambient exposure down to ISO 100, f/2.8, 1/125 sec. If closing the aperture is out of the question due to DOF considerations, you use the 2-stop ND filter, and adjust your flash power as in the first example, but reducing the ajustment by 2 stops due to the ND filter. So assuming your flash incident reading is still f/8, you will need to reduce the power only 1 stop.

PS: Keith types faster than I do!

 

[daggah]

In the second situation, I would use my ND3 filter and factor that into the calculation.

Also, I'm assuming that I'd base any other lights (in addition to the key light) calculated based off of the ratio of light I want compared to the key, right? I.e., if I was going for a more moody look, then maybe I'd figure out my key light and then set the fill to half of whatever the key is (or adjust distance, etc., etc.)?

 

[pgriz]

Yes, but remember the flashes are additive. If the second light is a full stop or more below the main light, you don't to have to make significant adjustments to the primary exposure, but if they are equal power, then the two together give you an additional stop of light, which you do have to adjust for.

 

[daggah]

Yes, but remember the flashes are additive. If the second light is a full stop or more below the main light, you don't to have to make significant adjustments to the primary exposure, but if they are equal power, then the two together give you an additional stop of light, which you do have to adjust for.

That would also depend a lot on the direction of the fill as well, right?

 

[pgriz]

Yes. However, if the two lights overlap in their coverage, then the additive effect happens. If you are, for example, having a main on the right at 45 degrees camera right, and a fill on the left also at 45 degrees, there will be some areas that overlap. Those are the areas where you need to be sure you're not overexposing.

 

[daggah]

Thanks for the info and feedback!