Help me understand using a light meter to balance ambient and fill flash.

kc4sox · Sep 17, 2014

First off my meter (new to me as of today) is a Sekonic L-478D. I get the whole "Two exposure" thing. And, I know how to meter for the ambient exposure. I want to be able to mix fill flash in with ambient at around 30% flash to ambient depending on the situation. And, I'm just a bit confused as to what is the proper methodology to obtain that.

To my understanding:

1. Set the desired ISO then meter for the ambient exposure using either T or A priority mode to establish the baseline. And to light the background to suit the look your going for.

2. Meter for the fill flash ( And this is where I am cloudy ) using the exact same settings ( T , A and ISO )? And then adjust power and / or distance to obtain the % of light being added by the flash to what I am looking for ? I also realize that A can be used to adjust the amount of flash.

Perhaps a stupid question but, it's been YEARS since I've used a meter and even then it was only to determine settings for studio lighting and not OCF in an outdoor environment. Man, if ever someone needed a mentor it's me

Thanks for helping a slow learner out !

Mike

tirediron · Sep 17, 2014

Nice meter - I've been looking at those and drooling, but I really can't justify it when my old Minolta Flash V still works so well. First of all, I would avoid the use of aperture or shutter priority since the camera can't account for the flash. I would use straight manual; I generally go with the lowest possible ISO (increase as necessary), 1/250 (max sync) and f5.6 - 8 (again, adjusting as necessary for optimum exposure and depth of field). Once you've metered for and set your ambient exposure, add flash as required. If you want a specific percentage of flash, your meter should have a setting which will read flash output as a percentage of ambient, however I prefer to adjust to get the look I want rather than a specific percentage.

Alexr25 · Sep 18, 2014

I agree 100% with tirediron, when you are trying to set fill-flash ratios its a lot easier to work in manual mode.

The thing to remember is that altering the shutter speed will have no effect on the flash exposure and will only effect your ambient exposure, whereas altering aperture and/or ISO will effect the overall exposure, both flash and ambient.

My way of setting fill exposure is meter the ambient and set your ISO, aperture and shutter for the effect you want, then meter the flash and adjust flash power or distance to give the desired lighting ratio. Finally meter the combined ambient and flash exposure and if necessary tweak the aperture for the correct exposure. With fill levels in the order of 30% this last step is optional as the flash is not going to make any significant difference to the overall exposure.

Village Idiot · Sep 18, 2014

You can't use aperture priority unless you're using a TTL or highspeed/hypersync mode of some sort. This is because the varying shutter speed could go over your camera's x-sync which will either cause the flash exposure to cover only a portion of the frame or to just not be a part of your final exposure at all.

pgriz · Sep 18, 2014

My approach: First meter your ambient. Say it's 1/250 at f/8 at ISO100.
Since aperture's going to be the common item between ambient exposure and the flash exposure, decide which aperture you want to have. Say, it's f/5.6. (Ambient is therefore 1/500 at f/5.6 at ISO100)
Next, decide if the ambient is equal in brightness to your subject, or darker. Let's say you want the background to be 1 stop darker. So now your exposure for ambient would be 1/1000 at f/5.6 at ISO 100.
Next, adjust your flash power (or distance to subject) until it gives you proper exposure at f/5.6.
Your camera has a sync speed maximum (anywhere from 1/60 to 1/250 usually). So if you have the high-speed flash implementation for your equipment, set it. Then your exposure will be 1/1000 at f/5.6 at ISO 100, with your main subject properly exposed, and your background 1 stop darker.

If you don't have equipment with high-speed flash capability, then you will need to adjust the aperture so that the sync speed (say it's 1/250) gives you the desired exposure. In this example, this would be 1/250 at f/11 at ISO 100. Now you need to increase the flash power (or adjust the distance) to give you four times more power than before, because your aperture is letting in four times less light than at f/5.6.

Now if you really, really wanted to use f/5.6 (assuming no high-speed flash capability of your equipment), then you'd need a 2-stop neutral density filter to drop the ambient exposure down to the sync speed (so, 1/250 at f/5.6 at ISO 100), but you'd still need the flash power at f/11 equivalent.

The above assumes that your main subject is lit only by the flash. If the main subject is ALSO lit by the ambient, and since light is additive, you'll have overexposure of the main subject by about 1/2 stop. So in this situation, you'll need to reduce your flash power a bit to get the right exposure.

Also, when working out these things, doing it in manual (both camera and flash) puts the variables under your control. You need to control BOTH the shutter speed (due to sync speed limitations) AND the aperture (since flash-power and DOF are dependent on that).

Rick50 · Sep 18, 2014

I agree with pgriz here but there is one more option. Set the camera in Manual mode as mentioned and the flash in ETTL. Then
adjust flash output power using flash exposure compensation.

Mike_E · Sep 18, 2014

All roads lead to Rome it's just which one to take.

If you are shooting eTTL then I'd do it Rick's way.

If you want to have the flash off camera (better) then measure ambient and manually set your camera. Having done that adjust your flash to 1/3 stop above ambient and then reset your aperture 1/3 stop up and leave the shutter speed alone.

For more info on this look up 'dragging the shutter'. If you haven't heard of this then it will be well worth your time to look into it.

WayneF · Sep 25, 2014

kc4sox said:
First off my meter (new to me as of today) is a Sekonic L-478D. I get the whole "Two exposure" thing. And, I know how to meter for the ambient exposure. I want to be able to mix fill flash in with ambient at around 30% flash to ambient depending on the situation. And, I'm just a bit confused as to what is the proper methodology to obtain that.

To my understanding:

1. Set the desired ISO then meter for the ambient exposure using either T or A priority mode to establish the baseline. And to light the background to suit the look your going for.

2. Meter for the fill flash ( And this is where I am cloudy ) using the exact same settings ( T , A and ISO )? And then adjust power and / or distance to obtain the % of light being added by the flash to what I am looking for ? I also realize that A can be used to adjust the amount of flash.

Perhaps a stupid question but, it's been YEARS since I've used a meter and even then it was only to determine settings for studio lighting and not OCF in an outdoor environment. Man, if ever someone needed a mentor it's me

Thanks for helping a slow learner out !

Mike

By definition, you are using manual flash mode if you are attempting to meter it.

You must use fixed ISO and fixed aperture A mode with manual flash. Manual flash cannot react to the camera changing aperture or ISO later, after metering.

FWIW, if using TTL flash, and metering with the camera, just setting -1 1/3 EV flash compensation is virtually same as 30% flash (as close as you can get).