How to calculate required flash power?

[omaha]

I'm trying to figure out some stuff for a shot I have planned. I should go out and do some testing, but thought I'd ask here first.

Its going to be a portrait taken in full daylight. Problem is, I want to use a very large aperture (like 3.8 or so).

So lets say I meter the sky and get something like f/22 at 1/125s. If I use a five stop ND filter, I'll get that down to f/4.

I'll be using two strobes on the subject. The way I calculate it, I'll need the main strobe to meter at f/22 to overcome the ND filter.

Is there a straightforward way to work out how much power that translates into? My strongest strobe is 450Ws. Sound like enough?

 

[MattKing]

Depends on what reflectors/light modifiers you are using.

Do you have a flash meter? If so, you can practice ahead of time to get it right for f/22, measure the flash to subject distance, and then set up in the field using the same distance.

Is the portrait backlit with the ambient light? If so, a large reflector would work as well or better than a flash.

 

[omaha]

Yep. Have a flash meter.

Main light will have a 21" beauty dish (which from experience seems pretty "efficient"). Secondary will be a reflective umbrella.

I like the idea of the reflector. My only concern is throwing so much light on the subject's face that they squint. The scene we have selected will have the camera facing pretty much exactly Northeast, so no matter what, the sun won't be directly behind the subject. I'd like to take this in the early morning for the light, but we usually don't get good clouds around here until the evening, so odds are that's when we'll shoot.

The more I think about this, I think I'm thinking too much about this. What I need to do is get out and shoot the damned shot.

Thanks for your thoughts!

 

[Chris G]

I agree with Matt, a reflector is a great way to go! I know that doesn't help answer the question, but I like the idea of simplifying, using minimal gear.

 

[trythis]

Use flash meter, and bracket!

 

[paul_c5x4]

My only concern is throwing so much light on the subject's face that they squint.

One little trick I use - Instruct the subject to screw their eyes up, and when you go "Boo", snap the eyes open.

Depending on how long the eyes remain shut, the pupils may dilate, and if you get the timing right, you end up with big bright eyes.

 

[Drew B.]

Yep. Have a flash meter.

Main light will have a 21" beauty dish (which from experience seems pretty "efficient"). Secondary will be a reflective umbrella.

I like the idea of the reflector. My only concern is throwing so much light on the subject's face that they squint. The scene we have selected will have the camera facing pretty much exactly Northeast, so no matter what, the sun won't be directly behind the subject. I'd like to take this in the early morning for the light, but we usually don't get good clouds around here until the evening, so odds are that's when we'll shoot.

The more I think about this, I think I'm thinking too much about this. What I need to do is get out and shoot the damned shot.

Thanks for your thoughts!

Can't you dial down the flash...find what works.

 

[Muihlinn]

As rule of thumb you can get a measure to have a correct exposure, then set the flashes to 2 stops less for fill-up light. From this basic setup you can work up and down.

 

[John Koehrer]

Power measured in Watt Seconds doesn't directly translate to f stop. It's only how much is available at the output when it's discharged.
Design of the reflector/flash tube can change between different brands of flash, so only testing will give accurate results for you.

It MAY be more straightforward to measure output of the flash at a given distance & calculate using the inverse square law.
A flash meter would simplify your life though.

 

[benjiboy]

Many modern digital flash meters will tell you how much as a percentage of the exposure reading is ambient light and how much is flash and after taking the reading you can scroll the shutter speed up and down or move the flash backwards or forwards on the lighting stand until you get the required balance, around 30% flash to ambient exposure works for me.