In a multiple strobe setup, how powerful must each strobe be?

Question

I am about to buy my first strobe set. I want to do portrait photography, both in a home studio and outdoors. No weddings, no events, no more than one model at a time.

I see that a complete set should include a key, a fill, a hair and a background light. I learned that fill is usually 1-2 stops less powerful than key. To my great surprise, hair light should be 0.5-2 stops more powerful, depending the hair color. Finally, for background it seems a speedlight is sufficient.

Given that the directionality of each light (the light modifier attached) and the distance to the subject may differ, these ratios do not give me a hint on how powerful strobes I should buy.

Could you please comment on the correctness of what I said and propose a setup (no brands, just the maximum power) that could cover my needs? I will need to carry them on location, therefore weight -- including that of the necessary stands -- will matter. Therefore, if some of the strobes need not be that powerful, I could save weight and money by choosing a weaker unit or even a speedlight.

Answer 1

I see that a complete set should include a key, a fill, a hair and a background light.

What a complete set includes is very subjective and depends a lot on the desired look. Given that this is your first set, I would shy away from buying that many units. Start with one or two and add them as necessary.

You didn't buy the "complete" set of lenses for your camera either, did you?

I learned that fill is usually 1-2 stops less powerful than key. To my great surprise, hair light should be 0.5-2 stops more powerful, depending the hair color. Finally, for background it seems a speedlight is sufficient.

You state this as if it's the one definite way to setup the strobes to take portraits. That's like saying portraits are photographed with an aperture of f/2.0, which is simply not true in general.

I doubt that your goal is to take the same picture over and over again. There are all kinds of different light setups that include more or fewer strobes, reflectors, flags, etc.

these ratios do not give me a hint on how powerful strobes I should buy

It looks like you are looking for some way to figure out the exact number that you should shop for for that particular light setup that you have in mind. But you should always have some more power that's eaten by the modifiers and the distance. Not shooting at full power usually means faster recycle times, which helps when working with people.

My guess is that any middle of the road strobe kit will work in terms of power.

I want to do portrait photography, both in a home studio and outdoors.

A problem related to power that you can run into is that a strobe is too powerful. Certain looks require the light (modifier) to be extremely close to the subject.If a wide open aperture should be used it can happen that the lowest power setting of a strobe requires a shorter shutter speed than the camera can handle. There's simply too much light and no way to reduce it by means of the camera settings alone and using an ND filter becomes necessary. This however is an edge case that you won't run into much unless you buy very powerful strobes.

I will need to carry them on location, therefore weight -- including that of the necessary stands -- will matter.

There are different types of strobes available.

• Some are a single unit that sits on the stand. This requires a stronger (heavier) stand because the unit includes everything.
• Others have the "lamps" separate and connect them to a power pack via a cable. The pack is heavy but can sit on the ground, which lowers the requirements for the stands. this seems to be a more advantageous configuration for you.

Then of course there are speedlights, which are the lightest.

Answer 2

The main or key is set high to simulate afternoon sun. The fill is subordinate and best placed at lens height near the camera. You are filling shadows as seen from the camera's viewpoint.

If main and fill are equal as to output, measure main to subject distance and multiply by 1.4. This computes fill distance that causes the fill to be subordinate by 1 f-stop. This is the "bread and butter" ratio = 3:1.

For more contrast, set the fill 2-stops subordinate, the ratio is 5:1. To achieve, fill double (2X multiplier) main distance.

For even more contrast, set the ratio at 9:1. The multiplier is 2.8.

Move the background lamp forward or backwards to achieve different background shades. Same for hair light. Distance works but best is flash meter followed by half for quarter or eight power setting on the flash unit.

Hope this helps!