How many watts for which type of light modifier?

Started Sep 9, 2012 | Discussions thread
Sailor Blue
Sailor Blue Forum Pro • Posts: 13,243
Re: Getting closer to the answer

goodlucksdk wrote:

Kind of, see questions I have are can I operate a 7' parabolic antenna on those lights for example? Is there a way to know instead of just trying? I am looking for a general thumb of rule, as indicated in my first post, to put it in terms of a mathematical function:

Required Wattage = f (Light source size, Distance, F-stop, etc)

There is no way to really nail down a specific formula that will work with all lights and all modifiers. Even two "identical" lights or modifiers bought from the same manufacturer will have different efficiencies. Once you go to different manufacturers the variability goes up even more.

The best you can get for a single type of diffuser is just a series of general rules of thumb.

When it comes to comparing the efficiency between softboxes, white reflective umbrellas, white shoot through umbrellas, and silver umbrellas with or without diffuser socks the only thing you can do is test.

For a single type of diffuser the power needed to shoot at the same ISO & aperture is roughly related to the relationship of areas of the diffusers. For example, roughly twice as much power is needed if you double the surface area of the face of a softbox.

The inverse square law strictly applies to point light sources, not diffuse ones, but it will roughly tell you the ratio of powers needed to maintain the same exposure. The inverse square law works best at larger distances with diffuse light sources.

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