Softbox size question and speedlight

Started Sep 6, 2013 | Discussions thread
WFulton Senior Member • Posts: 2,695
Re: Interesting fact.....

Barrie Davis wrote:

Assuming (1) the baffles and diffusers are similar in their light stopping density, and (2) the output power of the head in use is kept the same....... large and small softboxes produce substantially the SAME exposure at the subject when the subject-to-HEAD distance is also the same.

Notice.... measuring to the HEAD, not the face of the softbox.

I agree with this (inverse square law is computed from the light source), and it is also true of umbrellas, regarding how close we choke up the flash head on the umbrella shaft.  The difference up close is a brighter but tiny area, or  farther back is a slightly less bright but larger area.   The metered light intensity is the same, but one way is a small light (less soft) and one way is a larger more soft light.

And in a similar way, it seems dumb to put a speedlight in the softbox, because of the fresnel reflection in the head. The most we can accomplish is illuminating a small diameter spot on center of the softbox fabric. Some light comes out of course, but it just ain't as planned.

Whereas the speedlight in an umbrella works great - full shaft length allows us to actually fill the umbrella, and use all of its size.  Softboxes are for bare bulb studio lights, with 180 degree coverage, to fill the softbox.

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