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I just read "Lessons Learned on The Flash Bus Tour" and saw this image. In it, an assistant is holding some sort of white shoot-through diffuser (I got a similar one in my 5-in-one reflector kit) right in front of a soft box, which is lighting a portrait from above.

This is Joe McNally and David Hobby teaching at a workshop, so I assume there's a good reason for this. However, I don't know what it could be, or why it works.

  • My understanding is that the larger the effective area of the light source, the softer the light.
  • The softbox accomplishes this by bouncing the point-source light from the strobe around inside the box. When it exits, it's coming from a much larger "hole" (the front of the softbox), and from multiple angles, so it's nice and diffuse.
  • The hand-held diffuser doesn't increase the surface area of the light much at all, since it's being held so close to the soft box.
  • It would definitely reduce the amount of light, and I can understand needing that, but I'd think you could also do that by reducing the flash power (and saving your batteries).

So, what is the intent, results, and mechanism behind adding in the additional diffuser?

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2 Answers

up vote 8 down vote accepted

There are a few reasons that I'm aware of to combine soft-box and an additional diffuser:

  1. With smaller softboxes and hotter lights (read: flashes) the softbox isn't always able to completely diffuse the light source, which causes a 'hotspot'... Essentially the center of the diffusion 'square' is brighter than the edges. It's better than a 'nekkid' flash, but often you can still see the effects of that hotspot on the subject. There are several solutions (lower flash power, bigger box, move the flash further back in the box, etc.) but often the most expedient way to deal with the problem during a 'live' shoot is simply to throw another layer of diffusion in front of the box to soften out the hotspot.

  2. To lower the power of the light without having to rip open the box and fiddle with the flash. If you're within a stop(ish) of where you want the light to be and your light source is a manual flash where the controls are not easy to get at, it's often more expedient to simply lower the light by throwing another layer of diffusion in there than to monkey with the controls on the flash.

  3. It is possible to make a small box somewhat bigger (at the expense of power) by hanging a diffuser in front of it... Now you're correct that this is not what's going on with this particular picture (the diffuser is too close to the box to have much effect in this regard), but for the sake of 'completeness' I thought I'd mention it. :-)

For me personally, adding a diffuser is mostly a 'hack' manuver (not hack as in 'unprofessional,' hack as in 'hacker') when I have actual clients in the studio and I don't have the time (or don't want to take the time) to be fussing with the lights while they sit there and wait for me to get it right by 'dialing it in.' If the light is already 'pretty close' but I just need to dial it down 'a bit more,' or 'soften it slightly' it's usually a big time savings just to hack the solution, rather than fiddle with making it 'really really right' by dialing it in at the source, or moving the lights back and forth... again all while the client sits there and waits for me (extra not good if the client is in the short-attention-span set... AKA a child). :-)

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+1 - I like your answer better than mine... –  John Cavan Apr 28 '11 at 19:13
    
@John Cavan - Where is yours? –  ysap Apr 28 '11 at 23:19
    
@ysap - I removed it, Jay was far more complete and expressive, so why keep it around? :) –  John Cavan Apr 28 '11 at 23:24
    
@John Cavan - well, if everyone would do this, we'll end up with a concise version of Photo.SE ... Remember, this is a discussion forum, not an encyclopedia. Your answer could raise further discussions in the comments. –  ysap Apr 29 '11 at 0:41
1  
@ysap: Actually, according to the FAQ, a discussion forum is specifically not what photo-SE is... With that said, I understand (and agree with) the underlying sentiment of "multiple answers are better than a lone answer" that you were trying to convey. Just wanted to make sure we weren't inadvertently propagating misinformation... –  Jay Lance Photography Apr 29 '11 at 7:26
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I can think of a couple of reasons why I might do that:

  1. The minimum power on the strobe is still more light than I want and so extra diffusion is needed. It can happen... :)

  2. The light duration is what I want, but the amount of it is too much. If I reduce the power on the strobe, I reduce the duration of the light, so this lets me reduce the amount of light without adjusting the power.

I suspect there may be others, but these two immediately come to mind.

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Undeleted it under peer pressure... :) –  John Cavan Apr 29 '11 at 14:57
    
+1 for bowing to peer pressure. ;-) –  Jay Lance Photography Apr 30 '11 at 18:51
    
@Jay Lance - LOL, thanks for the bonus. ;) –  John Cavan May 1 '11 at 1:44
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protected by jrista Jan 27 '13 at 16:08

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