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I was shooting on the beach into the sun and using my sb-700 on camera as fill. The camera I was using, a D3200, has a max shutter speed of 1/4000 of a second and no HSS. So, I used an ND filter — I don't remember exactly; maybe ND350ish — to slow down my shutter speed below my native sync speed. (I was metering off the sunset and using flash to expose my subject).

I noticed, however, that when I was using an on camera softbox on top of my flash that even though I was shooting at +3 EV, my exposures were coming out considerably darker than the exposures I did not use a soft box for. Of course, those without the softbox were really harsh and constrasty and rather unappealing.

So, this experience let me to this question: how many exposure value stops should I budget for my softbox? And, more generally, do front diffusers on different softboxes present with different qualities; i.e. different color temperature, different transmittance?

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    It depends on the softbox. Run a simple test in studio and you should have an answer. – dpollitt Mar 16 '15 at 0:51
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    It would depend entirely on the particular softbox in question. Some eat more light, some eat less. One way to find out how much yours absorbs is to do a manual test under strictly controlled conditions using manual flash settings. – Michael C Mar 16 '15 at 0:51
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    @dpollitt Great minds think alike! – Michael C Mar 16 '15 at 0:52
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    It's worth noting that trying to spread light to a larger area in very little depth, such as with an on-camera softbox, generally requires more agressive diffusing, and is likely to be less efficient than a bulkier version. – junkyardsparkle Mar 16 '15 at 6:30
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    @user74091 Please capitalize and punctuate properly — thanks! – mattdm Mar 16 '15 at 19:44
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The problem you had is that your flash was at it's maximum output. No amount of flash exposure compensation will fix that.

For maximum flash effectiveness you want your shutter speed at exactly the maximum sync speed even if it does support HSS.

If that's still not bright enough at maximum flash power output, then you have options:

  1. Move the flash closer to the subject.
  2. Use more powerful lighting. An SB-910 for example.
  3. Remove any internal baffles in your softbox.
  4. Don't use the softbox at all.
  5. Shoot nearer sunrise or sunset when the sunlight isn't so bright.

http://neilvn.com/tangents/controlling-bright-daylight-with-direct-off-camera-flash/

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There is no general rule nor a formula to see how many stops will a soft box knock down. The only way to find it out is to use a light meter and test those cases.

You also have to consider the fact that same soft box mounted differently on the same light source (system flash, strobe) would have different results. The resulting light on the subject when the light modifier is closer to the light bulb will be different when the light modifier is further away from the light bulb.

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