There are several drawbacks with diffusion panels when compared to using a softbox or brolly.
1) A large percentage of the light is bounced back off the surface and so approximately twice as much power is needed to give the same level of illumination as a softbox/brolly. That is to say, you lose about a stop.
2) The light bouncing back off the screen can cause spurious reflections or colour casts by interacting with walls and objects that are not intended to be illuminated.
3) They take up a lot of room in use.
You also have to be careful what material you use as a diffuser - the wrong stuff can give colour casts and/or patterns.
The position of the light is also critical. Too close to the screen and you do not get enough diffusion (and the light becomes more of a point source). To far back from the screen and even less light is transmitted by the screen.
Having said all that, they can work reasonably well if you bear those points in mind and you have nothing better to hand. They actually work better for modifying sunlight.
A better (and easier) light modifier is to just use two white boards.
The boards can be any convenient size (6' to 8' for full length portraits), and made of a variety of materials.
Large sheets of expanded polystyrene work well, or flat boards covered with white paper - even just painted white with a suitable paint.
Fix the two panels together in a 'V' (like the diffuser panel set up).
Place the panels in the position you want light to come from, with the bottom of the 'V' furthest from the subject.
<o like that.
Place your light (two lights better) pointing in to the 'V' (with two lights have one at the top, one at the bottom).
You will get an effect like diffuse daylight.
The V shape stops light spill, makes the 'diffuser' directional and ensures that nearly all the light goes on the subject.
Any lights will work - strobe, tungsten, quartz...
Cheap and easy. I (and a lot of other photographers of my acquaintance) used the set-up successfully for years.
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