I would say your best bet for diffuse lighting would be an umbrella but if you don't want to spend the money make some wooden frames and attach white sheets to them. You can use these as both diffusers (if you aim the light through the sheet) or as reflectors. The issue with these is that you don't want to put your light too close to the sheet because it would pose a fire hazard with halogens being so hot, and also because the closer you put the lights to the sheet the more it will act as a point source without diffusing the light too well, giving you harder light.
Personally, I believe your best bet is to purchase some umbrellas. Cost will run about $40 USDs for a 45 inch umbrella. I don't think you would save all that much money building the reflector/diffuser and it would be more unwieldy. Plus, you need more studio room using them.
Good luck...hope this helped. |