"If you ‘diaper’ a Fresnel (hang diffusion over the ends of the barn-doors), you increase the source size from the 10-inch lens to about 20 inches across the doors. While this is a doubling of the source size, it will have very little effect on the apparent softness of the light if it is 20 or 30 feet from the subject.
If you ever walk onto the set of a car commercial, you will most often see huge top sources. Products like the ‘Fisher Box’ come in various sizes including 17 feet by 57 feet. In relation to a car on a stage at 15 or 20 feet trim, you can see that the source is much larger than the subject." – Lee Rose of Design Partners
-From Swami Candela's upcoming Focus On Design column
